Driver to drive?

John Fields · Jan 27, 2012

On Wed, 25 Jan 2012 20:36:24 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 25 Jan 2012 20:07:30 -0800 (PST), Tim Williams
tmoranwms@gmail.com> wrote:

On Jan 25, 9:55 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I don't exactly understand the situation. Got a sketch?

Does this involve tantalum caps?

A description should suffice [rum disclaimer inserted here]:

Have you tried this?

http://tinyurl.com/7xqed3l

Once you sip this, cognac has no purpose any more.

---
For you, perhaps, but a true gourmand would acquiesce to that
some prefer the grape, and some the cane, without incurring judgment
and its attending rancor.
--
JF

Lostgallifreyan · Jan 27, 2012

Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in
news:lsg5i71e178pmkp99md7c868f84o9ia00d@4ax.com:

On Thu, 26 Jan 2012 20:39:38 -0600, Lostgallifreyan
no-one@nowhere.net> wrote:

Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in
news:vh24i79bak8er70nscqnl3d4ip3d6e4sqq@4ax.com:

If it's of any use, I could maybe do the test
using 100 mV instead of 1V.

Go, or no go?

Put 10K in the feedback just to make sure it isn't latching up via the
input ESD structure.

Use a resistive divider at the input to establish a midpoint input.

Before any loading, verify that output is also midpoint.

Can you get your hands on a decade resistor box?

Start with it set at least 20K

Connect it between output and V+ (thru you ammeter)

Gradually reduce resistance value avoiding massive steps by backing up
to a high setting before down-ranging.

Record current at which output node lifts.

This CA3140 may well be a POS, but your query has stirred my mind to
improve my generalized OpAmp to include:

(1) Output dead-band
(2) Output current limiting, V+ and V- values independently set
(3) Supply currents
(4) Swing limits (including load effects)

All with just 2 simple TANH functions... convergence guaranteed... all
derivatives exist

...Jim Thompson

I can't go further on this. I just spent three hours with resistors,
tweezers, a magnifier, I can barely see to type now. No resistor box, just a
pin deck and various resistors, and op-amps that may or may not be broken now
for all I know.

What results I did get were weird. First, I got VERY few results, running ten
miles is less stressful.

Trust me, I know.

Sample 1:
V mA Ltch mA Lift
5.12 0.760
10.01 1.074
15.01 1.206
20.02 1.588
25.02 1.560
30.02 1.842
35.00 2.048 1.310

That's it, and it's enough for me to conclude that going further will not
yield a pattern to grasp at all.

First, that latchup still occurs, even with 10K for feedback resistor.
Second, when the lift starts to occur, it is sharp, at a lower current but
still a latchup, just a smaller one. There is nothing neat, linear or
predictable about any of this. With supply voltages above 20V, the current
FELL from a slightly higher peak before latchup occured, as resistance from
out to V+ was gently lowered.

If this hasn't tried your patience as much as mine, please can you take a
brief look here:
http://repairfaq.cis.upenn.edu/Misc/laserdps.htm#dpsldd317
It's my laser driver, using a CA3140 if that's all avaliable, good for over
200 KHz easily, with fairly good wave shapes. LT1215 was needed to get it
good to 1 MHz and beyond. (Sam Goldwasser has seen it and confirms that it
works). My question is, as CA3140 seems to justify the POS tag

.... can
you suggest amps that are more widely available than the LT1215 that might do
as well, or better? Bear in mind that unity gain or less for these amps (the
LM317 is doing the grunt work) seems to imply that fast slew rate is vital,
but high GBP is less so. (Other people using video amps are getting less fast
and shapely waveforms at 100 KHz in their drivers, than mine gets at 200 KHz
or more, even with the CA3140. Not bragging, this is purely to set context.
It's an unusual situation). I want to make this easy for people to build.
GEtting LT1215's isn't so easy, or as cheap, as CA3140, and the margin of
difference is enough that I hope to find other contenders better placed than
either to do this well. Whatever I try must run on a single rail supply as
well as having fast slew rate. Apparently not an easy pairing of requirements
to meet....

John Larkin · Jan 27, 2012

On Fri, 27 Jan 2012 16:36:47 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Wed, 25 Jan 2012 20:36:24 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 25 Jan 2012 20:07:30 -0800 (PST), Tim Williams
tmoranwms@gmail.com> wrote:

On Jan 25, 9:55 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I don't exactly understand the situation. Got a sketch?

Does this involve tantalum caps?

A description should suffice [rum disclaimer inserted here]:

Have you tried this?

http://tinyurl.com/7xqed3l

Once you sip this, cognac has no purpose any more.

---
For you, perhaps, but a true gourmand would acquiesce to that
some prefer the grape, and some the cane, without incurring judgment
and its attending rancor.

Whatever you say.

John

Lostgallifreyan · Jan 27, 2012

Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in
news:0hl3i7d947gvldlhm4sh9kij5d8hsksp39@4ax.com:

(I have the basic core running to specification. All I need is to add
the sink current.)

Despite my new wariness about this amp, and the intractability of getting any
clear picture of current sink limitations, I'm still pleased about that, and
keen to try the model.

It's worth having purely because that IC is so
widely used, and many people might still reach for one whenever they want a
very high resistance input and a single rail supply, especially as they
probably have one somewhere.

Jim Thompson · Jan 27, 2012

On Fri, 27 Jan 2012 16:51:39 -0600, Lostgallifreyan
<no-one@nowhere.net> wrote:

Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in
news:lsg5i71e178pmkp99md7c868f84o9ia00d@4ax.com:

On Thu, 26 Jan 2012 20:39:38 -0600, Lostgallifreyan
no-one@nowhere.net> wrote:

Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in
news:vh24i79bak8er70nscqnl3d4ip3d6e4sqq@4ax.com:

If it's of any use, I could maybe do the test
using 100 mV instead of 1V.

Go, or no go?

Put 10K in the feedback just to make sure it isn't latching up via the
input ESD structure.

Use a resistive divider at the input to establish a midpoint input.

Before any loading, verify that output is also midpoint.

Can you get your hands on a decade resistor box?

Start with it set at least 20K

Connect it between output and V+ (thru you ammeter)

Gradually reduce resistance value avoiding massive steps by backing up
to a high setting before down-ranging.

Record current at which output node lifts.

This CA3140 may well be a POS, but your query has stirred my mind to
improve my generalized OpAmp to include:

(1) Output dead-band
(2) Output current limiting, V+ and V- values independently set
(3) Supply currents
(4) Swing limits (including load effects)

All with just 2 simple TANH functions... convergence guaranteed... all
derivatives exist

...Jim Thompson

I can't go further on this. I just spent three hours with resistors,
tweezers, a magnifier, I can barely see to type now. No resistor box, just a
pin deck and various resistors, and op-amps that may or may not be broken now
for all I know.

What results I did get were weird. First, I got VERY few results, running ten
miles is less stressful. Trust me, I know.

Sample 1:
V mA Ltch mA Lift
5.12 0.760
10.01 1.074
15.01 1.206
20.02 1.588
25.02 1.560
30.02 1.842
35.00 2.048 1.310

That's it, and it's enough for me to conclude that going further will not
yield a pattern to grasp at all.

First, that latchup still occurs, even with 10K for feedback resistor.
Second, when the lift starts to occur, it is sharp, at a lower current but
still a latchup, just a smaller one. There is nothing neat, linear or
predictable about any of this. With supply voltages above 20V, the current
FELL from a slightly higher peak before latchup occured, as resistance from
out to V+ was gently lowered.

If this hasn't tried your patience as much as mine, please can you take a
brief look here:
http://repairfaq.cis.upenn.edu/Misc/laserdps.htm#dpsldd317
It's my laser driver, using a CA3140 if that's all avaliable, good for over
200 KHz easily, with fairly good wave shapes. LT1215 was needed to get it
good to 1 MHz and beyond. (Sam Goldwasser has seen it and confirms that it
works). My question is, as CA3140 seems to justify the POS tag .... can
you suggest amps that are more widely available than the LT1215 that might do
as well, or better? Bear in mind that unity gain or less for these amps (the
LM317 is doing the grunt work) seems to imply that fast slew rate is vital,
but high GBP is less so. (Other people using video amps are getting less fast
and shapely waveforms at 100 KHz in their drivers, than mine gets at 200 KHz
or more, even with the CA3140. Not bragging, this is purely to set context.
It's an unusual situation). I want to make this easy for people to build.
GEtting LT1215's isn't so easy, or as cheap, as CA3140, and the margin of
difference is enough that I hope to find other contenders better placed than
either to do this well. Whatever I try must run on a single rail supply as
well as having fast slew rate. Apparently not an easy pairing of requirements
to meet....

I'll post my subcircuit as it stands right now and you can see if it
remotely matches your measured performance.... unwrap the usual
suspects...

******************************************************************
****** PINS: 1 2 3 4 5 6 7 8 ******
..SUBCKT CA3140 NULL1 INN INP VN NULL5 OUT VP STROBE
Q_Q11 N_1 N_1 NULL5 CA3046_ORG
Q_Q12 N_2 N_1 NULL1 CA3046_ORG
C_CIN INP INN 4pF
GDC_I2 VP STROBE VALUE
{(1+TANH(2.2976*(2*V(VP,STROBE)/1V-1)))*200uA/2}
Q_Q17 VP STROBE N_3 CA3046_ORG
R_R11 N_4 N_5 20
R_R9 N_6 N_4 50
R_R10 N_7 N_4 1K
Q_Q19 N_8 N_7 N_5 CA3046_ORG
Q_QD7 VP VP N_6 CA3046_ORG
R_R8 N_3 N_9 1K
GDC_I3 N_3 VN VALUE {(1+TANH(2.2976*(2*V(N_3,VN)/1V-1)))*2uA/2}
Q_Q13 STROBE N_2 VN CA3046_ORG
R_R5 NULL1 VN 500
R_R4 NULL5 VN 500
F_F1 STROBE VN VF_F1 1
VF_F1 VP N_8 0V
Q_Q18 N_5 N_9 OUT CA3046_ORG
G_G2 OUT VN VALUE { (TANH(3.66*V(OUT, N_9)-1.83)+19/17)*8.5mA
}
C_C1 STROBE N_2 12pF
G_G1 N_2 N_1 VALUE { (TANH(2.2/188mV*V(INP, INN))+1)*25uA }
GDC_I1 VP N_2 VALUE
{(1+TANH(2.2976*(2*V(VP,N_2)/1V-1)))*50uA/2}
******************************************************************
..MODEL CA3046_ORG NPN IS=3.860200F BF=120 NF=1.04845 VAF=61.1026 IKF=
+ 50.000000M ISE=3.100000P NE=2.16533 BR=100.101000M NR=1.04845 ISC=0
+ NC=1 RB=214.644 RBM=214.644 RE=721.362980M RC=9.2065 CJE=1.249000P
+ VJE=899.999940M MJE=499.999970M TF=210.000000P XTF=1.85 VTF=1.585
+ ITF=50.000000M PTF=0 CJC=1.000000P VJC=749.999940M MJC=333.000000M
+ XCJC=499.999970M TR=10.000000N CJS=6.300000P VJS=749.999940M MJS=
+ 499.999970M XTB=1.5 EG=1.11 XTI=3 KF=0 AF=1 FC=499.999970M
******************************************************************
..ENDS CA3140
******************************************************************

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice

John Fields · Jan 28, 2012

On Fri, 27 Jan 2012 15:10:01 -0800, John Larkin
<jlarkin@highlandtechnology.com> wrote:

On Fri, 27 Jan 2012 16:36:47 -0600, John Fields
jfields@austininstruments.com> wrote:

On Wed, 25 Jan 2012 20:36:24 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 25 Jan 2012 20:07:30 -0800 (PST), Tim Williams
tmoranwms@gmail.com> wrote:

On Jan 25, 9:55 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I don't exactly understand the situation. Got a sketch?

Does this involve tantalum caps?

A description should suffice [rum disclaimer inserted here]:

Have you tried this?

http://tinyurl.com/7xqed3l

Once you sip this, cognac has no purpose any more.

---
For you, perhaps, but a true gourmand would acquiesce to that
some prefer the grape, and some the cane, without incurring judgment
and its attending rancor.

Whatever you say.

John

---
Thank you.

--
JF

Lostgallifreyan · Jan 28, 2012

Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in
news:mbd6i7t4ea1jr2tr3pddvvbtp1v5bh894o@4ax.com:

I'll post my subcircuit as it stands right now and you can see if it
remotely matches your measured performance.... unwrap the usual
suspects...

It has about -0.6V of output spiking not seen in real ones when a 5V
200 KHz square wave goes negative on the inverting input of a differential
amplifier, and a small kink in the slope when the input signal rises. This
kink occurs slightly earlier on the slope if the load is reduced from 1K to
10K. It's definitely slightly odd. Generally the frequency response seems
about right, judging by effects on square waves at 1 MHz, but they're spikier
than the real ones would be.

(All this bearing in mind that LT1215 was showing very close agreement
between model and built circuit in virtually identical context (only the
compensation caps were absent in the CA3240 version as it doesn't need
them, and this is true for the current model too, where they make no
difference to the wave other than a very slight recuction in that spiking).

Please let me know if you have suggestions for that laser driver's op-amps.
http://repairfaq.cis.upenn.edu/Misc/laserdps.htm#dpsldd317
I think you're right that the CA3140 (CA3240 in this case) is not to be
relied on, and I hope that the LT1215 is not the only game in town! It's
great but I imagine there may be a standard that is cheaper and easier to get
while being as good, and that maybe better ones exist too. I need a dual amp,
for single rail, with fast slew, ideally unconditionally stable at unity
gain. Something that makes a 1 MHz square have a bit more snap to its rise
and fall times. There may be amps that work well in this context that might
not be expected to, knowing that CA3140 even works at all well, and that
LT1215 works very well, might help prompt ideas from you because you know
amps a lot better than me. I Googled for months, and followed LT's and NS's
IC suggestions without coming up with much. Maybe there isn't, but if LT can
make the LT1215 I do hope someone made some good alternative to try.

Jim Thompson · Jan 28, 2012

On Fri, 27 Jan 2012 18:40:29 -0600, Lostgallifreyan
<no-one@nowhere.net> wrote:

Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in
news:mbd6i7t4ea1jr2tr3pddvvbtp1v5bh894o@4ax.com:

I'll post my subcircuit as it stands right now and you can see if it
remotely matches your measured performance.... unwrap the usual
suspects...

It has about -0.6V of output spiking not seen in real ones when a 5V
200 KHz square wave goes negative on the inverting input of a differential
amplifier, and a small kink in the slope when the input signal rises. This
kink occurs slightly earlier on the slope if the load is reduced from 1K to
10K. It's definitely slightly odd. Generally the frequency response seems
about right, judging by effects on square waves at 1 MHz, but they're spikier
than the real ones would be.

(All this bearing in mind that LT1215 was showing very close agreement
between model and built circuit in virtually identical context (only the
compensation caps were absent in the CA3240 version as it doesn't need
them, and this is true for the current model too, where they make no
difference to the wave other than a very slight recuction in that spiking).

Please let me know if you have suggestions for that laser driver's op-amps.
http://repairfaq.cis.upenn.edu/Misc/laserdps.htm#dpsldd317
I think you're right that the CA3140 (CA3240 in this case) is not to be
relied on, and I hope that the LT1215 is not the only game in town! It's
great but I imagine there may be a standard that is cheaper and easier to get
while being as good, and that maybe better ones exist too. I need a dual amp,
for single rail, with fast slew, ideally unconditionally stable at unity
gain. Something that makes a 1 MHz square have a bit more snap to its rise
and fall times. There may be amps that work well in this context that might
not be expected to, knowing that CA3140 even works at all well, and that
LT1215 works very well, might help prompt ideas from you because you know
amps a lot better than me. I Googled for months, and followed LT's and NS's
IC suggestions without coming up with much. Maybe there isn't, but if LT can
make the LT1215 I do hope someone made some good alternative to try.

What puzzled me is I couldn't get a match between DC open-loop gain
and the gain-bandwidth crossing point... I get about 11MHz GBW with
100dB at DC, so I generally don't trust the data sheet... which says
~4MHz. My 11MHz probably accounts for the "spikier" transient
behavior.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice

Lostgallifreyan · Jan 28, 2012

Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in
news:kgh6i7lktgbqb90va2oq8jh4fvbet02e45@4ax.com:

On Fri, 27 Jan 2012 18:40:29 -0600, Lostgallifreyan
no-one@nowhere.net> wrote:

Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote
in news:mbd6i7t4ea1jr2tr3pddvvbtp1v5bh894o@4ax.com:

I'll post my subcircuit as it stands right now and you can see if it
remotely matches your measured performance.... unwrap the usual
suspects...

It has about -0.6V of output spiking not seen in real ones when a 5V
200 KHz square wave goes negative on the inverting input of a
differential amplifier, and a small kink in the slope when the input
signal rises. This kink occurs slightly earlier on the slope if the load
is reduced from 1K to 10K. It's definitely slightly odd. Generally the
frequency response seems about right, judging by effects on square waves
at 1 MHz, but they're spikier than the real ones would be.

(All this bearing in mind that LT1215 was showing very close agreement
between model and built circuit in virtually identical context (only the
compensation caps were absent in the CA3240 version as it doesn't need
them, and this is true for the current model too, where they make no
difference to the wave other than a very slight recuction in that
spiking).

Please let me know if you have suggestions for that laser driver's
op-amps. http://repairfaq.cis.upenn.edu/Misc/laserdps.htm#dpsldd317
I think you're right that the CA3140 (CA3240 in this case) is not to be
relied on, and I hope that the LT1215 is not the only game in town! It's
great but I imagine there may be a standard that is cheaper and easier
to get while being as good, and that maybe better ones exist too. I need
a dual amp, for single rail, with fast slew, ideally unconditionally
stable at unity gain. Something that makes a 1 MHz square have a bit
more snap to its rise and fall times. There may be amps that work well
in this context that might not be expected to, knowing that CA3140 even
works at all well, and that LT1215 works very well, might help prompt
ideas from you because you know amps a lot better than me. I Googled for
months, and followed LT's and NS's IC suggestions without coming up with
much. Maybe there isn't, but if LT can make the LT1215 I do hope someone
made some good alternative to try.

What puzzled me is I couldn't get a match between DC open-loop gain
and the gain-bandwidth crossing point... I get about 11MHz GBW with
100dB at DC, so I generally don't trust the data sheet... which says
~4MHz. My 11MHz probably accounts for the "spikier" transient
behavior.

The negative-going overshoot is large, maybe 100 times what it would be in a
real one at 200 KHz. I deliberately slowed the input square wave slopes to
1ľs (not that much flat-time left after that, at 200 KHz), and it didn't help
much. In a context with very little parasitic inductance or capacitance no
op-amp should have this problem.

What DID help a lot was choosing the resistors in the first stage
differential amp of that laser driver circuit to be higher than those the
LT1215 wants for fast accuracy. Instead of 3K3 and 680R, I tried 33K and 6K8,
which removed the 'undershoot' but rounded off the wave far too much, so then
I tried 10K and 2K, (ratio close enough), and the result is very good. But as
far as I know, the CA3140 would not be that sensitive to these changes in
resistance.

What are the values of the zeners in that original schematic? Also, what are
the three or so most important spice parameters to be used in the internal
models for the BJT, MOS, and diode models?

Also, what other op-amp might work for that laser driver circuit I linked to?
(Single rail, fast slew, dual stage, pluggable replacement for CA3240,
LT1215...)

Jim Thompson · Jan 28, 2012

On Sat, 28 Jan 2012 08:11:33 -0600, Lostgallifreyan
<no-one@nowhere.net> wrote:

Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote in
news:kgh6i7lktgbqb90va2oq8jh4fvbet02e45@4ax.com:

On Fri, 27 Jan 2012 18:40:29 -0600, Lostgallifreyan
no-one@nowhere.net> wrote:

Jim Thompson <To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote
in news:mbd6i7t4ea1jr2tr3pddvvbtp1v5bh894o@4ax.com:

I'll post my subcircuit as it stands right now and you can see if it
remotely matches your measured performance.... unwrap the usual
suspects...

It has about -0.6V of output spiking not seen in real ones when a 5V
200 KHz square wave goes negative on the inverting input of a
differential amplifier, and a small kink in the slope when the input
signal rises. This kink occurs slightly earlier on the slope if the load
is reduced from 1K to 10K. It's definitely slightly odd. Generally the
frequency response seems about right, judging by effects on square waves
at 1 MHz, but they're spikier than the real ones would be.

(All this bearing in mind that LT1215 was showing very close agreement
between model and built circuit in virtually identical context (only the
compensation caps were absent in the CA3240 version as it doesn't need
them, and this is true for the current model too, where they make no
difference to the wave other than a very slight recuction in that
spiking).

Please let me know if you have suggestions for that laser driver's
op-amps. http://repairfaq.cis.upenn.edu/Misc/laserdps.htm#dpsldd317
I think you're right that the CA3140 (CA3240 in this case) is not to be
relied on, and I hope that the LT1215 is not the only game in town! It's
great but I imagine there may be a standard that is cheaper and easier
to get while being as good, and that maybe better ones exist too. I need
a dual amp, for single rail, with fast slew, ideally unconditionally
stable at unity gain. Something that makes a 1 MHz square have a bit
more snap to its rise and fall times. There may be amps that work well
in this context that might not be expected to, knowing that CA3140 even
works at all well, and that LT1215 works very well, might help prompt
ideas from you because you know amps a lot better than me. I Googled for
months, and followed LT's and NS's IC suggestions without coming up with
much. Maybe there isn't, but if LT can make the LT1215 I do hope someone
made some good alternative to try.

What puzzled me is I couldn't get a match between DC open-loop gain
and the gain-bandwidth crossing point... I get about 11MHz GBW with
100dB at DC, so I generally don't trust the data sheet... which says
~4MHz. My 11MHz probably accounts for the "spikier" transient
behavior.

The negative-going overshoot is large, maybe 100 times what it would be in a
real one at 200 KHz. I deliberately slowed the input square wave slopes to
1ľs (not that much flat-time left after that, at 200 KHz), and it didn't help
much. In a context with very little parasitic inductance or capacitance no
op-amp should have this problem.

What DID help a lot was choosing the resistors in the first stage
differential amp of that laser driver circuit to be higher than those the
LT1215 wants for fast accuracy. Instead of 3K3 and 680R, I tried 33K and 6K8,
which removed the 'undershoot' but rounded off the wave far too much, so then
I tried 10K and 2K, (ratio close enough), and the result is very good. But as
far as I know, the CA3140 would not be that sensitive to these changes in
resistance.

What are the values of the zeners in that original schematic? Also, what are
the three or so most important spice parameters to be used in the internal
models for the BJT, MOS, and diode models?

Also, what other op-amp might work for that laser driver circuit I linked to?
(Single rail, fast slew, dual stage, pluggable replacement for CA3240,
LT1215...)

The CA3140 datasheet has conflicting statements, low sink current, yet
18mA short to V+, so I scaled to that short current. Thus a
suggestion, in this line...

G_G2 OUT VN VALUE {(TANH(3.66*V(OUT, N_9)-1.83)+19/17)*8.5mA}
^^^^^
Reduce this value--------------------------------------^^^^^

Until peaking matches your measurements.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice

John Larkin · Jan 28, 2012

On Thu, 26 Jan 2012 17:50:24 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Wed, 25 Jan 2012 18:44:44 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 25 Jan 2012 19:26:26 -0600, John Fields
jfields@austininstruments.com> wrote:

On Tue, 24 Jan 2012 21:11:23 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi <tombiasi@optonline.net
wrote:

On Tue, 24 Jan 2012 15:56:48 -0800 (PST), Bill Bowden

I've seen LED flashlights with 2 white 3 volt LEDs wired directly in
parallel across two AA batteries.

-Bill

They rely heavily on the battery's internal resistance and luck.

LEDs have a current:voltage slope that's not a brick wall.

---
Pretty close, though, once you get past the knee.

It's usually the other way around: exponential at low currents, ohmic
at higher currents.

---
It never really gets ohmic unless you drive the junction hard enough
to short it,

Nonsense, unless you plan to quibble about the word "really."

and once you get past the knee -

As noted, diodes don't have a "knee" unless you arbitrarily define
one.

where a relatively large

voltage change results in a small current change - the slope changes
so that for a relatively small increase in voltage you get a large
increase in current.

No, that's backwards. Diodes, and LEDs, have current exponential on
voltage at low currents. At higher currents, the contact and bulk
resistivity start to dominate, and the voltage:current curve gets
nearly linear.

Just look at the curves on real led data sheets. The smaller parts
start to get ohmic at low currents, just a few mA. Bigger junctions
will stay exponential at higher currents, because they have less bulk
resistance.

This is a really tiny junction, so the v/i curve is a straight line at
operating currents:

http://vcclite.com/wp-content/files/VAOL-S8GT4-LED-0805-green.pdf

Bigger parts start to go ohmic at higher currents:

http://www.vishay.com/docs/81345/vlmp232.pdf

http://catalog.osram-os.com/jsp/download.jsp?rootPath=/media/&name=LA_LO_LY_E67F_Pb_free.pdf&docPath=Graphics/00057343_0.pdf&url=/media//_en/Graphics/00057343_0.pdf

Ordinary diodes do this, too. That's why diodes have some current
where their v:i curve has a zero temperature coefficient; the
exponential part has a negative TC but the bulk resistance TC is
positive. For small schottky diodes, that can be in the 10 mA
ballpark, so that can be useful.

--

John Larkin, President Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

josephkk · Jan 28, 2012

On Tue, 24 Jan 2012 11:54:18 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

On Mon, 23 Jan 2012 06:22:25 -0600, Lostgallifreyan
no-one@nowhere.net> wrote:

Lostgallifreyan <no-one@nowhere.net> wrote in
news:Xns9FE1C0DDF59A8zoodlewurdle@216.196.109.145:

[snip]

Ran your schematic... behavior is extraordinarily weird, output hangs
about 0.7V below ground when powered from +/-12V

Please recheck your netlist. Thanks!

...Jim Thompson

Is your test circuit in LTSpice form? I would like to fiddle with the
model in LTSpice.

?-)

John Fields · Jan 29, 2012

On Sat, 28 Jan 2012 11:58:15 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 26 Jan 2012 17:50:24 -0600, John Fields
jfields@austininstruments.com> wrote:

On Wed, 25 Jan 2012 18:44:44 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 25 Jan 2012 19:26:26 -0600, John Fields
jfields@austininstruments.com> wrote:

On Tue, 24 Jan 2012 21:11:23 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi <tombiasi@optonline.net
wrote:

On Tue, 24 Jan 2012 15:56:48 -0800 (PST), Bill Bowden

I've seen LED flashlights with 2 white 3 volt LEDs wired directly in
parallel across two AA batteries.

-Bill

They rely heavily on the battery's internal resistance and luck.

LEDs have a current:voltage slope that's not a brick wall.

---
Pretty close, though, once you get past the knee.

It's usually the other way around: exponential at low currents, ohmic
at higher currents.

---
It never really gets ohmic unless you drive the junction hard enough
to short it,

Nonsense, unless you plan to quibble about the word "really."

---
There's no quibbling about the word "really", the quibble is about
your assertion that a diode junction is ohmic at vaguely described
qualitative "higher currents".
---

and once you get past the knee -

As noted, diodes don't have a "knee" unless you arbitrarily define
one.

---
"As noted"???

I don't really think an "arbitrary" definition is necessary, since the
location of the knee has been with us for decades.
---

where a relatively large
voltage change results in a small current change - the slope changes
so that for a relatively small increase in voltage you get a large
increase in current.

No, that's backwards. Diodes, and LEDs, have current exponential on
voltage at low currents. At higher currents, the contact and bulk
resistivity start to dominate, and the voltage:current curve gets
nearly linear.

---
True enough, but "nearly linear" isn't quite the same as "ohmic", is
it?

And, it's just plain silly talk since it has very little to do with
what we're talking about, which is running LEDs from a voltage source.
Look at the V:I curve for a vanilla silicon diode at from zero volts
to where it lets, say, 1mA through the diode and you'll see that the
voltage across the diode, at that point, will be about 0.7V, mas o
menos.

Now run the voltage up to about 1.4V.

Will the current through the diode stop at 2mA?
---

Just look at the curves on real led data sheets. The smaller parts
start to get ohmic at low currents, just a few mA. Bigger junctions
will stay exponential at higher currents, because they have less bulk
resistance.

This is a really tiny junction, so the v/i curve is a straight line at
operating currents:

http://vcclite.com/wp-content/files/VAOL-S8GT4-LED-0805-green.pdf

---
Surely you can't be serious.

If you examine the Forward Current vs Forward Voltage curve with some
care, you'll find that with 10mA through the LED it drops about 1.9
volts, and with 20mA through it drops about 2 volts.

Simply using Ohm's law in both cases - in order to determine the
resistance of the LED in each case - yields for the first case:

E 1.9V
R = --- = ------- = 190 ohms
I 1e-2A

and for the second:

E 2.0V
R = --- = ------- = 100 ohms
I 2e-2A

then, since an ohmic load's resistance must be constant as the current
through it varies, that LED is clearly _not_ an ohmic load.
---

Bigger parts start to go ohmic at higher currents:

http://www.vishay.com/docs/81345/vlmp232.pdf

http://catalog.osram-os.com/jsp/download.jsp?rootPath=/media/&name=LA_LO_LY_E67F_Pb_free.pdf&docPath=Graphics/00057343_0.pdf&url=/media//_en/Graphics/00057343_0.pdf

Ordinary diodes do this, too. That's why diodes have some current
where their v:i curve has a zero temperature coefficient; the
exponential part has a negative TC but the bulk resistance TC is
positive. For small schottky diodes, that can be in the 10 mA
ballpark, so that can be useful.

---
Grasping at straws?

--
JF

krw@att.bizzzzzzzzzzzz · Jan 29, 2012

On Sat, 28 Jan 2012 18:43:09 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sat, 28 Jan 2012 11:58:15 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 26 Jan 2012 17:50:24 -0600, John Fields
jfields@austininstruments.com> wrote:

On Wed, 25 Jan 2012 18:44:44 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 25 Jan 2012 19:26:26 -0600, John Fields
jfields@austininstruments.com> wrote:

On Tue, 24 Jan 2012 21:11:23 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi <tombiasi@optonline.net
wrote:

On Tue, 24 Jan 2012 15:56:48 -0800 (PST), Bill Bowden

I've seen LED flashlights with 2 white 3 volt LEDs wired directly in
parallel across two AA batteries.

-Bill

They rely heavily on the battery's internal resistance and luck.

LEDs have a current:voltage slope that's not a brick wall.

---
Pretty close, though, once you get past the knee.

It's usually the other way around: exponential at low currents, ohmic
at higher currents.

---
It never really gets ohmic unless you drive the junction hard enough
to short it,

Nonsense, unless you plan to quibble about the word "really."

---
There's no quibbling about the word "really", the quibble is about
your assertion that a diode junction is ohmic at vaguely described
qualitative "higher currents".
---

and once you get past the knee -

As noted, diodes don't have a "knee" unless you arbitrarily define
one.

---
"As noted"???

I don't really think an "arbitrary" definition is necessary, since the
location of the knee has been with us for decades.

John, I told you there are people who believe there is a "knee" in an
exponential function. ;-)

---

where a relatively large
voltage change results in a small current change - the slope changes
so that for a relatively small increase in voltage you get a large
increase in current.

No, that's backwards. Diodes, and LEDs, have current exponential on
voltage at low currents. At higher currents, the contact and bulk
resistivity start to dominate, and the voltage:current curve gets
nearly linear.

---
True enough, but "nearly linear" isn't quite the same as "ohmic", is
it?

What do you think constant dI/DV means?

<...>

Jim Thompson · Jan 29, 2012

On Sat, 28 Jan 2012 18:43:09 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sat, 28 Jan 2012 11:58:15 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 26 Jan 2012 17:50:24 -0600, John Fields
jfields@austininstruments.com> wrote:

On Wed, 25 Jan 2012 18:44:44 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 25 Jan 2012 19:26:26 -0600, John Fields
jfields@austininstruments.com> wrote:

On Tue, 24 Jan 2012 21:11:23 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi <tombiasi@optonline.net
wrote:

On Tue, 24 Jan 2012 15:56:48 -0800 (PST), Bill Bowden

I've seen LED flashlights with 2 white 3 volt LEDs wired directly in
parallel across two AA batteries.

-Bill

They rely heavily on the battery's internal resistance and luck.

LEDs have a current:voltage slope that's not a brick wall.

---
Pretty close, though, once you get past the knee.

It's usually the other way around: exponential at low currents, ohmic
at higher currents.

---
It never really gets ohmic unless you drive the junction hard enough
to short it,

Nonsense, unless you plan to quibble about the word "really."

---
There's no quibbling about the word "really", the quibble is about
your assertion that a diode junction is ohmic at vaguely described
qualitative "higher currents".
---

and once you get past the knee -

As noted, diodes don't have a "knee" unless you arbitrarily define
one.

---
"As noted"???

I don't really think an "arbitrary" definition is necessary, since the
location of the knee has been with us for decades.
---

where a relatively large
voltage change results in a small current change - the slope changes
so that for a relatively small increase in voltage you get a large
increase in current.

No, that's backwards. Diodes, and LEDs, have current exponential on
voltage at low currents. At higher currents, the contact and bulk
resistivity start to dominate, and the voltage:current curve gets
nearly linear.

---
True enough, but "nearly linear" isn't quite the same as "ohmic", is
it?

And, it's just plain silly talk since it has very little to do with
what we're talking about, which is running LEDs from a voltage source.
Look at the V:I curve for a vanilla silicon diode at from zero volts
to where it lets, say, 1mA through the diode and you'll see that the
voltage across the diode, at that point, will be about 0.7V, mas o
menos.

Now run the voltage up to about 1.4V.

Will the current through the diode stop at 2mA?
---

Just look at the curves on real led data sheets. The smaller parts
start to get ohmic at low currents, just a few mA. Bigger junctions
will stay exponential at higher currents, because they have less bulk
resistance.

This is a really tiny junction, so the v/i curve is a straight line at
operating currents:

http://vcclite.com/wp-content/files/VAOL-S8GT4-LED-0805-green.pdf

---
Surely you can't be serious.

If you examine the Forward Current vs Forward Voltage curve with some
care, you'll find that with 10mA through the LED it drops about 1.9
volts, and with 20mA through it drops about 2 volts.

Simply using Ohm's law in both cases - in order to determine the
resistance of the LED in each case - yields for the first case:

E 1.9V
R = --- = ------- = 190 ohms
I 1e-2A

and for the second:

E 2.0V
R = --- = ------- = 100 ohms
I 2e-2A

then, since an ohmic load's resistance must be constant as the current
through it varies, that LED is clearly _not_ an ohmic load.
---

Bigger parts start to go ohmic at higher currents:

http://www.vishay.com/docs/81345/vlmp232.pdf

http://catalog.osram-os.com/jsp/download.jsp?rootPath=/media/&name=LA_LO_LY_E67F_Pb_free.pdf&docPath=Graphics/00057343_0.pdf&url=/media//_en/Graphics/00057343_0.pdf

Ordinary diodes do this, too. That's why diodes have some current
where their v:i curve has a zero temperature coefficient; the
exponential part has a negative TC but the bulk resistance TC is
positive. For small schottky diodes, that can be in the 10 mA
ballpark, so that can be useful.

---
Grasping at straws?
\

Let it go Fields. It's Larkin. Did you notice the new narcissistic
SIG ?

John Larkin · Jan 29, 2012

On Sat, 28 Jan 2012 20:11:51 -0500, "krw@att.bizzzzzzzzzzzz"
<krw@att.bizzzzzzzzzzzz> wrote:

On Sat, 28 Jan 2012 18:43:09 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sat, 28 Jan 2012 11:58:15 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 26 Jan 2012 17:50:24 -0600, John Fields
jfields@austininstruments.com> wrote:

On Wed, 25 Jan 2012 18:44:44 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 25 Jan 2012 19:26:26 -0600, John Fields
jfields@austininstruments.com> wrote:

On Tue, 24 Jan 2012 21:11:23 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi <tombiasi@optonline.net
wrote:

On Tue, 24 Jan 2012 15:56:48 -0800 (PST), Bill Bowden

I've seen LED flashlights with 2 white 3 volt LEDs wired directly in
parallel across two AA batteries.

-Bill

They rely heavily on the battery's internal resistance and luck.

LEDs have a current:voltage slope that's not a brick wall.

---
Pretty close, though, once you get past the knee.

It's usually the other way around: exponential at low currents, ohmic
at higher currents.

---
It never really gets ohmic unless you drive the junction hard enough
to short it,

Nonsense, unless you plan to quibble about the word "really."

---
There's no quibbling about the word "really", the quibble is about
your assertion that a diode junction is ohmic at vaguely described
qualitative "higher currents".
---

and once you get past the knee -

As noted, diodes don't have a "knee" unless you arbitrarily define
one.

---
"As noted"???

I don't really think an "arbitrary" definition is necessary, since the
location of the knee has been with us for decades.

John, I told you there are people who believe there is a "knee" in an
exponential function. ;-)

And there are people who think that the only kind of resistance is
E/I.

Some famous person once said "When all you know is Ohm's Law,
everything looks like a resistor."

Well, that was me, actually.

--

John Larkin, President Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

John Larkin · Jan 29, 2012

On Sat, 28 Jan 2012 18:43:09 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sat, 28 Jan 2012 11:58:15 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 26 Jan 2012 17:50:24 -0600, John Fields
jfields@austininstruments.com> wrote:

On Wed, 25 Jan 2012 18:44:44 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 25 Jan 2012 19:26:26 -0600, John Fields
jfields@austininstruments.com> wrote:

On Tue, 24 Jan 2012 21:11:23 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi <tombiasi@optonline.net
wrote:

On Tue, 24 Jan 2012 15:56:48 -0800 (PST), Bill Bowden

I've seen LED flashlights with 2 white 3 volt LEDs wired directly in
parallel across two AA batteries.

-Bill

They rely heavily on the battery's internal resistance and luck.

LEDs have a current:voltage slope that's not a brick wall.

---
Pretty close, though, once you get past the knee.

It's usually the other way around: exponential at low currents, ohmic
at higher currents.

---
It never really gets ohmic unless you drive the junction hard enough
to short it,

Nonsense, unless you plan to quibble about the word "really."

---
There's no quibbling about the word "really", the quibble is about
your assertion that a diode junction is ohmic at vaguely described
qualitative "higher currents".
---

and once you get past the knee -

As noted, diodes don't have a "knee" unless you arbitrarily define
one.

---
"As noted"???

I don't really think an "arbitrary" definition is necessary, since the
location of the knee has been with us for decades.
---

where a relatively large
voltage change results in a small current change - the slope changes
so that for a relatively small increase in voltage you get a large
increase in current.

No, that's backwards. Diodes, and LEDs, have current exponential on
voltage at low currents. At higher currents, the contact and bulk
resistivity start to dominate, and the voltage:current curve gets
nearly linear.

---
True enough, but "nearly linear" isn't quite the same as "ohmic", is
it?

And, it's just plain silly talk since it has very little to do with
what we're talking about, which is running LEDs from a voltage source.

Lots of consumer products do exactly that.

Look at the V:I curve for a vanilla silicon diode at from zero volts
to where it lets, say, 1mA through the diode and you'll see that the
voltage across the diode, at that point, will be about 0.7V, mas o
menos.

0.7 volts at 1 mA? Where can I get some of those cool silicon LEDs?

What color are they?

John

--

John Larkin, President Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com

Precision electronic instrumentation
Picosecond-resolution Digital Delay and Pulse generators
Custom timing and laser controllers
Photonics and fiberoptic TTL data links
VME analog, thermocouple, LVDT, synchro, tachometer
Multichannel arbitrary waveform generators

josephkk · Jan 29, 2012

On Wed, 25 Jan 2012 09:27:46 -0800 (PST), Robert Macy
<robert.a.macy@gmail.com> wrote:

On Jan 25, 9:48 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 25 Jan 2012 16:53:17 +1100, "Phil Allison" <phi...@tpg.com.au
wrote:

"John Larkin"
"Phil Allison"

"Tim Williams is so full of shit "

Yes, suffice it to say, even though (dry slug) tantalums tend to
have
ESR
comparable to (higher grade) electrolytics, they are far, far
simpler:

** Their failure modes are many and failures far more common too.

If you keep dV/dT down, they are very reliable.

** Absolute crap.

As fucking usual, Larkin has no idea of what he speaks and does not
give
a shit either.

I've used well over 100,000 tantalum caps in the last 6 years or so.
53,000 of 2.2u 20v alone. The only ones that failed were loaded
backwards or were on power rails that had high dV/dT available.

** So you have NOT seen the general failure rates with all brands of
tants
and across all types of equipment.

You know no-one who has and do not give a shit either.

Fuck off to hell - you rabid, septic psychopath.

I see very low failure rates in the equipmennt I design. I'm sure
there is badly designed gear that blows tantalum caps.

I'm not a repair tech, so I don't deal with a lot of equipment
designed by somebody else.

** SO SHUT THE FUCK UP

- YOU BLOODY IMBECILE !!!!!!!

.... Phil

I think your ESR tester doesn't work very well with low-uF parts.

John

Use your SoundCard and a little fussing and you can get down to
milliohms between the ranges of 1000Hz to 90kHz. Actually, 89kHz, but
can't get to 100kHz.

A little software and your Soundcard 24bit? running at 192kS/s dual
channel

I don't suppose that you have the source code for that do you? If so i
would like to have a copy.

?-)

Jim Yanik · Jan 29, 2012

John Fields <jfields@austininstruments.com> wrote in
news:v4l8i79lka6tno3o5v361mg0u4uctsb5bt@4ax.com:

On Sat, 28 Jan 2012 11:58:15 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 26 Jan 2012 17:50:24 -0600, John Fields
jfields@austininstruments.com> wrote:

On Wed, 25 Jan 2012 18:44:44 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 25 Jan 2012 19:26:26 -0600, John Fields
jfields@austininstruments.com> wrote:

On Tue, 24 Jan 2012 21:11:23 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi
tombiasi@optonline.net> wrote:

On Tue, 24 Jan 2012 15:56:48 -0800 (PST), Bill Bowden

I've seen LED flashlights with 2 white 3 volt LEDs wired
directly in parallel across two AA batteries.

-Bill

They rely heavily on the battery's internal resistance and luck.

LEDs have a current:voltage slope that's not a brick wall.

---
Pretty close, though, once you get past the knee.

It's usually the other way around: exponential at low currents,
ohmic at higher currents.

---
It never really gets ohmic unless you drive the junction hard enough
to short it,

Nonsense, unless you plan to quibble about the word "really."

---
There's no quibbling about the word "really", the quibble is about
your assertion that a diode junction is ohmic at vaguely described
qualitative "higher currents".
---

and once you get past the knee -

As noted, diodes don't have a "knee" unless you arbitrarily define
one.

---
"As noted"???

I don't really think an "arbitrary" definition is necessary, since the
location of the knee has been with us for decades.
---

where a relatively large
voltage change results in a small current change - the slope changes
so that for a relatively small increase in voltage you get a large
increase in current.

No, that's backwards. Diodes, and LEDs, have current exponential on
voltage at low currents. At higher currents, the contact and bulk
resistivity start to dominate, and the voltage:current curve gets
nearly linear.

---
True enough, but "nearly linear" isn't quite the same as "ohmic", is
it?

And, it's just plain silly talk since it has very little to do with
what we're talking about, which is running LEDs from a voltage source.
Look at the V:I curve for a vanilla silicon diode at from zero volts
to where it lets, say, 1mA through the diode and you'll see that the
voltage across the diode, at that point, will be about 0.7V, mas o
menos.

Now run the voltage up to about 1.4V.

Will the current through the diode stop at 2mA?
---

Just look at the curves on real led data sheets. The smaller parts
start to get ohmic at low currents, just a few mA. Bigger junctions
will stay exponential at higher currents, because they have less bulk
resistance.

This is a really tiny junction, so the v/i curve is a straight line at
operating currents:

http://vcclite.com/wp-content/files/VAOL-S8GT4-LED-0805-green.pdf

---
Surely you can't be serious.

If you examine the Forward Current vs Forward Voltage curve with some
care, you'll find that with 10mA through the LED it drops about 1.9
volts, and with 20mA through it drops about 2 volts.

Simply using Ohm's law in both cases - in order to determine the
resistance of the LED in each case - yields for the first case:

E 1.9V
R = --- = ------- = 190 ohms
I 1e-2A

and for the second:

E 2.0V
R = --- = ------- = 100 ohms
I 2e-2A

then, since an ohmic load's resistance must be constant as the current
through it varies, that LED is clearly _not_ an ohmic load.
---

Bigger parts start to go ohmic at higher currents:

http://www.vishay.com/docs/81345/vlmp232.pdf

http://catalog.osram-os.com/jsp/download.jsp?rootPath=/media/&name=LA_L
O_LY_E67F_Pb_free.pdf&docPath=Graphics/00057343_0.pdf&url=/media//_en/G
raphics/00057343_0.pdf

Ordinary diodes do this, too. That's why diodes have some current
where their v:i curve has a zero temperature coefficient; the
exponential part has a negative TC but the bulk resistance TC is
positive. For small schottky diodes, that can be in the 10 mA
ballpark, so that can be useful.

---
Grasping at straws?

the Harbor Freight giveaway LED flashlights have NINE LEDs in parallel,no
series resistor,and three AAA cells to power it. I have several of them.

--
Jim Yanik
jyanik
at
localnet
dot com

John Fields · Jan 29, 2012

On Sat, 28 Jan 2012 17:51:38 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sat, 28 Jan 2012 18:43:09 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sat, 28 Jan 2012 11:58:15 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Thu, 26 Jan 2012 17:50:24 -0600, John Fields
jfields@austininstruments.com> wrote:

On Wed, 25 Jan 2012 18:44:44 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Wed, 25 Jan 2012 19:26:26 -0600, John Fields
jfields@austininstruments.com> wrote:

On Tue, 24 Jan 2012 21:11:23 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi <tombiasi@optonline.net
wrote:

On Tue, 24 Jan 2012 15:56:48 -0800 (PST), Bill Bowden

I've seen LED flashlights with 2 white 3 volt LEDs wired directly in
parallel across two AA batteries.

-Bill

They rely heavily on the battery's internal resistance and luck.

LEDs have a current:voltage slope that's not a brick wall.

---
Pretty close, though, once you get past the knee.

It's usually the other way around: exponential at low currents, ohmic
at higher currents.

---
It never really gets ohmic unless you drive the junction hard enough
to short it,

Nonsense, unless you plan to quibble about the word "really."

---
There's no quibbling about the word "really", the quibble is about
your assertion that a diode junction is ohmic at vaguely described
qualitative "higher currents".
---

and once you get past the knee -

As noted, diodes don't have a "knee" unless you arbitrarily define
one.

---
"As noted"???

I don't really think an "arbitrary" definition is necessary, since the
location of the knee has been with us for decades.
---

where a relatively large
voltage change results in a small current change - the slope changes
so that for a relatively small increase in voltage you get a large
increase in current.

No, that's backwards. Diodes, and LEDs, have current exponential on
voltage at low currents. At higher currents, the contact and bulk
resistivity start to dominate, and the voltage:current curve gets
nearly linear.

---
True enough, but "nearly linear" isn't quite the same as "ohmic", is
it?

And, it's just plain silly talk since it has very little to do with
what we're talking about, which is running LEDs from a voltage source.

Lots of consumer products do exactly that.

---
Sure, and if they're designed properly, the resistance of the LED and
the internal resistance of the battery will limit the current through
the LED to safe levels.

If not, LED life will be reduced; sometimes drastically.
---

Look at the V:I curve for a vanilla silicon diode at from zero volts
to where it lets, say, 1mA through the diode and you'll see that the
voltage across the diode, at that point, will be about 0.7V, mas o
menos.

Now run the voltage up to about 1.4V.
Will the current through the diode stop at 2mA?

0.7 volts at 1 mA? Where can I get some of those cool silicon LEDs?
What color are they?

---
More silliness, since since when is a "vanilla silicon diode" an LED?

I've got a vanilla 1N4148 on the bench with 1.0000 mA through it and
0.618 V across it, so what do you think will happen if I double the
voltage across it?

--
JF

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