Driver to drive?

[John Larkin]

On Sun, 29 Jan 2012 10:43:10 -0600, Jim Yanik <jyanik@abuse.gov>
wrote:

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.

It would be more efficient (uniform brightness vs battery voltage,
longer battery life) if it used a switcher, but it's cheap, so it
doesn't.

If the LEDs are purchased in quantity, all from the same wafers, they
will share current pretty well without added resistance. Thermal
runaway is not an issue once they get into their ohmic ranges, which
most LEDs do at their design operating current.

The idea that you can't parallel diodes is silly. Motorola sold high
current paralleled-diode rectifier arrays decades ago.


--

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

 

[My Name Is Tzu How Do You]

On Sun, 29 Jan 2012 09:47:03 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

The idea that you can't parallel diodes is silly. Motorola sold high
current paralleled-diode rectifier arrays decades ago.

Yes, but they were thermally bonded together or the spec for their use
in parallel required thermal equality in operation.

 

[John Larkin]

On Sun, 29 Jan 2012 10:44:58 -0600, John Fields
<jfields@austininstruments.com> wrote:

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?

Something like 350 mA. But there's a lot of variation between
different manufacturers' parts.

If it's on a PC board, with a bit of pad+trace to heatsink the leads,
it should survive.

At that voltage, it will be well into its ohmic range, namely the
current:voltage line will be straight, not exponential. That should be
close to the zero TC point too.

Without the series resistance component, pure exponential, increasing
the diode voltage from 0.6 to 1.2 would increase the current by a
factor of about 10^10.

If you paralleled a bunch of 1N4148s from the same reel, and ran them
at, say, 100 mA or so, I'd expect pretty good current sharing.


--

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]

On Sun, 29 Jan 2012 09:59:03 -0800, My Name Is Tzu How Do You Do
<Tzu@hereforlongtime.org> wrote:

On Sun, 29 Jan 2012 09:47:03 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

The idea that you can't parallel diodes is silly. Motorola sold high
current paralleled-diode rectifier arrays decades ago.

Yes, but they were thermally bonded together or the spec for their use
in parallel required thermal equality in operation.

The ones that I'm thinking of were soldered to a common heat sink
plate. But they needn't be. At high currents, diodes can have zero TCs
and, in effect, have internal ballasting resistors.


--

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

 

[My Name Is Tzu How Do You]

On Sun, 29 Jan 2012 10:21:48 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

The ones that I'm thinking of were soldered to a common heat sink
plate.

NO SHIT, SHERLOCK!

 

[John Fields]

On Sun, 29 Jan 2012 09:47:03 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 10:43:10 -0600, Jim Yanik <jyanik@abuse.gov
wrote:

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.

It would be more efficient (uniform brightness vs battery voltage,
longer battery life) if it used a switcher, but it's cheap, so it
doesn't.

If the LEDs are purchased in quantity, all from the same wafers, they
will share current pretty well without added resistance.

---
"Pretty well"?

Is that a new technical term?
---

Thermal
runaway is not an issue once they get into their ohmic ranges, which
most LEDs do at their design operating current.

The idea that you can't parallel diodes is silly. Motorola sold high
current paralleled-diode rectifier arrays decades ago.

---
Apples and oranges, plus they took great pains to match the forward
voltages of the individual rectifiers in the array and to keep them
isothermal.

From Motorola's 1980 Silicon Rectifier Data Manual:

news:r94bi79k5pqkbc58l9foi01th6pln62tkv@4ax.com

--
JF

 

[John Larkin]

On Sun, 29 Jan 2012 12:56:21 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sun, 29 Jan 2012 09:47:03 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 10:43:10 -0600, Jim Yanik <jyanik@abuse.gov
wrote:

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.

It would be more efficient (uniform brightness vs battery voltage,
longer battery life) if it used a switcher, but it's cheap, so it
doesn't.

If the LEDs are purchased in quantity, all from the same wafers, they
will share current pretty well without added resistance.

---
"Pretty well"?

Is that a new technical term?

It means "well enough to be useful."

You might try being useful yourself, instead of just whining all the
time.


--

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

 

[My Name Is Tzu How Do You]

On Sun, 29 Jan 2012 11:26:41 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

You might try being useful yourself, instead of just whining all the
time.

He is. He told you Motorola performed the matching and culling at the
time of manufacture. It has nothing to do with "coming off the same
die", or anything else. It has to do with REAL performance AFTER
manufacture. And they culled together matched sets based on REAL
performance numbers.

You are apparently in the dark a bit more than you think you are.

 

[John Larkin]

On Sun, 29 Jan 2012 11:33:46 -0800, My Name Is Tzu How Do You Do
<Tzu@hereforlongtime.org> wrote:

On Sun, 29 Jan 2012 11:26:41 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

You might try being useful yourself, instead of just whining all the
time.


He is. He told you Motorola performed the matching and culling at the
time of manufacture.

Hardly insider information. When else would they possibly do it?


It has nothing to do with "coming off the same

die", or anything else.

It sure does. Parts off the same die tend to be very similar. More so
nowadays, since silicon processing has improved a lot in the last few
decades.

I find that small parts, like LEDs, tend to be very repeatable,
especially ones off the same reel. We once made a deal with Mouser to
sample us ten parts each off a bunch of different reels of rectifiers,
with the understanding that they would test the samples and they would
then sell us the reels that we liked, for Grehkov (drift step-recovery
diode) use. That worked great. Parts from different reels were very
different, but we fould one reel that had all good snappers.


It has to do with REAL performance AFTER

manufacture. And they culled together matched sets based on REAL
performance numbers.

Duh.

You are apparently in the dark a bit more than you think you are.

Well, I do this for a living, and build stuff that works.


--

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]

On Sun, 29 Jan 2012 11:44:28 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 11:33:46 -0800, My Name Is Tzu How Do You Do
Tzu@hereforlongtime.org> wrote:

On Sun, 29 Jan 2012 11:26:41 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

You might try being useful yourself, instead of just whining all the
time.


He is. He told you Motorola performed the matching and culling at the
time of manufacture.

Hardly insider information. When else would they possibly do it?


It has nothing to do with "coming off the same
die", or anything else.

It sure does. Parts off the same die tend to be very similar. More so
nowadays, since silicon processing has improved a lot in the last few
decades.

I find that small parts, like LEDs, tend to be very repeatable,
especially ones off the same reel. We once made a deal with Mouser to
sample us ten parts each off a bunch of different reels of rectifiers,
with the understanding that they would test the samples and they would

no, ^^^^ we did the testing



--

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 Fields]

On Sun, 29 Jan 2012 10:18:28 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 10:44:58 -0600, John Fields
jfields@austininstruments.com> wrote:


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?

Something like 350 mA. But there's a lot of variation between
different manufacturers' parts.

---
Snipped irrelevant pontificating.

--
JF

 

[John Fields]

On Sun, 29 Jan 2012 11:26:41 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 12:56:21 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 29 Jan 2012 09:47:03 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 10:43:10 -0600, Jim Yanik <jyanik@abuse.gov
wrote:

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.

It would be more efficient (uniform brightness vs battery voltage,
longer battery life) if it used a switcher, but it's cheap, so it
doesn't.

If the LEDs are purchased in quantity, all from the same wafers, they
will share current pretty well without added resistance.

---
"Pretty well"?

Is that a new technical term?

It means "well enough to be useful."

You might try being useful yourself, instead of just whining all the
time.

---
As usual, you try to trivialize criticism by belittling it.

As far as being useful goes, you really ought to take a look at all
the crap you post before you make disparaging remarks.

--
JF

 

[John Larkin]

On Sun, 29 Jan 2012 13:53:22 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sun, 29 Jan 2012 10:18:28 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 10:44:58 -0600, John Fields
jfields@austininstruments.com> wrote:


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?

Something like 350 mA. But there's a lot of variation between
different manufacturers' parts.

What did you measure?

---
Snipped irrelevant pontificating.

Electronics is irrelevant to you?

The LT Spice model for the 1N4148 has an Rs value of 0.568 ohms, which
looks a tad low to me.


--

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]

On Sun, 29 Jan 2012 14:12:00 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sun, 29 Jan 2012 11:26:41 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 12:56:21 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 29 Jan 2012 09:47:03 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 10:43:10 -0600, Jim Yanik <jyanik@abuse.gov
wrote:

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.

It would be more efficient (uniform brightness vs battery voltage,
longer battery life) if it used a switcher, but it's cheap, so it
doesn't.

If the LEDs are purchased in quantity, all from the same wafers, they
will share current pretty well without added resistance.

---
"Pretty well"?

Is that a new technical term?

It means "well enough to be useful."

You might try being useful yourself, instead of just whining all the
time.

---
As usual, you try to trivialize criticism by belittling it.

As far as being useful goes, you really ought to take a look at all
the crap you post before you make disparaging remarks.

What current did you measure on the 1N4148?


--

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

 

[Fred Bloggs]

On Jan 29, 12:47 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 10:43:10 -0600, Jim Yanik <jya...@abuse.gov
wrote:





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

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

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

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

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

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

On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi
tombi...@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.

It would be more efficient (uniform brightness vs battery voltage,
longer battery life) if it used a switcher, but it's cheap, so it
doesn't.

There's a reason why those cheap white LED flashlights are made by
battery manufacturers, but who can complain about the price, they're
almost free. There's also a reason why the cheapies are all powered by
AAAs and button cell batteries and that is the relatively high battery
internal resistances. The white LED luminous output versus drive
current is practically stationary at its peak which is very convenient
for design purposes, and design point, if your battery impedance is
fairly well characterized. A variation of +/-15 % does not make much
of visible difference. They also don't seem all that concerned about
driving the LED with up to 4x maximum continuous IF rating, that
current does not last long enough to damage anything. The story
changes if you move to a more serious power source, the current is
almost always regulated.
Please view in a fixed-width font such as Courier.

..
..
..
..
.. From load line:
..
..
..
..
.. V - V
.. BATT LED
.. I = ------------
.. LED R + R
.. BATT LED
..
..
..
.. from which:
..
..
..
.. dI d(R + R ) dR
.. LED BATT LED BATT
.. ----- = (-) -------------- ~ (-) ------
.. I R + R R
.. LED BATT LED BATT
..
..
..
.. single digit percentage sensitivity
..

 

[My Name Is Tzu How Do You]

On Sun, 29 Jan 2012 11:44:28 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

Well, I do this for a living, and build stuff that works.

Then why take a stupid stance that they do not need to be thermally
coupled?

 

[John Fields]

On Sun, 29 Jan 2012 12:18:40 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 13:53:22 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 29 Jan 2012 10:18:28 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 10:44:58 -0600, John Fields
jfields@austininstruments.com> wrote:


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?

Something like 350 mA. But there's a lot of variation between
different manufacturers' parts.

What did you measure?

---
712mA.
---

---
Snipped irrelevant pontificating.

Electronics is irrelevant to you?

---
No, irrelevant pontificating about it is.
---

The LT Spice model for the 1N4148 has an Rs value of 0.568 ohms, which
looks a tad low to me.

---
Assuming 750mA puts it into the linear region, then Rs = Vf/If ~ 1.7
ohms, so I agree.

How's that for a first?

--
JF

 

[John Fields]

On Sun, 29 Jan 2012 12:37:30 -0800, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 14:12:00 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 29 Jan 2012 11:26:41 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 12:56:21 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 29 Jan 2012 09:47:03 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 10:43:10 -0600, Jim Yanik <jyanik@abuse.gov
wrote:

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.

It would be more efficient (uniform brightness vs battery voltage,
longer battery life) if it used a switcher, but it's cheap, so it
doesn't.

If the LEDs are purchased in quantity, all from the same wafers, they
will share current pretty well without added resistance.

---
"Pretty well"?

Is that a new technical term?

It means "well enough to be useful."

You might try being useful yourself, instead of just whining all the
time.

---
As usual, you try to trivialize criticism by belittling it.

As far as being useful goes, you really ought to take a look at all
the crap you post before you make disparaging remarks.

What current did you measure on the 1N4148?

---
712mA.

--
JF

 

[John Larkin]

On Sun, 29 Jan 2012 13:18:13 -0800, My Name Is Tzu How Do You Do
<Tzu@hereforlongtime.org> wrote:

On Sun, 29 Jan 2012 11:44:28 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:


Well, I do this for a living, and build stuff that works.

Then why take a stupid stance that they do not need to be thermally
coupled?

I explained that. At low currents, diodes have negative TCs and can go
into thermal runaway if paralleled (and, obviously, poorly heat sunk.)
At high currents, they become ohmic and have more positive TCs, so
there's no thermal runaway mechanism.

That's why people sell flashlights that have multiple LEDs and no
resistors. Because it works.

And multiple diodes soldered to a common heatsunk baseplate really
aren't "thermally coupled" very well... not much better than if they
were separate diodes on the same heat sink.

Whether diodes can be usefully paralleled is an engineering issue that
has to be evaluated for specific cases. But a hard rule to never
parallel diodes, because you heard it somewhere, doesn't make sense.

Thermal runaway is a bigger issue for transistors.



--

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]

On Sun, 29 Jan 2012 15:33:40 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Sun, 29 Jan 2012 12:18:40 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 13:53:22 -0600, John Fields
jfields@austininstruments.com> wrote:

On Sun, 29 Jan 2012 10:18:28 -0800, John Larkin
jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

On Sun, 29 Jan 2012 10:44:58 -0600, John Fields
jfields@austininstruments.com> wrote:


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?

Something like 350 mA. But there's a lot of variation between
different manufacturers' parts.

What did you measure?

---
712mA.
---

---
Snipped irrelevant pontificating.

Electronics is irrelevant to you?

---
No, irrelevant pontificating about it is.
---

The LT Spice model for the 1N4148 has an Rs value of 0.568 ohms, which
looks a tad low to me.

---
Assuming 750mA puts it into the linear region, then Rs = Vf/If ~ 1.7
ohms, so I agree.

That's the static resistance. A more useful value is dV/dI, the slope
of the curve in the linear region. That's the Spice 0.568 value. The
value from several 1N4148 data sheets is more like 0.7 ohms.

How's that for a first?

LT Spice shows 0.64 amps at 1.236 volts. Most diode data sheets show
about half that current.


--

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