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John Larkin
Guest
Guest
Sun Jan 29, 2012 3:06 am
On Sat, 28 Jan 2012 16:31:37 -0800 (PST), Bill Bowden
<bperryb_at_bowdenshobbycircuits.info> wrote:
Quote:
On Jan 28, 11:59 am, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 25 Jan 2012 11:58:29 -0800 (PST), fungus <to...@artlum.com
wrote:
On Jan 25, 5:54 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Constant-current drive is better of course, but CV can be usable.
In the context of the thread:
Using CV to run LEDs in parallel near their
optimum current is risky (the first circuit in
the thread).
Well, lots of people do it and get away with it.
John
I executed a red LED to see what it would withstand. The LED just
starts to glow at about 1.47 volts and drops 2 volts at 160mA. At 2.7
volts the current is 200mA and brightness is fairly constant over the
range of 20Ma to 200mA.
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 25 Jan 2012 11:58:29 -0800 (PST), fungus <to...@artlum.com
wrote:
On Jan 25, 5:54 pm, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
Constant-current drive is better of course, but CV can be usable.
In the context of the thread:
Using CV to run LEDs in parallel near their
optimum current is risky (the first circuit in
the thread).
Well, lots of people do it and get away with it.
John
I executed a red LED to see what it would withstand. The LED just
starts to glow at about 1.47 volts and drops 2 volts at 160mA. At 2.7
volts the current is 200mA and brightness is fairly constant over the
range of 20Ma to 200mA.
Yeah, efficiency drops at high currents, partially due to heating.
At 230mA the voltage rises to almost 5 volts
Quote:
and the brightness falls off. The LED dies a little above that, but it
lived a happy life.
lived a happy life.
Something small like this
http://www.kingbrightusa.com/images/catalog/SPEC/APTD1608QBC-D.pdf
could be reasonably run from a couple of alkaline cells.
--
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
Guest
Guest
Sun Jan 29, 2012 3:31 am
On Sat, 28 Jan 2012 18:14:25 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon_at_On-My-Web-Site.com> wrote:
Quote:
On Sat, 28 Jan 2012 18:43:09 -0600, John Fields
jfields_at_austininstruments.com> wrote:
On Sat, 28 Jan 2012 11:58:15 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Thu, 26 Jan 2012 17:50:24 -0600, John Fields
jfields_at_austininstruments.com> wrote:
On Wed, 25 Jan 2012 18:44:44 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 25 Jan 2012 19:26:26 -0600, John Fields
jfields_at_austininstruments.com> wrote:
On Tue, 24 Jan 2012 21:11:23 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi <tombiasi_at_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 ?
...Jim Thompson
jfields_at_austininstruments.com> wrote:
On Sat, 28 Jan 2012 11:58:15 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Thu, 26 Jan 2012 17:50:24 -0600, John Fields
jfields_at_austininstruments.com> wrote:
On Wed, 25 Jan 2012 18:44:44 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Wed, 25 Jan 2012 19:26:26 -0600, John Fields
jfields_at_austininstruments.com> wrote:
On Tue, 24 Jan 2012 21:11:23 -0800, John Larkin
jjlarkin_at_highNOTlandTHIStechnologyPART.com> wrote:
On Tue, 24 Jan 2012 20:17:17 -0500, Tom Biasi <tombiasi_at_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 ?
...Jim Thompson
If you have anything intelligent to say on topic, give it a try.
--
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
Guest
Tue Feb 07, 2012 6:41 pm
On Mon, 6 Feb 2012 19:08:45 -0800 (PST), dagmargoodboat_at_yahoo.com
wrote:
Quote:
On Feb 6, 1:03 pm, John Larkin <jlar...@highlandtechnology.com> wrote:
On Sun, 05 Feb 2012 10:57:00 -0800, Bart!
B_at_rt_The_Sheriff_Is_A_Nig***!.org> wrote:
On Sun, 05 Feb 2012 09:47:40 -0800, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I don't know either.
The first time you have spoken the truth in this group in quite some
time.
How could I know how your flashlight works?
You could find out and tell us. That could be interesting, especially
the non-invasive ways. There could be all sorts of interesting
tangents. Really.
Go for it.
Okay, Nym's had his shot.
There are several LED flashlight schemes. The crudest connect 3
series cells to the LEDs, relying on the cells' series resistance and
the LEDs resistance. Some add series resistors. If you parallel
enough LEDs they can handle 3xAAA in series, but as soon as one LED
fails, the rest rapidly die.
Boost converter schemes apply either constant voltage, constant
voltage from a current mode boost, or current mode boost, in order of
sophistication.
More sophisticated schemes modulate the converter, for multiple power
settings.
None use 555s AFAIK.
On Sun, 05 Feb 2012 10:57:00 -0800, Bart!
B_at_rt_The_Sheriff_Is_A_Nig***!.org> wrote:
On Sun, 05 Feb 2012 09:47:40 -0800, John Larkin
jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
I don't know either.
The first time you have spoken the truth in this group in quite some
time.
How could I know how your flashlight works?
You could find out and tell us. That could be interesting, especially
the non-invasive ways. There could be all sorts of interesting
tangents. Really.
Go for it.
Okay, Nym's had his shot.
There are several LED flashlight schemes. The crudest connect 3
series cells to the LEDs, relying on the cells' series resistance and
the LEDs resistance. Some add series resistors. If you parallel
enough LEDs they can handle 3xAAA in series, but as soon as one LED
fails, the rest rapidly die.
Boost converter schemes apply either constant voltage, constant
voltage from a current mode boost, or current mode boost, in order of
sophistication.
More sophisticated schemes modulate the converter, for multiple power
settings.
None use 555s AFAIK.
That's because, as everyone knows, 555s are obsolete.
Michael A. Terrell
Guest
Guest
Wed Feb 08, 2012 7:23 pm
krw_at_att.bizzz wrote:
Quote:
That's because, as everyone knows, 555s are obsolete.
So are Slomans. Museum pieces, in fact.
--
You can't have a sense of humor, if you have no sense.
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