I understand the points about calculating the series resistance for an LED and the battery voltage, and about using a small resistor just to be safe even if the battery voltage is "about right" for the LED.
The general advice "do not connect LEDs in parallel" refers to this sort of circuit:
"Adam Funk" schreef in bericht news: snipped-for-privacy@news.ducksburg.com...
LEDs are current drive devices so you will always need to limit the current somehow. You will sometimes meet LEDs driven directly from a battery but then the internal resistance of that battery accounts for the current limiting. Sometimes you see LEDs directly in parallel which may do for LEDs from the same batch as long as they are not driven to their limits. It is nevertheless considered bad practice.
Usually you will not use three resistors if you can do with two of them. If the LEDs differ widely they may also influence each other. Nevertheless there may be some use for this circuit. For instance you may want to spread the dissipated heat.
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It never really gets ohmic unless you drive the junction hard enough
to short it, and once you get past the knee - 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.
Nonsense, unless you plan to quibble about the word "really."
As noted, diodes don't have a "knee" unless you arbitrarily define one.
where a relatively large
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:
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Bigger parts start to go ohmic at higher currents:
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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.
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John Larkin, President Highland Technology Inc
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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".
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>> and once you get past the knee -
>
>As noted, diodes don't have a "knee" unless you arbitrarily define
>one.
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.
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John Larkin, President Highland Technology Inc
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0.7 volts at 1 mA? Where can I get some of those cool silicon LEDs?
What color are they?
John
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John Larkin, President Highland Technology Inc
www.highlandtechnology.com jlarkin at highlandtechnology dot com
Precision electronic instrumentation
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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.
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
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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
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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
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