LEDs in parallel

"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.

petrus bitbyter

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

-Bill

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

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

LEDs have a current:voltage slope that's not a brick wall. They can be run at constant voltage, and often are.

The dynamic impedance of a flashlight-type LED is a lot higher than the impedance of an AA battery.

John

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

John

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:

Bigger parts start to go ohmic at higher currents:

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

What do you think constant dI/DV means?

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.

Lots of consumer products do exactly that.

What color are they?

John

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

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.

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.

NO S*IT, SHERLOCK!

It means "well enough to be useful."

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