Running leds on AC

Received wisdom is that one should use an inverse-parallel diode or perhaps a series diode of adequate PIV, but what actually happens if you don't but current is limited?

I found this at

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talking about diodes in general.

"Thus the peak inverse voltage (p.i.v.) is quoted in their specification as the voltage below which they will not breakdown. The reverse breakdown of diodes has its uses as a voltage reference or for protection purposes. It is possible to recover from the breakdown situation as long as the diode has not been overheated, i.e. burnt out, by excessive power dissipation. Thus a series resistor must be used to limit the current flow through the diode."

True or false?

Boldly going, I've a 2v yellow (5v max PIV) running off a 55v transformer, with 2.35k of series resistance. Calculated If about 22mA. Still alive after

4 hours, but will it do long-term damage?

Meter reading shows 35v across the resistor, so average current (both ways)

15mA and a fairly high avalanche(?) voltage.
Reply to
cpemma
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I do know that most GaN and InGaN LEDs (UV, violet, blue, non-yellowish green, white, pink, and anything non-red by Nichia) suffer damage in the form of partial shorts (resistance often around a hundred to several hundred ohms) when they break down. And that one aging mechanism in some chemistries appears to be from a combination of heat and electric field in the chip - and this may progress faster during reverse breakdown than during normal forward-biased use. I would add a parallel diode, and in addition a series one to reduce current consumption and heating of the dropping resistor. I would not use just a series diode since it may leak enough current to cause breakdown or result in excessive reverse bias that some types may be aged excessively by if the temperature is elevated. Or put two LEDs in antiparallel.

- Don Klipstein ( snipped-for-privacy@misty.com)

Reply to
Don Klipstein

In the past year or so someone posted a URL to a study regarding LED use in auto applications. It said that some LEDs survived a very harsh environment that had high voltage spikes in the reverse direction. But that doesn't mean that they should be exposed to those high reverse voltages. Like Don K. says, use another diode (even a much cheaper red LED) in parallel to the LED, with its cathode connected the the LED's anode (antiparallel). If it is run off an AC power line, use a capacitor to filter out high voltage spikes such as those from lightning.

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Reply to
Watson A.Name - 'Watt Sun'

Don't get me wrong, I'm not advocating this as a recommendable practice ;-) Just a bit surprised the led didn't quickly put its legs in the air. (It's still running :)

It arose from somebody who'd fitted a bell-push with a led, and it didn't work. I thought it was the 12v ac supply that had killed the led. Turns out his 430R dropper resistor was a 430k and the led was fine. Monkey curiosity took over.

Without a 'scope I can't see exactly what's happening, but from the meter readings I'm guessing a fairly high reverse voltage drop and a more modest reverse current, still giving considerably more VI heat in the reverse direction than in the forward.

Reply to
cpemma

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