Running leds on AC

[cpemma]

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
http://alpha400.ee.unsw.edu.au/elec1011/resource/diode/sect_6/dde-s6t.htm
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.

 

[Don Klipstein]

In article <bgk5ep$lk8$1@newsg2.svr.pol.co.uk>, cpemma wrote:

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
http://alpha400.ee.unsw.edu.au/elec1011/resource/diode/sect_6/dde-s6t.htm
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?

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 (don@misty.com)

 

[Watson A.Name - 'Watt Sun]

In article <slrnbiriq4.3s9.don@manx.misty.com>, don@manx.misty.com
mentioned...

In article <bgk5ep$lk8$1@newsg2.svr.pol.co.uk>, cpemma wrote:
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
http://alpha400.ee.unsw.edu.au/elec1011/resource/diode/sect_6/dde-s6t.htm
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?

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 (don@misty.com)

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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[cpemma]

Watson A.Name - 'Watt Sun' wrote:

In article <slrnbiriq4.3s9.don@manx.misty.com>, don@manx.misty.com
mentioned...
In article <bgk5ep$lk8$1@newsg2.svr.pol.co.uk>, cpemma wrote:
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
http://alpha400.ee.unsw.edu.au/elec1011/resource/diode/sect_6/dde-
s6t.htm 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?

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 (don@misty.com)

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.

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.