Electronics Forum / Basics / June 2007

As long as you are sure that the reverse voltage is not more
than the rated Vr, the LED will be fine.  The series
resistor doesn't help, because if the voltage is high enough
for it to do anything (meaning that the resistor is
controlling the reverse current), the LED is already damaged.

 Usually, no problems, especially if resistor-limited.

 In my experience, it is common for LEDs to be rated for max. 5 volts
reverse voltage and have a high rate of doing nothing wrong with 12 volts
reverse.
 Problem here:  If anything goes wrongo, blooie or blammo then it's on
the one making products with LEDs being "abused".

 Some things to keep in mind:

1.  Many LEDs can have reduced power dissipating capability in reverse
breakdown mode.  The reverse breakdown site may concentrate into a
localized hotspot akin to what happens in BJTs during "forward bias second
breakdown".
 However, I suspect a resistor limiting power dissipation in reverse
breakdown to a milliwatt or two or probably a few will give adequate
protection.  Sorry, my warranty is limited to what you paid me to post
this!  :)  :)
 For 1 milliwatt dissipated into the LED at worst-case reverse breakdown
voltage given a 10K series resistor, LED voltage is a mere 3.16 volts and
supply voltage is a mere 6.32 volts.  Should just a few milliwatts be
capable of blowing a reverse-biased LED, I think that a 10K resistor can
easily fail to be adequate protection with supply voltage above 9-12
volts, may or may not be OK at 15 volts.

2.  Reverse breakdown is "dangerous at any speed" in most GaN and InGaN
LEDs.  This means most blue, white, blue-green, non-yellowish-green,
violet, UV, purple and pink ones.  Such LEDs are even static-sensitive,
with some having 1,000 and 400 volt ratings that I suspect are based on
"human body model", and some can be even worse.  I have personally damaged
some such LEDs by failure to adhere to usual static protection practice.
 I suspect reverse breakdown causes some sort of damaging electrolysis
that results in a "resistive short".
 This is an area where a 10K resistor gives little or no protection.

3.  I have heard of some high power LEDs, specifically some by Lumileds,
having a reverse protection diode.  The reverse protection diode may be an
ordinary red LED.  Should you be using LEDs that actually have a reverse
protection diode (which I think is less usual), then reverse current must
be limited to an amount that will not damage the reverse protection diode.
 I suspect a 10K resistor will do well in this area.

- Don Klipstein (don@misty.com)

A 10,000 ohm resistor?  that seems high for a led forward bias
resistor and any reasonable power supply voltage.

Just play it safe and put a diode in there to protect it against
reverse bias.

Most leds are rugged enough to take some current limited reverse
voltage so it may work fine, but if the led costs money or is hard to
change out, a 5 cent diode makes more sense.

No.

No need.

Graham