Is there any problems with applying a reverse V that is <= Vr of an
LED? I'll limit it through a 10K resistor also....
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 (
[email protected])
