Is this bad for my led?

[landshark92]

Hi guys, Im new to the forum and lovin all the information floating around here You wouldn't believe how many pages I have open at once right now.

Anyways, I was playing with my voltmeter and I connected it to a cree led. The meter was in the ohms setting and I didn't get anything, no reading so I switched polarity and to my surprise, the led lit up. It was not very bright and I could look directly at it without hurting my eyes. I don't know what voltage or current was being sent to the led. Do you think that could have damaged the led?

Also I was concerned about connecting the led backwards. I know diodes don't like that but I didn't think the ohm setting on my meter would have sent out any power at all. Do you think that was bad for the led?

This is the meter I am using
http://www.sears.com/shc/s/p_10153_12605_03482141000P

Any enlightenment is much appreciated.

 

[LukeA]

You didn't hurt the LED. The reverse breakdown voltage of a power LED is roughly -3V. Your multimeter on the Ohm setting puts out much less than that. The way a lot of multimeters measure resistance is to put out a fixed voltage and measure what current, if any, they get back after passing it through the circuit. From that they calculate the resistance in Ohms. That little bit of power dissipated is what you see as light. A normal resistor would dissipate that power as a small amount of heat.

Reverse polarity isn't inherently bad for diodes, in fact it's exactly why they exist. Diodes function like electrical check-valves. LEDs are very weeny as far as being actual diodes though, with their low reverse breakdown voltages.

 

[JohnR66]

Reverse beakdown is typically rated at 5 volts unless some protection diode is employed. To see what would happen I cranked up the reverse voltage of unprotected LEDs to 30 volts and let them sit for a few minutes. With most quality LEDs, nothing would happen. With cheapo LEDs, some would start conducting a small current of a few ma.

When forward biased again, these cheap LEDs would continue to "dark conduct" same as if they suffered ESD. turning up the current to normal operating levels (20ma or so) or giving them a quick flash of higher current would sometimes restore normal operation. Most were permanently damaged.

As for the meter, its output is likely too low to cause any sort of damage. In fact, any LED that shines dimly at low current is a healthy LED with no "dark" conduction channels.

 

[uk_caver]

FWIW,
Seoul P4s seem to have a regular diode reverse-connected across them, and so will conduct significantly from ~0.6V reverse voltage.

This came to my attention when connecting reverse voltage to a P4 and homemade driver, when a fair amount of current started flowing.
Although the controlling electronics in the driver were protected against reverse voltage, the driver had a FET output stage, and both the FET and the P4 acted as regular diodes when reverse voltage was applied.

Luxeon I/III, and Cree XRE/XPE/XPG don't seem to have this arrangement.

 

[landshark92]

Thanks for the thorough replies. You guys have allayed my biggest fears. And now I have a way to test if my emitter is fried or not.

Oh, to be a complete newbie in a new hobby is a very fun time. Thanks for shedding some light on the matter