Hello everyone.
I have just been blowing up a lot of red LEDs when I try to test them (such as after making small RGB panels). This is probably common sense, but having released a whole bunch of magic smoke, I figured I would relate the experience just in case any other beginner has the same 'bright' idea I had.
I just recently got a Rigol DP832 - my first real lab bench power supply. Well, the DP832 is really cool in that you can just program in 15mA or 20mA, and viola, you have an instant LED tester, right? No messing with LM317 circuits or resistors! (Who doesn't make use of a $400 power supply to make $.02 LEDs light up!
)
Well, maybe, but only if you are careful.
After making a whole bunch of RGB LED panels, I wanted to quick sweep them with the power supply to test all the LED connections. (If everything lights up, then I know the surface mount soldering is OK.)
So, I do what any good beginner does, I set the voltage at 15V (four LEDs in series per color), set the current at 20mA, and figure I am great - the power supply will regulate to 20mA, and I will be safe as I try all the different color channels. Turn the output on, and commence to go down the line with the probe. Blue works, Green works, but those Red channels, man the LEDs must have been a bad batch! They work once, maybe twice, but they all seem to be dying! What crap!
After lots of
, it turns out that those Red LEDs just can't stand being over-voltaged to double their Vf for the very brief time it takes the current regulator to kick in. (The Red LED chain only took about ~8.2V for it to reach 20mA current, compared to the Blue @12.8V and Green @13.6V. The delta between my chosen voltage of 15V and Vf was much smaller on Green and Blue, which is likely what saved them from my stupidity.)
One the one board that I didn't blow up, I started with the output of the power supply off, hooked up the Red chains, and turned the output on. Viola! No troublesome red channel! When allowed to ramp from 0V (vice having to clamp from 15V), the Red LEDs never exceeded their ratings.
Moral of the story: Blowing up LEDs is a great way to get more familiar with your brand new power supply, the peak detect function on your DMM, and getting clued into the world of 'duh'!
TL/DR: Even if your power supply has a CC mode, never test an LED by connecting an active output with a high voltage to said LED. Instead, start with the output disabled, wire the LED, and turn on the output to allow the CC mode to kick in before you over voltage the LED.