Led driver

[Kamlesh Barot]

I've old led driver - 10x3W having input 85-277v...0.25A max & output of DC 18-35 V....650ma Max power factor>95%, working satisfactorily for some time, with 3 each green, red, 2 each white and blue LED diodes connected in series.

Today I reflew 4 old red and 2 blue old and 2 blue new LEDs with 2 new green LEDs connected in series to the same driver. 6 of them went dead on switching them on. What was wrong? The new LEDs were having the specs 3.2v at 250~350 ma, while the old were the same.

Now I've ordered new 10 Red leds with 2.0-2.8 v and 400 ma. Should I connect all reds on the driver or try 3 each red, blue green like the first time in series?

 

[DIWdiver]

A number of things could explain what happened.

- Bad LEDs
- a few LEDs wired backward
- connecting LEDs after the power supply is on, or briefly disconnecting them while power is on
- OH YEAH! How about a 650 mA driver on a 350 mA LED?

 

[Kamlesh Barot]

- Bad LEDs

I can understand

- a few LEDs wired backward

But that should only stop the current passing, how'd it blow off?

- connecting LEDs after the power supply is on, or briefly disconnecting them while power is on

This wasn't so

- OH YEAH! How about a 650 mA driver on a 350 mA LED?

Isn't that better or it should've been a <350 mA driver?

 

[evilc66]

Most cheap LEDs don't have reverse bias protection diodes on them, so wiring them backwards has a strong likelihood of killing them. Pushing almost double the current through the LED will also have a strong likelihood of killing them. If an LED has a max current rating of 350mA, then that's the max current you should supply to them. Some higher quality LEDs can be pushed harder with higher currents, but that also generally requires better cooling to keep the die temperatures at reasonable levels.

 

[mds82]

most of the time color LED's have a different forward voltage than white ones also.

 

[DIWdiver]

Yes, the driver should be rated no more than the lowest LED rating.

Unlike most other power supply situations, the LED driver determines the current, and the LED determines the voltage. So no matter what LED you hook to a 650 mA driver, it will try to push 650 mA through it. If you have an XP-L, which can be driven up to 3000 mA, you aren't taking advantage of the emitter's capabilities. If you have a little T1 style, you'll probably instaflash it. With the ones you've got, you are seriously overpowering them.

As evilc66 points out, most emitters do not have reverse polarity protection. Being diodes, they should not allow current through in this case. Unfortunately, the max reverse voltage they can withstand is pretty low, unlike most of the diodes we use as diodes. Max ratings of around 5V are pretty common. Your driver could easily provide enough voltage to break them down and destroy them.

Two things you can do with the driver and emitters you have:
1. Dim the driver so it provides only 350 mA. May or may not be possible with your driver, and you always run the risk of turning it up too high.
2. Run two strings of emitters in parallel. As long as you make the strings with the same emitters, say two red, two blue, two green in each string, and all of the emitters are matched (all the reds come from the same batch, all the greens come from the same batch, etc) this should work fine. You could run one old blue and one new blue in each string. As long as they are matched in pairs down the strings.

 

[Kamlesh Barot]

Excellent, thanks a bunch!

 

[Lexel]

If you run there 2 led strings parallel add a balance resistor in both paths that compensates voltage differences
at 325mA a 10 Ohm should work well