LED Buck Driver

[john2k]

I have a buck type LED driver that adjusts voltage output depending on whether I wire LED's in series or parallel. If i wire in parallel the voltage output is always 3.2V with a constant current output of 350mA, but if I wire them in series the voltage increases by 3.2V per LED. The intensity of the LED's seem rather bright and i am thinking if there is any way to reduce the intensity? maybe by adding a resistor of some kind on the output positive wire of the driver before it connects to the parallel LED's?

If so what kind of resistor would be recommended?

thanks

 

[CocaCola]

Well you really can't predict the outcome of a self adjusting circuit when you start adding things unless you know the circuit... What does the datasheet say about adjustment? A resistor might or might not work, the circuit might try to overcompensate or who knows what... Have you tried a series diode, to drop off a little voltage, it might not be as 'noticed' by the self adjustment circuitry...

 

[john2k]

Well you really can't predict the outcome of a self adjusting circuit when you start adding things unless you know the circuit... What does the datasheet say about adjustment? A resistor might or might not work, the circuit might try to overcompensate or who knows what... Have you tried a series diode, to drop off a little voltage, it might not be as 'noticed' by the self adjustment circuitry...

I agree with you about the fact that the circuit might try to overcompensate because it's adjusting all the time and by adding something that it's not designed for might just kill it prematurely.

Another question, if a few LED's wired in parallel had something wrong with them and were on the edge of dying out, would a LED driver dimming them completely be a normal behaviour of a driver? reason I ask is that up until now, i thought one of my drivers failed because it was dimming out the LEDs and then after 40 mins or so eventually would dim all the way out. But I have just realised the LED's are completely dead and I think the driver is actually fine because it actually still works. Just wondering if it is normal for the driver to dim out the LED's if something was wrong with it and why?

 

[BobK]

When you parallel the LEDs they are sharing the 350ma current (possibly not equally).

When they are in series each is seeing 350ma, so yes, they should be quite a bit brighter. This is the preferred way to wire multiple LEDs to a constant current source. Of course the input voltage must be higher than the combined forward voltages of the LEDS in series.

If you want to reduce the brightness, you need to reduce the current through the LEDs. If the current output is not adjustable, a resistor in parallel with the LEDs would do it, just measure the voltage without the resistor, then calculate the resistance needed to take say half the current. But this is, of course wasteful.

Most likely there is a sense resistor in the constant current driver, increasing this resistor would reduce the current.

Bob