HI, I want to make a 40 X 3W led array (28 Red, 5 Blue, 5 UV, 2 Green) , 35 of the LEDS has a forward voltage of 3.6V and other 5 UV Leds are 3.2-3.4V. All of the LED's max currrent is 700mA. I want to make the array 10 leds in a series string and 4 sets of series strings in parallel. Each 10 LED series string has 9 x 3.6fV LED's and 1 x 3.4fV LED's. Is it possible to run the different forward voltages together at 35.8V per string? The power is coming from a 38Vdc power supply to a 5A buck module. When i try power each string at 34-35.8V 600mA. Im finding the ends are bright but some are dim and few not working, (which are the 3.4v ones and some 3.6v ones). The String seems to flicker on and off,sometimes all the LEDs in the string,sometimes only a few. If anyone is able to assist, id be very appreciative.
to be honest this might be too specific for GC, i have never seen anything like this get answered here, maybe try a LED Lighting site.....?????? from what little i know, the 35 LED's won't run with the 5 UV's, different wattage and volts and mA. at very least u would need 2 drivers and isolate the UV's......??????
You'll want to run all in a series string. That is what keeps them all at the same drive current. Parallel would have to big a margin for error to balance them. What you're doing is fine and normal. If I had to guess your problem is what they call the voltage dropout on a DC to DC buck supply. If you put in 38V you can only get out say 36 or 35 or 34V depending on the circuit. If you're forward voltage for your circuit is 36V at a given current you might not be making that voltage. If you're not making the full voltage the LEDs have whats called a "forward voltage" you have to make this voltage to even make light but current will still pass through if you don't make it. Meaning all the LEDs will be conducting but some may not turn on or be bright. The changes you see are probably due to different temperature conditions and input/output conditions from the supply. Try putting a hot air gun under the board on one barely turning on. This should increase the temp at the die and lower the voltage turn on and the LED should get brighter. Or just simply take a volt meter and measure input voltage or voltage across LEDs.