[SOLVED] Regulating a regulated LED driver

Hello board,

I have tried searching for this to no avail. I purchased a LED driver for a set of 20 LEDs. The Vf is 3.5v and the drivers current source is 350 mA with a Vmax of 142v

this is great since it powers my LEDs with little complication. However I would like a way to regulate the intensity. In traditional voltage sources a series resistor would suffice, but how does one proceed in this case?

Thank you very much for any input you can give.

P.S. the led driver is 633-1116-ND at Digikey.

What you have is a constant current source of 350ma.
It doesn't matter if you put a resistor in series, the circuit automatically raises the voltage to continue giving 350ma to the circuit until it reaches the maximum voltage of 142 volts but for that voltage you need huge watts for the resistor.
What you can do is put a resistor in parallel with every led, the total current will always be 350ma but it will divide in this network and some current will pass through the resistor and the rest through the led.
Take a look at the simulation below, i have a constant current source of 350ma, the leds are set to vf=3,5v (as you said) and you can see that with a 40 ohm resistor the led draws only 0,21A and the rest goes through the resistor.

Please note that the resistor watt is 0.137 x 30 = 4.11 W
No matter how bright the leds you will always have the same mains consumption but depending on the resistor value you will generate more light or heat.



Alex

Thank you very much for taking your time to do the simulation. This should work perfectly.

From this I gather that we should be able to use a high wattage, high resistance resistor in parallel with the whole 25 led system?

I would love to perform this simulations myself, are you using any open source software?

Thanks again,

Rob

first of all i made a mistake, the watts of the resistor are P=V*I (V=I*R) so we transform it to P=I*I*R

so the watts in the previous case were 0.137 * 0.137 * 30 = 0.56 (big difference)

My simulation program is nor open source, sorry.

I have also simulated 25 leds and with a value of 1K you will have 0.25A for the leds, you will need a resistor of 11.5W

Please note that simulation data and results are not 100% accurate so you will need to do a test and measure a real circuit to be sure.




Alex

Go to Analog Circuit Simulation Software from SIMetrix Technologies - SPICE and download the demo version if you want a free simulator. It is fully functional for small circuits.

Others are LTspice & Tina which are free.

Keith