PWM Driver, Transistor Question

Hello,
Im trying to set up an RGB led light. Ive got luxeon led's, Led Dynamics Buck Puck luxeon drivers, and a PWM circuit I built.

The PWM circuit is a 74hc14 set up as an oscillator (four oscillators, really. one fast for pwm, and three for automatic slow fading of 3 channels) and an lm324 comparitor to make the square wave pwm..
The circuit works great for lighting regular led's. Full control from full off to full bright.

The luxeon drivers I have, have an output pin that puts out 5v, and an input pin. at 0volts on the input the led is full bright, at 5 volts the led is off.

I hooked my pwm circuit to the 5v, but the circuit takes away about 1 volt, so the output isn't quite enough to give full control of my luxeon driver.

SO, I decided to use a transistor. I figure'd I'd put the output of my pwm to the base, and have the transistor pass the 5 volts to my luxeon driver.
I thought this would be simple, but just can't get it to work.

The problem I am having is that the luxeon never goes all the way out, and never goes to full power. I've tried every transistor I can find. npn, pnp 2907 2904 4403 4401 3702 3904 etc etc etc..... and they all do the exact same thing. Ive put a resistor in line w/ the base, and got the luxeon to go all the way out, but when it should have been fully bright, it was just barely lit.

I have read every PWM schematic I can find on the web, and they are all basically the same.. the pwm drives an output transistor/mosfet or something, and that drives the light or motor or whatever. I can't figure out where Im going wrong.

Any help would be greatly appreciated!

so you are using the PWM waveform into the 0-5V input correct? If so, it may get complicated. The input is looking for a voltage between 0 and 5 volts, the PWM is outputting a squarewave. try a cap across the input to the LED driver and ground, and a blocking diode from the PWM driver. What you are trying do do is change the digital signal into an analog signal.

Good luck

Jeff

well the input on my driver will accept a range from 0-5 and vary the current that it is supplying to the led accordingly.
I just want feed the control pin on the driver the 5v PWM signal, so that it is always supplying the full current.

I thought I could just put the 5v on one side of the transistor, the control input on the other, and my pwm output to the base of the transistor. I can't get it to work, though.

Can you show schematics? It's much easier to troubleshoot when there's a drawing.

Well, the pwm circuit I am using is

http://www.solarbotics.net/library/circuits/pix/QLF7.gif
Ive made some modifications to that circuit, but I dont think its anything that should effect this.


The led driver is
http://www.leddynamics.com/LuxDrive/content/Page_Buck.html

I basically want the led outputs on that circuit to power the Control input on the luxeon driver.

Have you tried the circuit shown in Fig. 14 of the data sheet for the driver? "Pulse/Strobe input 5V=On", although the 'logic will be backwards', that is a high input will turn the LED off.

Anyway, it should work just fine....

Doug Owen

I tried a circuit similar to that one.. but not exactly. That may be my problem /ubbthreads/images/graemlins/wink.gif
I've also figured out that if I use an lm339 w/a small pull up resistor instead of the lm324 it seems to work better. Still not fully on and off... but close. I may just need to tweak my pull up resistor value.

Well.. back to the breadboard

Stick with it, the Fig. 14 circuit should work. Be sure your low out is low enough to be sure the transistor is off (maybe even put a resistor from base to emitter) and that the base gets enough drive to saturate the transistor with a high input and you should be Jake.

Doug Owen