Im am building a lighting system for my room that uses 5 chains of high-current leds, and 1 high-current RGB led. I was wondering if the ULN2803 could pass the pwm frequency from the softpwm libary to dim the leds. Is it feasible? Or should i grab a bunch of driving transisters.
The ULN2803 is a bunch of transistors, just all in one place. The switching time isn't really specified in the datasheet but I don't think you'll have any trouble switching in the kHz range, which is definitely fast enough for dimming (in fact staying below 1 kHz is probably most power-efficient).
I'd be more concerned about actual current levels and how hot the ULN2803 is going to get.
Well im pumping 100mA at 20v to up to 5 strips, the most that will normally be activated at once is 2 or 3 at half power, as one strip puts out 50,000mcd at full power. Each of the leds in the rgb puts out 4,000mcd. I plan to only have the leds at full blast to wake me up in the morning, with a bright flash, then normal, flashing every 5 minuites 'till i get up. Im putting in this system because i want dimmability, while being green. The total wattage at full blast will be only 10-15w, less then one dimmable cfl. Heatsinking is simple if needed, as I can tie it into my computers severly underloaded liquid cooling system.
An sorrey for the confusion, i only glanced the datasheet(possibally the wrong one) and thought it was a opti-isolater, hence the wondering about swicthing rate.
(you don't need the stuff at the bottom of the schematic)
Just remember that the stated current handeling capacity pr channel of the 2003 does not mean that you can sink that much current on each channel at the same time. The total heat dissipation capacity of the 2003 is much to low to handle that.
Remember also that you can parallel 2 (or more) channels to get higher current handeling, but just remember what i wrot above.
I would go for 2 or 3 2003's to be on the safe side.
The max total current put on the entire chip at once will be 500mA for one second. Normal total usage will be 100-200mA, and useage during colored lighting will be 50-100mA. The chip will be well cooled.
If that is the case i think you are on the safe side.
I've used the ULN200x chip reliably for years, both in resistive senario's and inductive with no problems, each channel supposedly handles 500ma, i've moved over to TTL level mosfets for high current apps now