Refresh my memory please. I want to drive a regular RGB 1W LED. So no built-in controller. The best option I can come up with is to drive it with an external WS2801 or LPD8806. Since I only need a single channel and nothing high speed, it would make sense to use the WS2801 (of which I have several hundreds still). That combined with an NPN on each channel should do the trick if I recall.
Anyone see a glaring issue here?
Sounds correct.
Use those 2801 chips up one at a time!
LOL, I have over 600 of them …
Do you know the current & voltage needed for these LEDs? I didn’t see that info, or even a link or part number for the LED.
They’re your average run of the mill RGB chip LED mounted on an aluminum substrate. Voltage between 2.0V to 3.4V depending on color, 350mA/ch. The WS2801 can drive them just fine with external NPNs, something I did eons ago. Just needed to make sure I wasn’t forgetting something obvious. Now I just need to “deadbug” one of these ICs as they’re all SMD versions.
The other option would be to use the analogwrite function in this example. Just use the PWM pins and mosfets or even NPNs.
http://github.com - FastLED
So how do you get 350mA @ 2.0 to 3.4v from your power supply? Just different resistors (and ignore temperature effects), or a constant-current driver? If the latter, is an NPN transistor needed or does your constant-current chip or circuit accept logic level PWM?
It’s all in the datasheet Zeph. The WS2801 can work both as a constant voltage or constant current configuration. With the LEDs all needing 350mA, configure it for constant current, and use a high enough voltage to cover the highest range on the LEDs. In my case, I’m going for a 12V rail.
Ah, thanks. It looks like it can only do “constant current” at 50ma even with an external transistor to allow higher voltage, but it could handle higher current in “constant voltage mode” with an external transistor, right? So you are planning on the latter?
Cool, I didn’t realize it had that mode.
Nope, I’m running constant current mode, driving a 10W RGB LED that requires 700mA per channel. The voltage doesn’t need to be constant. While the recommended voltage for this LED is 12V, I can drop that down to 10V before I lose the blue channel (as it works at up around 9V), or I can crank it up to well past 12V. Current gets limited to 700mA regardless of voltage.
The 50mA you are referring to is specific to it being in constant voltage with no external circuitry. Adding external NPNs allows you to use either constant voltage or constant current.
Take a look at this method.