Hey all, this is my first time on the forums. I've had an Arduino for a pretty short time, and I've been messing around with PWM control of LED's, control over DMX, etc.
The next project I'm looking to mess with is creating a basic model for a pixel driver using the WS2811 three-channel PWM IC. I'm interested in the pixel strings places like Adafruit carry, but I'm looking at being able to control much larger pixels than what I'm seeing (upwards of dozens of RGB LEDs). To that end I'm looking at having a WS2811 IC driving three beefy MOSFETS via optocouplers, that way I can attach the driver to any sized pixel I would want to. I was hoping to throw up my basic schematic and see if anyone sees anything that's going to cause an explosion before I construct my first test (parts are currently on order).
Eventually I'd step up to a 24V supply, and power to the pixel would go direct, not via the driver board.
I haven't examined every aspect of your design but you will deffinitely need some pulldown resistors on the gates of the mosfets. Your opto's will charge the gates but there is nothing to discharge the gate for the off period. There are some elaborate mosfet driver designs out there, but usually a simple resistor from each gate to ground will suffice and stop the mosfet from potentially staying latched on due to gate charge. Mosfets have a very high gate input impedance so it could take several seconds or more for the gate voltage to leak away without them. Also, without any pulldowns the high input impedence will leave the mosfet very susceptable to EMI (electro magnetic interference) in the enviroment which could have the effect of them switching on at random if electrical or RF noise is close.
I have only looked very breifly for a few seconds at the diagram so can not comment on the rest but the gate drive jumped straight out at me.
Something like 10K - 100K resistors should work fine atleast as a starting point. The value shouldn't be too critical in this applicacion, just whatever you have in that sort of range in your junk box.
Don't forget, although you have a GND indicated on the drawing you have no actual connection terminal for connecting the ground/0v to your power supply etc. I'm sure that's just taken for granted but if you made it up on a PCB you would have to bodge that connection on somehow later on.
I had forgot the pull downs when I threw the schematic together, my bad. So out of curiosity you are saying that the gate charge would drain 'back' through a pull down to ground? I guess I just assumed that charge would have bled to ground out the source (though not why I had them left off :p)
Parts should be arriving this week to give this circuit a test.
Aristobulous:
So out of curiosity you are saying that the gate charge would drain 'back' through a pull down to ground?
Essentially yes. The gate on a mosfet can be thought of as a capacitor, holding the voltage that is put on it, with respect to ground. It may discharge eventually, through natural leakage, but the resistor will speed the process up. We are talking picofarads of equivalent capacitance, so it's not a lot of charge but it is enough to potentially hold the device on, given the high gate input resistance (i said impedence earlier, my mistake, been awake all night) there is very little in the way of leakage to pull that small charge back to ground and shut the gate off. It may not even be a problem but it is certainly good practice to not leave the gate input effectively floating after the voltage has been removed. It's similar to leaving unused input pins floating on a logic chip for instance, the logic state is unpredictable and just picks up noise or other static charges in the vacinity. Not to mention the floating input could actually be damaged by the high voltage of static electricity.
I hope that helps explain it, to the best of my abilities anyway. I'm sure there are people who can explain it in much more detail, right down to atomic level but hopefully this will suffice enough to get your circuit working close to what you expect.
I have no experience with the WS2811 to be able to comment on that part but i'd be interested to hear your results. I assume it's PWM so the opto's should couple that right through to the mosfets unless it's some absurd frequency above the maximum limits for the opto's and fets. I doubt it on an LED driver though.
