Thank you for taking the time. I didn't know I wasn't automatically subscribed to notifications (did I miss a checkbox when signing up?) so I didn't notice at first.
basically run the frequency up high enough and the transistor won't turn off.
Thanks for the explanation. Would you say that what you describe would account for the ~30% increased power draw when going from 15 to 60 Hz? I would have imagined that adding 45 "extra long" turn off times per second still wouldn't add up to a lot of extra on-time per second. If the 1% duty actually resulted in an on-time of 10 ms/s then these 45 turn off times would have to add up to about 3 ms to account for a 30% increased power draw, right? (
Edit, answering my own question: Well, no because the power board has some overhead, doh. But even if I subtract the 3 W of overhead, then the proportional increase in power draw will be even greater (42%) and the 45 turn off times would have to add up to ~4 ms.)
My initial impression was that the turn on/turn off times were down in the nano-to-microsecond range, but it seems to me that this "extra long" turn-off time would have to approach about 60 microseconds or so (3000 us / 45). Is that a reasonable order of magnitude for the phenomenon you're describing?
As I interpret this you were driving the transistor base via a 220 Ohm resistor, which seems very low. How was the transistor wired e.g. as common emitter with a resistor as collector load (if so what value resistor), or as an emitter follower? A simple schematic would help.
The simple schematic is basically exactly what was illustrated in the first post (minus the proper symbols) – I used no other components. I'm assuming there's only one sensible way to hook up the transistor in the specified series, with the specified components (and with the 3.3 V rail), so that's the way I did it.
To be honest I haven't really built any transistor circuit before so I just made sure I could dim a single LED with PWM via a transistor, and then substituted the LED with the LED driver to see if PWM was the way to go. I didn't put more thought into it because I didn't know enough anyway. Apparently it both worked, and failed.
What circuit design pattern(s) would you suggest I look up in order to do it properly with a transistor?
Why do you feel you need the transistor at all, if you're just driving a digital input that works without it?
I don't feel I need it now since I've since found the solution to the problem, but at the time I wanted some sort of indirection between my Pi and any unknown future mistake I was going to make. At the time I 1) didn't know whether the power board actually expected a PWM signal on the dimming control pin, 2) didn't have the service manual for the power board (i.e. didn't know what was connected to the control pin), 3) didn't (and still don't) have the datasheet for the LED driver on the power board (which turns out to be all that was connected to the control pin). It seemed like a good idea to let the transistor break instead of my Pi in case I messed up, I guess.