After a long wait, I got my Teensy Load boards, assembled one last weekend, and just had a testing session. Some things work, some don’t. The most critical thing (current limiting) doesn’t work, which is a bit rubbish, but it’s a chance to do some troubleshooting and work out what’s wrong. (This is the first project I’ve had that didn’t work first time, so I’m due some learnin’.)
What does work:
- Talking to the DACs from the Teensy over I2C to set current demand and voltage limit.
- Measuring current and voltage using the Teensy ADCs.
What doens’t work is current limiting: the current gradually creeps up until the power supply I’m using for testing starts cutting out. The gate of the current control FET starts off around Vdd then ambles down to some intermediate value instead of going low to switch the current off. The current set point at the non-inverting input of the current control op amp is about 3 mV, and the voltage measured across the sense resistor at the non-inverting input creeps up to about 25 mV without the FET switching off. (I suspect that those stupidly small voltages are part of my problem…)
Anyway, it looks quite pretty. Here’s the scene in my shed this afternoon (confused looking Ian out of shot!):
and here’s close-up of the little thing. You can see the Teensy slung underneath the board, the weird looking sense resistor I bought, the two DACs for setting the current demand and voltage limit, and my high-class hardware store standoffs and improvised stabiliser (OSH Park send you three boards, so why not make use of them?).
