Posts tagged: treadmill

Aug 05 2011

Bothersome circuit

Very tired and about to go to bed, but traced and ID’d most of the parts on the old treadmill motor controller board.

It drives a motor rated for 180v DC, but my measurements say it’s putting out 330v DC which would make sense for rectified UK mains. I suspect on looking at the board that for UK use they dial the speed control trim-pot down to half so the PWM signal never goes above 50%.

Still means the motor’s running at twice the voltage it should though, which’d explain the overheating issues.

I don’t understand the circuit fully though. For one thing there’s a half-bridge rectifier essentially bridging the motor terminals. Surely that’d mean it’s perpetually shorted out? I don’t understand it, but suspect it might be something for dealing with back-EMF from the motor.

I also don’t understand how the MOSFET’s driving it. It all seems to be coming through a 5Watt resistor and some very thin traces, especially since the motor’s 2-2.5HP (1400Watt+). Maybe it’s because it’s late, maybe I’m just not seeing it and it’s just the trigger. Just can’t see it right now. But the circuit does work (even if the fuses and current breaker don’t. If something shorts it ALLWAYS knocks out the 30Amp breaker for the workshop).

It’s a single-sided PCB. Here’s my notes on it. A few figures are missing, but all components are marked.

I’m planning on making a much simpler control circuit, but it does involve a very large 220-110v transformer to get the proper 180v DC.

It’s been a good day. Castings done, experiments done, oscilloscope usable again, found artial pressure cooker for new vacuum degass chamber, some clearing up done.

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