pwm detection switch

246

02:45:17

A failsafe mechanism which will only drive a heater (the lamp) if PWM current is detected through the fan (the DC motor).

If either switch above the sense resistor is open, no current will flow and the comparator output is low. If the fan's path is uninhibited, current flows through the sense resistor and the comparator outputs PWM.

A smoothing capacitor on the output maintains the output gate above the threshold voltage if the fan is on, switching the load on.

published 5 years ago

eekee

5 years ago

Neat! Also works with a DC supply to the fan.

dsball

5 years ago

Yes, 100% duty cycle PWM is DC :) I am not certain this would work in practice though. If an open occurs, noise on the resistor could still potentially enable the OA output. It might also be tricky sourcing an op amp which functions that close to the low rail. One option might be a current sense amplifier an instrumentation amplifier, but that would be even more sensitive to noise. I haven't put it in practice though, my worries could be completely unfounded :) I'm working on a more resilient solution but it will be rather more complicated. I'll add a link if/when I finish it.

dsball

5 years ago

https://everycircuit.com/circuit/6400987127087104 Quite a bit more complicated, but more resilient and tests for decreased current caused by a disconnected fan or corrosion (not just an open), or increased current caused by a stalled fan motor.

eekee

5 years ago

Rail to rail *input* is a selling point for some op-amps. Not sure how common it is. I guess you're right about the noise. It would need reduced gain to defeat that.

eekee

5 years ago

Huh, noise in 50mR should be tiny, but anyway, have a generic differential amp with controllable gain: https://everycircuit.com/circuit/4827915190272000

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