I’ve finally forced myself to layout a small board. It’s been far too long anyway.
It consists of these building blocks: reference voltage, comparator, RC low-pass, gain.
There are better methods for the job, e.g. using a digital one-shot to create pulses independent of the triggering waveform… yes. As usual I just wanted something that works in this special case, something simple, small and cheap.
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[PDF]
To make sure the standard firmware can be used without recompiling it, this board uses the same ADC-input (PC5, pin #28) as the voltage-based sensing.
To avoid an electrical short, you must remove R3 (10k) before adding this board!
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R3 is in the bottom left quadrant, right above the red capacitor.
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Empty spot:
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Parts and assembly:
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Preparing the spacers:
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Test fit:
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Done with soldering:
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Making adjustments:
A word of warning: this is a serious health hazard, as it involves making adjustments to a device with exposed 240V AC. Use an isolation-transformer and / or let somebody with proper qualifications do it. I shall not be held responsible for any harm that might come your way.
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[x: 1ms/div – y: 50mV/div]
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[x: 0.5ms/div – y: 50mV/div]
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[x: 1ms/div – y: 1V/div]
The “trick” is to adjust Vref in such a way that the signal after the comparator looks as shown above. There should not be any significant changes to the signal’s shape when the fan-speed is changed.
The “gain” should be adjusted to result in a stable output voltage of about 2.2V or so. This should translate to a displayed value of about 900 in fan-test mode. The reference voltage for the ADC is 2.5V!
DONE.