faking out fan tachometer for my inverter

Thread Starter


Joined Mar 4, 2018
I have an inverter that I use for solar power. It has 3 box fans 2 of which are 150cfm and the other one has a tach wire and only spins around 60 cfm. Since there are 2 power boards full of fets that I don't want blown I don't trust a low cfm fan to cool the other board. So basically it's power board, Toroid coil, power board in that order and control boards mounted on top of the last power board. Since the control board controls the low cfm fan and expects to see a tach signal, or goes into alarm, I need a way of giving it a signal when needed.

I was thinking of using something like an NE555 timer with a pot to give the correct frequency when that fan is turned on by the control board. Not sure of the output voltage for the fan yet but I can make a regulator to get the voltage down. I've been putting off learning to use my mini DSC as well.

Any ideas folks on a diagram?



Joined Aug 1, 2013
If all you want to do is create a false tach signal, a 555 probably will be all you need.

What are the voltage and current markings on the hub of the fan you want to emulate?

Manufacturer / model number / photo?

Where does the tach signal go? For a standard 12 Vdc fan, the tach signal is generated by an open-collector hall effect sensor. The signal can be internally pulled up to the fan operating voltage, internally pulled up but to 12 V but clipped at 5 V by a zener, or brought out as an open collector. So we need to know more about the environment of the signal. Input straight into a 5 V logic device? Something else?