Reverse PWM signal Polarity

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amrg

Joined Sep 24, 2018
1
Hello,
Just wanted to start by saying that I came here for help to finish my personal truck build. My knowledge in electronics is fairly basic (lots of stuff off of this site and youtube) but I understand basic circuitry.
I have an aftermarket PWM cooling fan installed on my truck. Originally I had it controlled using its own temp related controller. I am currently trying to get it controlled directly from the ecu but I am having issues running it properly. Turns out whenever I command 10% duty cycle at the ecu the fan goes to 90% and when i command 90% the fan goes to 10%. I did more research and it turns out i need to reverse the polarity to get it to function properly.
Is their a way to easily invert that signal given the signal voltage is 5V while the battery source is 12V?
 

wayneh

Joined Sep 9, 2010
17,496
Is their a way to easily invert that signal given the signal voltage is 5V while the battery source is 12V?
Are you certain that the PWM signal presented to the fan is 5V? That complicates things but only slightly. Getting a 5V DC supply from an automotive system is super easy these days. I'd just use one of those cheap 12V-to-USB adapters.

I'm wondering if the PWM signal is really switching between ground and +5V, or is in fact switching between open and ground (for instance open collector).
 

Alec_t

Joined Sep 17, 2013
14,280
It could be as simple as this:-
PWM_invert.PNG
This should work whether the signal is from an open-collector source or a push-pull source.
D1 is to block any negative-going load-dump spikes.
D2 is a 4.7V zener to clamp the output voltage.
 
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