martin_h Posted March 23, 2016 Share Posted March 23, 2016 I am trying to build a fan/motor controller I have a circuit board that outputs a PWM signal and am using the attached circuit.... But because the PWM output is open collector the control action is reversed ie a pwm of 0 is full on and a pwm of 255 is off. Can anyone suggest anyway of inverting the control signal. I have tried using a single transistor as an inverter but for some reason it doesn't work....welll it works if I short the first transistor base to +ve, but the PWM doesn't drive it Any help gratefully received. Link to comment Share on other sites More sharing options...
Icosahedron Posted March 23, 2016 Share Posted March 23, 2016 (edited) Since the PWM output is open collector, a pull-up resistor will be required on the output to switch the BC108 on. I wouldn't expect a BC108 to survive having its base shorted to +12V with emitter connected to GND or did you mean to say that you shorted the 10k resistor to +12V? Edited March 23, 2016 by Icosahedron Half asleep. Link to comment Share on other sites More sharing options...
Knighty2112 Posted March 24, 2016 Share Posted March 24, 2016 Been a while since I did some electronics, but couldn't you use a flip flop to invert the signal from PWM? Link to comment Share on other sites More sharing options...
martin_h Posted March 24, 2016 Author Share Posted March 24, 2016 8 hours ago, Icosahedron said: Since the PWM output is open collector, a pull-up resistor will be required on the output to switch the BC108 on. I wouldn't expect a BC108 to survive having its base shorted to +12V with emitter connected to GND or did you mean to say that you shorted the 10k resistor to +12V? I did wonder about the pull up resistor......... and yes I shorted it through its base resistor, there are a few things I can still remember from the dim and distant past. Link to comment Share on other sites More sharing options...
gajjer Posted March 24, 2016 Share Posted March 24, 2016 Hi Martin If the drive is open collector, you need a pull up to the first transistor. The 10k will do. The input can then be connected directly to the output of the PWM. cheers gaj Link to comment Share on other sites More sharing options...
gajjer Posted March 24, 2016 Share Posted March 24, 2016 Hi again. Also, you only need one of those 1k resistors. cheers gaj Link to comment Share on other sites More sharing options...
Dr_Ju_ju Posted March 24, 2016 Share Posted March 24, 2016 BTW, PWM control of a fan may well result in the fan 'singing\whining' due to the square wave nature of the applied signal, it would be better to a true variable DC controller, e.g. something based on the LM317 chip.... Link to comment Share on other sites More sharing options...
martin_h Posted March 24, 2016 Author Share Posted March 24, 2016 Many thanks to all, the revised circuit works well, as for a singing fan, I'm not to bothered about that, its only a small PC fan that will be sat outside and probably only ever running at 1/4 power. Link to comment Share on other sites More sharing options...
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