Jump to content

Narrowband

Help!! electronics expert needed


martin_h

Recommended Posts

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.

circuit 01.jpg

inverted circuit.jpg

Link to comment
Share on other sites

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?

Link to comment
Share on other sites

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

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

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

Archived

This topic is now archived and is closed to further replies.

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.