Demultiplexer to switch between fixed resistor values to adjust duty cycle on pwm.

WBahn

Joined Mar 31, 2012
29,829
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Thread Starter

destructo_low

Joined Jul 17, 2012
3
I am toying with the idea of using a 1-4 demultiplexer to switch between fixed resistor values to adjust the duty cycle of a pwm.

I plan to use a Texas Instruments CD4556BM96

The pwm currently has a potentiometer, but I need four fixed values for four individual fan speeds. I'll set the potentiometer where I want it and measure for the fixed values I'll need before I unsolder the potentiometer and hard wire in my circuit.

I am no electronics guru. My first concern is that the resistors will all terminate at one point in the circuit (where the potentiometer currently terminates). Will the power feeding through the active resistor try to run back up the other three resistors into the demultiplexer's outputs? If so, could I install diodes between the demultiplexer and the resistors?

My second concern is the switching time between each of the four resistors. If you switch between fan speeds quickly, the resistors will each be connected and disconnected in rapid succession. I don't know if demultiplexers are "make" before "break" or "break" before "make". At any point will there be no resistance as the demultiplexer switches, at any point will there be high resistance as the demultiplexer switches, or is this nothing to worry about?

I hope that all made sense in my limited jargon. If this is in an incorrect subforum or could be placed into a better subforum, please do so.

Thanks,
destructo_low
 
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