How to change the range of voltage?

I am doing a project to use dsPIC as controller to output PWM signal to control motors. I can use driver IC (I used L293D actually) to simply let motors to rotate, but I cannot control the voltage range to control the motor to rotate with all range of speed.

The output of dsPIC PWM pins is in the range of 0V~5V, but for motors, the input range is different.

Assume the range is -10V~10V, I want to change the 0~5V output to -10V~10V (0V->-10V, 5V->10V). Is there a good way to achieve this?

Answer

Assumptions:

• The circuit has +10V and -10V supply rails available


• Isolation of the dsPIC from the motor power / ground rails is desirable, to prevent back-EMF from frying the dsPIC

The following arrangement would provide arbitrary voltage switching, isolated from the controller side of the circuit:

^{simulate this circuit – Schematic created using CircuitLab}

This will provide an inverted PWM signal to power the motor.

• The PWM signal switches the optocoupler's internal LED on and off.


• This turns the output phototransistor of the optocoupler on and off correspondingly.


• When the PWM signal is high, the opto's transistor conducts, pulling the MOSFET gate low, and thus it behaves as an open switch.


• When the PWM signal is low, the opto's transistor does not conduct, so the MOSFET's gate is pulled high, it behaves as a closed switch, allowing current through.


• The diode across the motor prevents back-EMF from frying the MOSFET.