How to change the range of voltage?

Question

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 1

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.

Answer 2

You have not mentioned any way of driving current into the motors. Boosting the voltage out of the micro pin is not the problem, the problem is the micro pin can only source a tiny bit of current (5mA maybe) whereas even a small motor may draw an amp or more.

By the sound of it you are using a DC motor (not a stepper motor), so the usual way of driving this is with an H-Bridge.

Google will throw up hundreds of motor driver chips, circuits, etc.