How do I control the speed of a 24 volt DC motor with a 5 volt signal?

paudiol · 24-01-2014 08:06PM #1

Hi guys, I need your expertise here. I am trying to control the speed of a 24 volt dc motor for a project. To do this I am using a MOSFET transistor. I am using the 5 volt output from a NI myDAQ microcontroller. When I connect up the circuit, the MOSFET allows roughly 13 volts to pass through the motor without a gate signal. When I apply a 5 volt signal to the gate of the MOSFET transistor it sends 24 volts to the motor and remains at this even when the 5 volt signal is removed. My plan was to use PWM on the gate signal to control the speed of the motor. I was hoping that the motor would be at 0 volts and ramp up to 24 volts with a 5 volt gate signal. It is a 3.6 amp motor and the MOSFET is a IRF520.

http://pdf1.alldatasheet.com/datasheet-pdf/view/22389/STMICROELECTRONICS/IRF520.html

http://www.ebay.ie/itm/Brushless-DC24V-Magnetic-Drive-Pump-640GPH-Motor-Speed-Variable-0-100-50C-246...

http://www.electronics-tutorials.ws/io/io_7.html

For the last link the circuit I am using is the Unipolar transistor switch circuit.
Any more info needed I will be glad to supply. Thanks

paudiol · 24-01-2014 08:10PM

I have attached the circuit diagram I am using.

PWM Circuit Diagram.JPG

pljudge321 · 26-01-2014 11:55AM

You using a five volt signal to dry drive a mosfet which doesn't reach its saturation region until Vgs reaches about 10V, look at the FETs datasheet and look for its output characteristic.

In addition a microcontroller probably won't be able to source and sink enough current to properly drive the mosfet gate in any reasonable fashion. Use a proper gate driver and drive its logic input using the microcontroller. You'll need to use another power source to power the gate drive, or use the 24V rail and use a zener clamp to avoid damaging the gate of the FET by applying too high a voltage to it.

How do I control the speed of a 24 volt DC motor with a 5 volt signal?

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