control dc motor with mydaq

[JimMurphy]

Hello All.

I am trying to control the speed of a 24v dc motor with a mydaq. I have gotten conflicting information about how best to control the motor, with a few people telling me to use a mosfet. This is something I know very little about. Can anyone recommend a specific type of mosfet that I could use (there are thousands!) or an alternative meythod of controlling it?

Many thanks

Jim

[DanHarryman]

Hello Jim 

Firstly I'd like to thank you for your post. When controlling a DC motor there are a few things to consider. As you are using the myDAQ you are unable to supply the 24Volts needed so as you've heard you will need to drive it using something like a MOSFET or Op Amp. When using one of these though it's important to know if you want the direction of the motor to be one way or two. If you want to spin the motor in a single direction I would use a PWM signal from the myDAQ to a MOSFET or amplifier. When picking a specific MOSFET I would look at one with a high slew rate so you can still apply a reliable PWM signal to the motor. 

If you wanted to use an analogue signal instead you will want to use two transistors in a Push Pull configuration but then as long as you drive them with DC or slow changing values the slew rate should be less of an issue. 

Let me know if this helps at all. 

Dan.H 

National Instruments UK

Applications Engineer 

[JimMurphy]

Hello Dan,

Thanks a million for the reply. I am indeed running the motor one way only. It's actually a motor speed control I am trying to do, which is monitored/corrected by an encoder on the belt. I will absolutely be using pwm to control the speed, so relays etc were out. I'll look further into op-amps and see if I can put something together.

Thanks again for replying,

Jim

[DanHarryman]

Hey Jim

Thats no problem im happy to help. It's also worth noting that you can use a motor driver IC. These might be a little harder to get hold of but should do the job perfectly as its what they're designed for. One that should work is a lm 298 but you might wan to check the data sheet. Let me know how you get on.

Thanks

Dan