Op amp / Mosfet question.

Jimdagym

Hello all,

I also posted this in the electrical forum. I apologise for the crossposting, thought there could be valuable help in both fora.

Anyone have any experience with these? I am looking to control the speed of a 24v dc motor, running in one direction, with a 5v output from a mydaq. Its for a project and I am stumped at this bit as I know sfa about both!
Trying to even buy samples to try is a minefield. RS sell about 10000 types of each
Any help is much appreciated.

Jim

pljudge321

So you are probably going to use PWM to control the effective voltage the DC motor sees. Whats your design brief, do you have to build all the components from scratch? Also I've never used a MyDaq so a bit of info on what it is would be helpful.

DublinDilbert

I assume your using an NI-DAQ, so you don't have much drive current available on your digital outputs, so it would be best to use a mosfet.

Have a read up here:-
http://www.electronics-tutorials.ws/io/io_7.html

Make sure you include a fly back path (diode) for the inductive energy in the motor.

If you need mosfets let me know, i've a few knocking about. BTW Radionics will sell individual MOSFETs.

Jimdagym

pljudge321 wrote: »

So you are probably going to use PWM to control the effective voltage the DC motor sees. Whats your design brief, do you have to build all the components from scratch? Also I've never used a MyDaq so a bit of info on what it is would be helpful.

Yeah, exactly, using PWM to control speed, belt surface encoder to give feedback to sytem / correction to motor (closed loop).
The brief is to design and build it. Dont have to build everything from scratch, but its all out of my pocked so i cant buy everything complete either! The mydaq is National instruments data acquisition controller. Made to work with labview if you know that.
(mechatronics in sligo)

DublinDilbert wrote: »

I assume your using an NI-DAQ, so you don't have much drive current available on your digital outputs, so it would be best to use a mosfet.

Have a read up here:-
http://www.electronics-tutorials.ws/io/io_7.html

Make sure you include a fly back path (diode) for the inductive energy in the motor.

If you need mosfets let me know, i've a few knocking about. BTW Radionics will sell individual MOSFETs.

Thanks for the link. I am a fitter by trade so some of the electrics I am still only learning!
What i meant by radionics sell 10000 mosfets I meant different types. I am struggling to pick an appropriate one to use. Thank you for the offer though.
The thinking was that the daq controls the mosfet and the mosfet runs the motor. But I have never used a mosfet before. It looks like it should be mounted on a circuit board but I am not using one. Would I need to build one to include the diode?
I was also asking on the ni forums and one response said to use an ic like a lm298, but that has me equally lost :pac:
If i knew then what I know now I would have built this smaller in scale and ran a sub 5v motor to control it directly off the daq. FWIW I am using a 24v, 24w motor. 1.35A AFAIR

I appreciate your replies guys. I honestly do.

Knifey Spoony

Do you want to be able to control the direction of the motor as well as its speed? 'Cause controlling the direction would require a h-bridge, which at its most basic, is just four MOSFET's. You can pick up h-bridges in IC form easily enough, that includes various forms of protection for the circuit powering the h-bridge (fly-back diodes). Just make sure that the h-bridge or MOSFET that you end up getting is able to deal with the voltage and current requirements of the motor.


EDIT: never mind the above, I just re-read the OP and saw the motor needs to only go in one direction. D'oh!

Irish_Elect_Eng

You can use a breadboard to prototype your circuit:

Like these from Maplin.

DublinDilbert

Your motor is relatively small, I've worked on 1000A motor controllers.

I assume your using labview so you'll need a vi that can generate the pwm signal. With the ni daq you can only generate a software pwm, a quick google and you'll find a vi.

Just build your circuit on vero board, the likes of maplin will have that. It's just got holes at 0.1 inch pitch and coper strips/tracks that you can solder stuff to. Yes best to include a diode on this for the "fly back"

The link I posted above basically gives you the circuit you need. Mosfets are very easy to use, don't be daunted.