4
\$\begingroup\$

I'm trying to drive a DC motor using an external voltage source and control it using a transistor switch. It doesn't seem to be working during the simulation though. I know I must be doing something incredibly stupid.

I have it built in circuitlab here.

Thanks

enter image description here

enter image description here

\$\endgroup\$
  • 2
    \$\begingroup\$ Is there any reason for the output being at the positive side of the 12V power supply? Why are you feeding a AC voltage as the control signal? From the data you provide us is not clear what you intend to do. \$\endgroup\$ – Bruno Ferreira Jul 16 '12 at 21:57
  • 1
    \$\begingroup\$ Bruno is exactly right -- if you plot the voltage at the drain terminal of M1 (add a node name there and click it), or if you plot the current going into R1 (click on one of the terminals of the resistor which you'll see when you hover over them), you'll see that your switch is working just fine! \$\endgroup\$ – compumike Jul 16 '12 at 22:07
  • \$\begingroup\$ oh right, the output should be on the other side of the resistor. The AC voltage is just a simulation of a control signal from a microcontroller. I guess it could have been a square wave. \$\endgroup\$ – waspinator Jul 16 '12 at 22:27
  • \$\begingroup\$ Comment on your R1; normally resistance of motor is far lower than 10K ohm. \$\endgroup\$ – JeeShen Lee Jul 17 '12 at 1:02
4
\$\begingroup\$

Maybe what you want is something like this:

Circuit

Signals

Notice that the output is inverted relative to the control signal because when the FET is "on" it behaves as a closed switch making the motor run (assuming that L1 is you motor).

I'm not sure why you have R1 in series with L1, I just leave it there.

\$\endgroup\$
  • \$\begingroup\$ I modeled the motor as a resistor and inductor in series. Not sure if that makes sense or not, but I guess it shouldn't matter. I guess I was just measuring the output at the wrong place. Would I still need D2 and R2 if the control is a microcontroller? Thanks \$\endgroup\$ – waspinator Jul 16 '12 at 22:24
  • 1
    \$\begingroup\$ @waspinator If the control signal comes from a microcontroller you can omit D2 but you should keep R2 (to protect the FET) since FETs doesn't like their gates floating. \$\endgroup\$ – Bruno Ferreira Jul 16 '12 at 22:32
  • 1
    \$\begingroup\$ Ah. If your motor is modeled by R1 and L1, you'd be interested in the current through the pair, right? \$\endgroup\$ – RJR Jul 17 '12 at 0:22
  • \$\begingroup\$ ya. I think where Bruno put the output does that. \$\endgroup\$ – waspinator Jul 17 '12 at 3:25

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.