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schematic

simulate this circuit – Schematic created using CircuitLab

I am trying to design a basic circuit for controlling both the direction and speed of a permanent magnet DC motor. The way I intend to do this is to use the circuit above (component values are arbitrary) however I have a couple of questions to ask:

  • Do I need to add an opto-isolator between the Gate resistor and the PWM signal?

  • If I want to add a fuse for safety purposes in this circuit would I place it between my 24VDC source and the Drain of my mosfet?

  • What addition could I add to this circuit in order to change the direction of rotation of the motor?

  • Is there a possibility of changing both the direction and speed of the motor using a wigwag potentiometer (similar to the ones used on mobility scooters) without building something too complicated?

These are the motor parameters:

power rating: 0.43kW

Voltage rating: 24VDC

Full load Current: 18A

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  • \$\begingroup\$ Have you done the math? \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 7 '16 at 0:26
  • \$\begingroup\$ Have you checked how to correctly use an N-type MOSFET (assuming the IRF530 is correct) for PWM switching? Have you done much research into motor driving? \$\endgroup\$ – user2943160 Sep 7 '16 at 0:34
  • \$\begingroup\$ @user2943160 Yes I have done a bit of research on H-bridges and it is not as easy as people on the internet make it seem to be especially when you're dealing with high power applications. The values I have chosen in this schematic are arbitrary I was just wondering where it would be best to start. \$\endgroup\$ – GrapeApe Sep 7 '16 at 0:37
  • \$\begingroup\$ @GrapeApe You haven't drawn an H-bridge, here. Currently your question is also overly broad: you want to cover MOSFET gate drive, circuit protection, complete motor control, and implementing a specific user interface. Can you confine the scope of the question and get yourself a starting point on this design? \$\endgroup\$ – user2943160 Sep 7 '16 at 0:51
  • \$\begingroup\$ @GrapeApe the MOSFET is best after the load. \$\endgroup\$ – Bradman175 Sep 7 '16 at 0:56
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It looks like you're missing a few core principles about switching electronics and motor control. I'll go through your schematic and help you focus on what you need to learn.

  1. The MOSFET in your schematic is an N-channel type. To turn on an N-channel FET, the gate pin must be a couple of volts* higher than the source pin. In your circuit, if M1 is on, than the source pin will be approximately 24V. So the gate pin would need to be a couple of voltage above 24V. I doubt your PWM signal goes that high, so M1 will never turn on.

(*)I'm intentionally using inexact language to keep the explanation simple.

  1. As with all things electronic, there are several options for solving #1. Here's a couple of options listed in order of complexity: You could (a) use an N-channel FET on the low-side of the motor, (b) use a P-channel FET instead, or (c) use the N-channel FET on the high-side with a boost capacitor PWM drive. However, none of these options will give you directional control of the motor.

schematic

simulate this circuit – Schematic created using CircuitLab

  1. In order to get directional control of the motor, you will need to be able to reverse the current. The simplest way to do that using solid state components is the H-bridge. The most complex part of an H-drive at 24V is the drive circuitry for the high-side FETs. But that's out of scope of this question. D1 in your schematic is good for one-directional control, but will cause failure if you leave it in for bi-directional control.

schematic

simulate this circuit

Alternatively, if it doesn't need to be solid state, you can use mechanic switches and/or relays:

schematic

simulate this circuit

  1. You don't need opto-isolators to drive a motor. However, if the logic side of your device is particularly sensitive to noise or you're doing very high precision analog readings, any effort to isolate an electric motor from the rest of your circuit can only help.

  2. A fuse isn't a bad idea, of course. There are other ways to protect the circuit as well. If you do want to use a fuse, choose a slow blow fuse. Surge currents in DC motors can be very high for short periods of time. The fuse can be anywhere in series with the 24V source.

I can't address your question about wigwag potentiometer because I'm not familiar with what that is.

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  • \$\begingroup\$ What I mean by a 'wigwag' is basically a center-tapped potentiometer. They're commonly used as a throttle control (speed control) on mobility scooters. Thanks for the clear and simple answer anyway I'll accept this answer but I want to see what other ideas people have first. \$\endgroup\$ – GrapeApe Sep 7 '16 at 2:52
  • \$\begingroup\$ a "wigwag" is generally a potentiometer + a self centering mechanism. It seems the motor scooter community have their own vernacular. \$\endgroup\$ – Spoon Sep 7 '16 at 4:10
  • \$\begingroup\$ @GrapeApe Then yes, you could use a pot to control the motor the way you want. But that is done easiest with a layer of programmable logic in between. For example, a microcontroller reading ADC from the pot and translating that to motor commands. \$\endgroup\$ – Dan Laks Sep 7 '16 at 5:53

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