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enter image description here

Software engineer dabbling in electronics here.

I created this circuit to control a linear actuator using the states: Full speed forward, full speed backward, and off.

The switches S1 and S2 are pins on a raspberry pi.

The MOSFETs T1 thru T4 are suitably rated N-channel MOSFETs.

M is a DC motor that can be driven with +12 V (forward) and -12 V (backward). Other speeds not needed.

R1 and R2 and 1M resistors to discharge the MOSFET gates.

All connections to the 12 V power supply go through a single fuse rated 3 A, the motor draws 2 A.

An interlock preventing the opening of both switches simultaneously with a bit of safety margin is implemented in software (not perfect but seems okay, a bug would blow the fuse preventing catastrophic failure).

I think this is okay, the resistance of each MOSFET is sufficient to keep each circuit closed when the corresponding switch is in the 'off' state.

But I'm green, did I miss anything?

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  • \$\begingroup\$ Are your switches driven with PWM? Full speed, ok ... but within what time? Current may be too high ... \$\endgroup\$
    – Antonio51
    Commented Apr 17 at 12:06
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    \$\begingroup\$ you shouldn't ask for a review of a circuit without attaching the schematic. \$\endgroup\$
    – LordTeddy
    Commented Apr 17 at 12:35
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    \$\begingroup\$ "did I miss anything?" Yep the schematic you are referring to. \$\endgroup\$
    – Lundin
    Commented Apr 17 at 12:43
  • \$\begingroup\$ @Lundin Hahahaha. Yeah sorry. Fixed \$\endgroup\$
    – cmc
    Commented Apr 17 at 13:04
  • \$\begingroup\$ @Antonio51 The motor draws 2A at peak, and the mosfets are rated 50A. \$\endgroup\$
    – cmc
    Commented Apr 17 at 13:07

1 Answer 1

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Basically it's highly naïve and fails to understand how a MOSFET works. The problem in a nutshell is that the upper MOSFETs are driven as source followers and, given that the gate voltage cannot be higher than 5 volts, then the source cannot switch to a voltage higher than 5 volts minus the gate-source threshold voltage. This means maybe 2 to 4 volts peak on the output.

It's not worth reviewing anything else about it because it'll perform so badly that it just isn't worth any more effort. Having said that, it's a regular rookie/newbie mistake so, you are in company.

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  • \$\begingroup\$ So maybe the best solution is that they switch this with some "H bridge" motor driver IC with all of this built-in? Including overcurrent protection and other goodies like that. \$\endgroup\$
    – Lundin
    Commented Apr 17 at 13:28
  • \$\begingroup\$ @Lundin it's got to be the right one but yes, for sure. Or, depending on the MOSFET type they may get away with an 18 volt control signal. \$\endgroup\$
    – Andy aka
    Commented Apr 17 at 13:49
  • \$\begingroup\$ @Lundin I'll look up an H bridge IC! Rolling my own would be nice for learning reasons but is not required. Andyaka Thanks for the feedback! Holding back for now until I understand what you wrote \$\endgroup\$
    – cmc
    Commented Apr 17 at 14:24
  • \$\begingroup\$ @cmc just ask if something is not graspable. \$\endgroup\$
    – Andy aka
    Commented Apr 17 at 14:40
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    \$\begingroup\$ Oh I was going to edit this one, but a new one makes more sense. My currently favored idea: T1,T2 as p-channels driven by small N-channels, rest as-is, schematic coming up in new post. \$\endgroup\$
    – cmc
    Commented Apr 18 at 7:08

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