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I built this circuit and it seems to work if i use a the same power supply to the motor that is powering the IC. I am using an LED in place of the motor for testing. But the circuit does not work if i use a separate power supply for the motor(LED). I am using a 12 volt power supply for the IC and a 9 volt battery for the motor(LED). I am trying to control a 90 volt DC motor and plan to rectify 110VAC to power the motor. My question is should I be able to power the motor with a separate supply? Does the ground for the MOSFET need to be the same ground that the IC uses? I am using a IRF630 for the MOSFET.

  • \$\begingroup\$ I don't see any diode (or snubber circuit) in your motor, it looks bad from the first view. \$\endgroup\$ – R Djorane Apr 15 '15 at 15:20

Yes, the ground for the MOSFET must be at the same voltage as the 555 ground. The MOSFET is turned on by Vgs- voltage from the gate to the source. If the voltage difference gets too large (say because of inductance in the direct connection) you can kill the MOSFET or the 555. A small resistor in series with the gate- perhaps 20 ohms- can slow the switching transitions slightly and reduce the di/dt.

You need a catch diode across the motor or you'll kill the MOSFET as soon as you connect a motor.

You should not attempt to operate this circuit with much less than 12V supply- certainly not the 3-18V supply shown on the schematic. You likely should implement an undervoltage lockout circuit to prevent the MOSFET from being partially turned on under any possible conditions.

Be careful with this kind of circuitry on the mains- an isolation transformer is highly recommended for testing but it's still potentially lethal. Also, connecting an earth-referenced oscilloscope or similar instrument can result in some fireworks and damage to the test device and the instrument.

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  • \$\begingroup\$ The circuit is working when i tied both grounds together using the 12volt power supply and a 9volt battery driving the LED. I then tried with the 12volt power supply and a rectified 110volt supply. Without the motor I connected my Multi-meter where the motor would be but the voltage reading did not vary. Should i be able to see a voltage change using this method? \$\endgroup\$ – Steve Apr 17 '15 at 17:56
  • \$\begingroup\$ A multimeter is not a good proxy for a load. If you're using an isolation transformer it should work with a resistive load. If not, it's dangerous and maybe your 12V supply is grounded. \$\endgroup\$ – Spehro Pefhany Apr 17 '15 at 20:36

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