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I want to control a DC motor using a P-channel MOSFET.

I tried the motor with a 5VDC power supply. It works perfectly and speeds up in no time. I measured the current flowing through, it's around 0.15 A.

Now, I wanted to include a MOSFET. I used FQP12P20. I connected the source to +5V, the gate to GND, and the drain to the same motor. The motor still works, but it takes seconds to speed up. The current flowing through and the voltage on the motor is the same. What could be the problem?

Thanks, lszabi

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You need to use a MOSFET with a logic level gate. The MOSFET you are using is only slightly 'on' with 5V on the gate. It's not guaranteed to conduct more than 0.25mA at 5V Vgs.

Get one that has a guaranteed Rds(on) at 4.5V gate-to-source voltage. For a motor with a run current of 150mA, maybe a few amperes at start-up, so < 100m\$\Omega\$ should be okay.

For example, a NDP6020P which is guaranteed to have less than 80m\$\Omega\$ with 4.5V drive even when very hot (125°C) and is more likely to be ~40m\$\Omega\$ at room temperature.

enter image description here

Alternately you could re-design your circuit to provide a -5V gate drive wrt ground (-10V relative to the source) but even so that (high voltage) MOSFET you have isn't all that suitable with an Rds(on) that is about 10x higher. What it's good for is high voltage (-200V rating).

enter image description here

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  • \$\begingroup\$ So what you're saying that it's the too high resistance in the MOSFET what causes the problem? But why are the voltage and the current the same then? \$\endgroup\$
    – lszabi
    Aug 13, 2014 at 11:54
  • \$\begingroup\$ It's too high mostly because the gate voltage is too low for that MOSFET. I don't believe the current and voltage at the motor terminals are the same, otherwise the motor would behave the same. Voltage is being dropped across the MOSFET. \$\endgroup\$ Aug 13, 2014 at 12:03
  • \$\begingroup\$ You're right, the voltage drops when I connect the motor. I'll try this mosfet you recommended. I also have PMV185XN and NX2301P, would they be suitable for driving a motor? \$\endgroup\$
    – lszabi
    Aug 13, 2014 at 12:17
  • \$\begingroup\$ The little SOT-23 ones you mention (especially the NX2301P) are probably okay but they'll take a hit when the motor first turns on. You might want to measure the motor resistance when it's just sitting there to see what the peak start-up current or stall current will be and make sure it can't destroy the transistor. It's a lot harder to damage a TO-220 part. \$\endgroup\$ Aug 13, 2014 at 14:34
  • \$\begingroup\$ Ok, thanks for all the help. Worst case I'll try some additional circuits or other parts. \$\endgroup\$
    – lszabi
    Aug 13, 2014 at 15:14

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