I was doing some experiments with mosfets I have at home. Everything works but there's something I can't really explain yet.


simulate this circuit – Schematic created using CircuitLab

The schematic is quite basic, the gate is connected to a dangling wire and I connect it manually to positive current to open the gate.

The load I have is a DC motor.

When the gate is connected to V+, the motor turns, when I disconnect the gate goes back to gnd and the motor stops.

The strange moment is that when the gate is to ground, the scope displays some kind of sine/triangle wave. The motor doesn't turn. I guess I'm sensing a voltage because the MOSFET allow a very very small amount of current to leak from the source to the drain.

My power source is of 7v and the voltage on the scope goes from 0v to 10v when the gate is closed. The frequency of the wave is of 100hz. I don't think it's AC noise because it should be of 60hz and go below 0v and probably not as high as 10v.

Is it normal to see something else than 0v when the gate is connected to ground? I was expecting to see maybe a really low voltage as I read that MOSFETs would behave like power resistor when the gate isn't completely open.

  • \$\begingroup\$ I removed the npn tag. You're not dealing with an NPN transistor here, or am I missing something? \$\endgroup\$ – Marcus Müller Jul 19 '18 at 13:40
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    \$\begingroup\$ Gates are control pins and are neither open nor closed. The drain-source channel can be regarded as being open-circuit or closed (like a relay contact) as a first approximation. Your question is unclear in that respect and it's unclear where your scope is connected. \$\endgroup\$ – Andy aka Jul 19 '18 at 13:42
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    \$\begingroup\$ Does your motor have a internal free wheeling diode or is it missing? \$\endgroup\$ – po.pe Jul 19 '18 at 14:00
  • \$\begingroup\$ @Andyaka the scope is connected as explained on the schematic with Osc+ and Osc gnd. What's not clear? \$\endgroup\$ – Loïc Faure-Lacroix Jul 19 '18 at 14:57
  • \$\begingroup\$ @Humpawumpa it's a dc brushed motor. \$\endgroup\$ – Loïc Faure-Lacroix Jul 19 '18 at 14:58

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