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Here the gate voltage is 5V and lots of the 12V input voltage stays across drain and source.

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Here the gate voltage is 12V and still some voltage stays across drain and source but this time smaller.

I knew that gate voltage just opens the path between drain and source making them act as short circuit. But here, I saw that

1) Apparently drain to source is not short circuted because there is some voltage drop in both cases. Why voltage drops across drain and source?

2) Why applying 5V or 12V to gate makes a difference? Isn't only job of gate voltage is to open/close the drain-source connection?

p.s : both are simulation results using proteus.

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  • \$\begingroup\$ You've created a source follower. Look at the Vgs value. You'll find a graph in the datasheet that shows what happens as that varies. \$\endgroup\$ – Phil G Oct 18 at 18:59
  • \$\begingroup\$ @PhilG is it about not connecting source to ground directly? Because after your comment I just remembered that and connected motor before drain instead of after source and connected source to ground. Now the voltage across the motor is 12V either I close SW1 or SW2. \$\endgroup\$ – muyustan Oct 18 at 19:08
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    \$\begingroup\$ I knew that gate voltage just opens the path between drain and source What gate voltage? The gate voltage with respect to the source voltage (Vgs) is determining it, not the gate voltage with respect to ground. \$\endgroup\$ – Huisman Oct 18 at 19:09
  • \$\begingroup\$ @Huisman I guess you are right and the problem is about that, see my comment above. So, in order to use an N channel mosfet as switch, it should be used such that it's source is connected to ground directly and the load is connected to drain, right? \$\endgroup\$ – muyustan Oct 18 at 19:11
  • \$\begingroup\$ It's not a requirement but it makes it much easier to drive if you use it that way, yes. There can be cases where you would want to use it as a high-side switch, but this isn't one of them. \$\endgroup\$ – Hearth Oct 18 at 19:54

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