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I am learning about electronics and am having a hard time finding a normally closed (i.e. on) switch using transistors and FETs. Am I violating any rules with the circuit below which is using an enhancement mode N-channel MOSFET with an NPN driver transistor? Can I simply pull up the FET's gate high with R1 to keep the switch on (by default)?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ This circuit is fine. You have the right of it. \$\endgroup\$
    – pgvoorhees
    Oct 10, 2016 at 20:49

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Yes, that's perfectly valid (should be a base resistor on the BJT, of course).

Another method you might use could be to use a P-channel MOSFET with a pull-down resistor (just the complement of your circuit). However this does not work if the power is removed from the micro since the protection circuit will drag down the output pin. Your circuit also allows the micro to switch a higher voltage than its supply (for example a 3.3V output switching a 5V or 12V circuit).

Edit: Added are two additional circuits- one using another NPN transistor which will work with power removed from the micro and using an opto as you suggested. Both are high-side switches and the opto one adds isolation while actually simplifying the circuit.

schematic

simulate this circuit – Schematic created using CircuitLab

A third approach is to use a depletion mode MOSFET or a JFET but that's a lot trickier because you have to come up with a negative voltage to turn the transistor off. It does work in the absence of any significant voltage supply though. Another disadvantage is that power devices are unavailable in P-channel type.

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  • \$\begingroup\$ Well, for a lot of low-current applications, PMOS is available. But when very low Rds is needed, you have to go to NMOS. For up to 500mA or maybe 2A, PMOS can probably be used if it is easier. \$\endgroup\$
    – user57037
    Oct 10, 2016 at 21:50
  • \$\begingroup\$ @mkeith I've never seen anything depletion mode that is P-channel other than JFET and RF parts. If you have a source (pun intended) please share. \$\endgroup\$ Oct 10, 2016 at 21:52
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    \$\begingroup\$ Available enhancement mode PMOS transistors actually seem to be better than similarly priced NMOS for low voltages- flying against physics considerations of carrier mobility. Perhaps because they've been optimized as load switches. \$\endgroup\$ Oct 10, 2016 at 22:25
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    \$\begingroup\$ @mkeith Drat, Oh, well, it would have been nice. There are depletion mode opto SSRs which work in either direction but they are very sluggish. \$\endgroup\$ Oct 11, 2016 at 0:34
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    \$\begingroup\$ What mkeith said. It's best to wait a day or so before accepting an answer- to do otherwise discourages potential better answers. \$\endgroup\$ Oct 11, 2016 at 6:13

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