I am new to transistors and I need help finding the type that I need. I am looking for a transistor that allows current to pass when the base is connected to ground, and doesn't allow the current to flow when it is connected to power. What transistor would this be?

Edit: looking to connect to positive line

  • 3
    \$\begingroup\$ It's PNP or PMOS. \$\endgroup\$
    – Eugene Sh.
    Nov 20, 2020 at 19:42
  • \$\begingroup\$ If the gate voltage is -ve, but Vdd is +ve, it could be an N-type JFET. \$\endgroup\$
    – user16324
    Nov 20, 2020 at 19:43
  • \$\begingroup\$ @BrianDrummond, or if "power" is a negative voltage it could be NMOS or NPN. \$\endgroup\$
    – The Photon
    Nov 20, 2020 at 19:56
  • 1
    \$\begingroup\$ Need more detail. What is "power"? 5V? 20V? 867.5309V? What are the other two terminals of the transistor connected to? Is one connected to power and the other to the load? What? A schematic, even hand-written and photographed, with just a 3-wire blob marked "unknown transistor" would help. \$\endgroup\$
    – TimWescott
    Nov 20, 2020 at 20:52
  • 1
    \$\begingroup\$ A very warm welcome to the site and great to see a new person venture into electronics. As well as the good answer below, you can find plenty of information and examples on the internet. As a start, try searching for 'npn switching circuits beginner'. Enjoy learning :-) \$\endgroup\$
    – TonyM
    Nov 23, 2020 at 16:16

1 Answer 1


I suspect you haven't found the comments particularly informative, so I'll give you some circuits.

In general, Eugene Sh is correct. You can use either of these


simulate this circuit – Schematic created using CircuitLab

A few notes: R1 should be about 10 times the load resistance. This ensures that the PNP transistor is fully on, and using much more than this will limit the voltage applied to the load AND increase the power dissipated by the transistor.

V2 should not be more than about 20 volts - if it is, you may destroy the MOSFET when you ground the control. If you are working with a higher voltage, you'll need to increase the voltage applied to the control line when you "ground" it to turn on the circuit.

V2 should be more than Vgs(th), the turn-on threshold of the MOSFET, which for "regular" MOSFETs will run about 5 volts, although more is better, and a good target is about 10 to 12 volts. If you look around, you can find "logic-level" MOSFETs which have lower turn-on voltages, but you must be careful not to use the wrong variety by mistake.


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