Is there a Transistor Switch where it is a closed circuit when off?

Thread Starter


Joined Oct 22, 2011
For example, A NPN BJT switch is open when the base collector voltage is too low, but is there an transistor where the switch is closed when the transistor is off and open when voltage is high? Thanks.


Joined Aug 8, 2005
It is possible to use a PNP as a voltage follower, where the xstr is on when the base is pulled low, and off when pushed high (must be equal to voltage on collector).


Joined Nov 23, 2012
I agree with NoMurphy, just be careful if you are controlling the PNP with a microcontroller. The output pins of the uController might not reach a high enough voltage to turn the PNP off completely. Most microcontrollers output will be about 4.2 to 4.5 volts (roughtly) where the PNP will just be turning on. If that is the case, post a followup if needed.


Joined Mar 31, 2012
You might look at depletion mode MOSFETs. Depending on exactly what you are doing, a PFET might also work.