transistor help

[nuttaphong191]

I am looking for a transistor that when its base or gate is not powered, it completes the circuit and when its base or gate is powered it breaks the circuit. Is posible to get this kind of transistor? Thanks

 

[bananasiong]

PNP, on when the base grounded, off when the base approaching supplied voltage.

 

[audioguru]

We talked about this recently on other websites.
A Jfet is a depletion-mode device that conducts when its gate-source voltage is zero. Adding the voltage turns it off like a vacuum tube.
It doesn't conduct much current.

Somebody showed that there are depletion-mode Mosfets that are used as RF amplifiers. They also don't conduct much.

 

[ThermalRunaway]

PNP, on when the base grounded, off when the base approaching supplied voltage.

A PNP transistor starts to turn on when the base is 0.7V below the emitter voltage, and off when the base is less than 0.7V below the emitter voltage. I've basically said the same as you there bananasiong, except I've added the relevance of the emitter voltage.

Brian

 

[EDGE]

Yea Go With The Depletion Modedevice If You Can Spec It
Edge!

 

[Roff]

I suspect that we might find a better solution if we knew what the OP was trying to accomplish.

 

[Gordz]

You could used an PNP that is biased on, and when you apply current to the base it will turn off. Or you could use the N.C. contacts of a relay.

 

[ThermalRunaway]

You could used an PNP that is biased on, and when you apply current to the base it will turn off. Or you could use the N.C. contacts of a relay.

You could also accomplish the same thing with a normal NPN. When it's not conducting the output will be high, and this could be used as the trigger for another circuit that controls a relay. There are probably thousands of solutions to such an open-ended question; which of these is the best depends on what the OP wants to do.

Brian

 

[Gordz]

You could also accomplish the same thing with a normal NPN. Brian

Indeed you can, the OP asked for suggestions and I gave one, as did you.

 

[ThermalRunaway]

@Gordz;

I wasn't critisising your suggestion, I was merely adding to it.

Brian

 

[Gordz]

Not at all, I remember, way back, wanting to build things I could not explain properly with things I did not understand fully. When you are starting out it is harder to explain why something can't be built the in the manner you want it to. The OP mentions gates yet does not know transistors