Normally On Switch for NPN Transistor

[Raymond31415]

"Normally On" switch

I'm making a computer component and I have constant power going to a npn transistor, I need a "normally on" switch to cut power to the npn transistor at different times. Anyone know a good website that discusses them.

 

[berkeman]

I'm making a computer component and I have constant power going to a npn transistor, I need a "normally on" switch to cut power to the npn transistor at different times. Anyone know a good website that discusses them.

Welcome back, Raymond.

Just about any switch will do. You would put it in series between the power source and the base drive resistor to the transistor, and also be sure to put a resistor from the base to ground, to get a good turn-off when you open the switch. Even a SPST switch would work.

Keep in mind that a mechanical switch will "bounce" when it closes, so the turn-on of the transistor will be a little choppy for a few milliseconds, if that matters in your circuit. There are ways to "debounce" the switch, and we can show you how to do that if needed.

 

[Raymond31415]

To be more specific, the component will be used in computer processors and random access memory so it can't be mechanical but solid state, and as small as transistors get.
I've read about transistors online but I haven't seen one that is "normally on".

 

[berkeman]

To be more specific, the component will be used in computer processors and random access memory so it can't be mechanical but solid state, and as small as transistors get.
I've read about transistors online but I haven't seen one that is "normally on".

Ah, that's different. There are many ways to do what you are asking, but it would help to know more about what the end goal is. Do you want to switch power on and off to a subcircuit? How much power? What voltage and current? Or are you switching the voltage on and off for something else?

There are packaged ICs that are used for high-side power switches for subcircuits, and they work well, if that's what you are intending to do.

 

[Raymond31415]

Yes it is going to a subcircuit, I need constant power to it but I need to shut it off regularly, and I only have to cut power to one npn transistor. I'm not sure how much power would go to it, as low as possible for the smallest transistors to operate.

 

[berkeman]

Yes it is going to a subcircuit, I need constant power to it but I need to shut it off regularly, and I only have to cut power to one npn transistor. I'm not sure how much power would go to it, as low as possible for the smallest transistors to operate.

So you need to drop the collector voltage of the NPN transistor to zero? Or you need to stop supplying base current to it? I'm still not clear on what you are trying to do. How is that NPN transistor connected in the circuit, and what exactly do you want to shut off?

 

[Raymond31415]

Yes i need to drop the collector voltage to zero, the base current is not important. I've read about npn and pnp transistors and how those work, but if you know of a website that talks about "normally on" switches it would help a lot.

 

[berkeman]

Yes i need to drop the collector voltage to zero, the base current is not important. I've read about npn and pnp transistors and how those work, but if you know of a website that talks about "normally on" switches it would help a lot.

"Normally on" doesn't really fit the situation. You just want a way to turn on and off the supply voltage to a transistor collector (I think). You would normally just do that with another transistor in the path of the NPN transistor's collector.

It still would help to understand how this NPN transistor is connected. The most often-used configuration, called common-emitter, has the emitter grounded (or similar), the base driven with a signal, and the collector is connected to the load and to a power supply (through the load or a separate resistor). If this is your configuration, there may be a simpler way to drop the voltage to the load. Like, you can just OR in another signal to the base drive, and shut the NPN transistor off that way. Much simpler than messing with the power supply to the transistor's collector.