Transistor as a switch

[mattjd2699]

What kind of transistor acts most like a switch contact fully on or fully off??? I have a notebook and want to turn on the cooling fan when it gets warm and back off when it is cool.........

 

[Yevgenip]

I think it is called a shmit-trigger. There is no "perfect" Transistor. You can always consider a relay.

 

[Kevin Weddle]

A transistor switches between two voltages. Since it is a current controlled device it can't actually act like a switch. You have to set the current with the voltage at the emitter base junction. However many circuits do utilize the cutoff and saturation feature. I have operated a transistor in the saturation region where the voltage across the transistor remains constant for a given amount of current. This is the closest approximation I have for a switch. Cutoff is easy, you just reverse the emitter base junction.

 

[audioguru2]

Cutoff is easy, you just reverse the emitter base junction.

Hi Kevin,
It might be complicated to reverse the junction.
Didn't you know that a transistor will be cutoff if you simply stop its base current? You could do it by shorting its base to its emitter with something like another transistor, or if its emitter is grounded, drive its base feed also to ground.
It is dangerous to have a reverse bias voltage on a silicon transistor's base-emitter junction. The absolute maximum voltage rating for the reverse-biased junction is only 6V to 8V and exceeding it will cause the junction to breakdown with damage.

Hi Matt,
Sure you can use a transistor as a switch for your fan. Simply provide base current to turn it on and stop the base current (like above) to turn it off. A Mosfet is better (and more expensive) but you won't notice the difference unless your fan is enormous.

You can make a simple Pulse-Width-Modulation circuit to control the speed of the fan, controlled by a temperature sensor. It would operate smoothly and quietly instead of suddenly switching.

 

[MP1]

Matt, there is a temperature controlled computer fan project in our projects section if this is what you are looking for.

MP

 

[hotwaterwizard2]

An SCR works great as a Latching switch and a Triac Works Best as a toggle

View attachment 36418

 

[ante1]

Hi John,

I think S2 should be a NC not NO as drawn here.

 

[EnigmaOne]

Actually, a MOSFET would be an excellent choice for what you are considering here.