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# transistor switch

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#### Josh

##### New Member
how do you use transistors to make a momentary switch an on/off switch?
Cheers for help.

Josh.

Unlike a standard mechanical switch, the transistor can control the amount of flow by varying the base voltage. I will try to explain transitor's switch function in a very simple manner.

Think of a building with people standing outside (the emitter). The 'base' can be thought of as a door, that when opened, allows the people to move inside (the collector). The wider the door is opened, the more people it will let in;but it takes energy to pull the door open,like someone pulling on it. The same principle applys to the transistor. without energy applied to the base, the door stays shut and the current (the people) can not move from the emitter to the collector. However, as a potential is applied to the base, the door opens. The more potential, the wider the door opens,thus a greater flow of current. In this fashion,transistor act as a switch.

This is also how transistor amplify;by using a small potential at the base, they can move a much larger potential from the emitter to the collector. In essence, you have amplified the small base potential. So, that's it. Pretty simple. eh?

However, as a potential is applied to the base, the door opens. The more potential, the wider the door opens,thus a greater flow of current.

Ravi - I hope I am not being rude, but I have to correct you on a technicality, although I am quite sure that you understand the operation of a transistor reasonably well - a transistor is a current transducer, not a voltage (potential) transducer - thus the more CURRENT you allow to flow throw the base, the more current can flow through the collector. This is why we often insert a base resistor - to limit the current through the base.

In contrast, a MOSFET is a voltage/current transducer - the greater the voltage on the gate, the greater the drain current.

BJT (bipolar junction transistor) = current controlled base
FET (field effect transistor) = voltage controller base

I would say you are both right...

Ravi referred to emitter, base and collector. So he was referring to a BJT.

hi,

there are three regions in transistor CE operation. a switch uses the two regions called cuttoff and saturation region. to make momentary switch let flow enough current so that transistor goes in to saturation region. in this case VCE drops extremely low.

the on off switch is a bistable multivibrator . any elementry electronics book will provide you the required info.

abhishek

Thanks everyone. so what's the easiest way of making a momentary switch an on/off switch? using an IC? How?

Thanks.

Josh.

The easiest way to make a solid-state switch using a transistor in saturation region. Like for ex. use BC547, connect 12V Vcc throgh 1k Load resistor (Rc). Connect 10k base resistor and ground the emitter. When you apply postiove voltage to the base, the transistor will go into saturation and current will start flowing through load resistor Rc. when no voltage is applied to base, no load current will flow.

Using IC. try CMOS switches like ADG411, CD4066 etc.

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