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Transistor Switch

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We have seen that some output devices need a high current to operate properly. The PICmicro chip cannot deliver sufficient current so a driver circuit, also called a buffer, is needed between the PICmicro and the output device. The output signal from the PICmicro is used to control the current that the driver allows to flow through the output device.

The simplest form of driver is the transistor switch.

The picture shows four common transistors. These vary in the total current they can deliver to the load, and how much current they take from the PICmicro chip in order to operate. The current gain of a transistor measures how much current is drawn from the PICmicro chip, in this case, for a given current delivered to the load. For example, a transistor with a current gain of 100 needs only a current of 8mA from the PICmicro to control a load current of 800mA.

The symbol for the transistor, and the names of its three legs are given in the next diagram.

Transistors are manufactured in a number of different packages, some metal cased, and some plastic. To use a transistor, you need to know the pin-out for the package used. The next picture shows details of the packages and the pin-outs for the four transistors shown earlier.

The circuit diagram shows how a transistor can be used to switch on a load. The load could be a buzzer, motor, solenoid etc.

If the load is a motor or a solenoid, then the diode will be needed to protect the transistor from the high voltage surge when the load is switched off. The base resistor, R, is usually around 1 kilohm to 10 kilohms.

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