TRANSISTORS:

A transistor is a device which acts as an electrical switch but has no moving parts. Current flow through the transistor can be controlled mechanically by a set of contacts or electrically by reversing the circuit polarity.

A transistor is made up of three small sections of material fused together like a wafer or Sandwich and placed in a container. The sections are referred to as "N" or "P" type material. The arrangement of the sections determines the transistor's polarity. The transistor may be known as a PNP transistor or as a NPN transistor according to the positions of the wafers. The middle wafer serves as the base and therefore dictates the polarity.

The transistor symbol is illustrated on the chart. The circle represents the container. The letters represent the three elements of which the transistor is composed -- E is the Emitter, C is the Collector, and B is the base. The arrow on the Emitter symbol indicates the direction of current flow through the transistor.

There are two important factors relative to the manner in which the transistor works. First: the Emitter-Collector circuit is the main current-carrying circuit. Second: current flow through the Emitter-Collector circuit is possible ONLY when there is current flow through the Emitter-Base circuit. Although the current flow in the base circuit may be only a fraction of the current flow in the Collector circuit, the Collector circuit cannot exist without the base circuit. It follows then, that an interruption of the light current flow in the base circuit will cause a stoppage in the heavy current flow in the Collector circuit in this manner the best circuit "triggers" the transistor and turn it ON or OFF.

As previously stated, the transistor is electrically controlled through the base circuit. A set of mechanical contact points, in series in the Emitter-Base circuit, can readily carry the light current flow in this circuit and serve to "make" and "break" the Base circuit.

Another method of electrical control is performed by reversing the current flow in the base circuit. The base circuit will carry current only when the proper voltage range and polarity is applied. When incorrect voltage or reversed polarity is applied the Base circuit is turned OFF and current flow in the Collector circuit is consequently stopped.

TRANSISTORS