A BC337 transistor is a three-terminal current control device used for general purposes. Based on the semiconductor type used, the BC337 transistor is a negative-positive-negative (NPN) type because the main terminals use a negative (N)-type material. The transistor schematic for an NPN transistor indicates the emitter with an arrow pointing away from the transistor. It is known as a bipolar junction transistor because it uses a positive (P)-type semiconductor material sandwiched between two N-type materials. Semiconductors, such as silicon and germanium, are used in the BC337 transistor to control the flow of electrical charges by allowing a control current to vary the conductivity of the transistor’s main terminals.
This NPN transistor has a base or the control terminal, an emitter terminal, and a collector terminal. On the base-to-emitter junction, the amount of current flow determines the collector current. The ratio of the collector current to the base current is known as beta. If a transistor has a beta of 100, a current of 1 milliampere (mA) through the base-emitter produces a current of 100 mA in the collector. The collector could be connected to a passive load, which could be a resistor, a tuned circuit, an audio transformer, or a relay’s electromagnetic coil, onto a direct current (DC) power supply.
The collector-to-emitter junction of the BC337 transistor may act as a switch for simple DC switching operations. If a 12-volt (V) relay coil requires 100 mA to be fully energized, the 100 mA may be produced by a 12 volts direct current (VDC) supply in series with the relay coil and the collector-to-emitter junction of the transistor. Given a beta of 100, it takes only 1 mA at the base to produce the 100 mA at the collector. The 1 mA may be produced at the base when a dark condition is shown on a phototransistor. This effect may be used to turn on a light bulb via a relay when the room gets dark.
There are many transistor characteristics to consider when using a BC337 transistor. The collector to emitter voltage should be below the maximum collector voltage, and the current on the collector should not exceed the maximum collector current specified for the device. A transistor array carries several transistors inside one package, a scheme that is very useful for designs that use several of the same transistor. In 8-bit computing, for instance, the bus drive may require eight transistors. A transistor array can help minimize total package count, which can simplify final circuit assembly.