#TRANSISTOR SWITCH MANUAL#

The switching action of a transistor can be at a very high frequency, like the radio, TV, and communication signals with megahertz frequencies.

#TRANSISTOR SWITCH MANUAL#

Although it is a very simple example, it shows the principle of the way a transistor can be used for this type of application.Ī similar circuit can be used for other applications such as counting people entering a library or the cars passing through a pathway by replacing the manual switch by a photocell (a resistor whose resistance changes with light and can be used as a light sensor), or by an automatic switch of a different type.

The scenario shown in Figure 1 is a practical example of the switching action of a transistor.

In such a case, if the voltage at point A is measured it must be 0 V.Ĭomparing with when the switch was open, now the voltage difference across the load is 25 V, and, therefore, the load is energized (note that when the load is connected to point A, the current through the emitter is the sum of the current through the 200 Ω resistor and the load current). The current flow of 125 mA causes a voltage drop of 25 V in the 200 Ω resistor. Ignoring the small voltage drops in the transistor, the C-E current is 125 mA (25 ÷ 200, based on Ohm’s law). The magnitude of this current depends on the resistance R, which is 200 Ω here. As mentioned earlier, this action turns on the internal collector-emitter connection and causes an electric current to flow from collector to the emitter. The voltage difference across the load is zero and the load is not energized.Ĭlosing the switch lets a current flow between the base and emitter. If the voltage at point A (where the load is connected) is measured, it shows 25 V because there is no voltage drop in the 200 Ω resistor (due to carrying no current). Consequently, there is no current between the collector and the emitter because the transistor acts as an insulator. The base-emitter junction is forward biased, but when the switch is open there is no current between the base and the emitter. The collector-base junction is reverse biased, as required, and a current never flows between the collector and the base. In the circuit shown in Figure 1, an NPN transistor is shown. In addition to taking a much larger space and making noise by each relay when making contacts, after a while bugs could also make nests in the warm space between them and gradually disrupt the operation (the term bug and debug in computer terminology stems from here). In the past, these relay systems were common.

Note that in a complicated system, such as the operation of an elevator or a train, there can be hundreds or sometimes more than a thousand relays.

Transistor switch