08-16-2017, 09:37 PM
Transistor as a Switch
When used as an AC signal amplifier, the transistors Base biasing voltage is applied in such a way that it always operates within its active region, that is the linear part of the output characteristics curves are used.
However, both the NPN & PNP type bipolar transistors can be made to operate as ON/OFF type solid state switch by biasing the transistors Base terminal differently to that for a signal amplifier.
Solid state switches are one of the main applications for the use of transistor to switch a DC output ON or OFF . Some output devices, such as LED s only require a few milliamps at logic level DC voltages and can therefore be driven directly by the output of a logic gate. However, high power devices such as motors, solenoids or lamps, often require more power than that supplied by an ordinary logic gate so transistor switches are used.
If the circuit uses the Bipolar Transistor as a Switch, then the biasing of the transistor, either NPN or PNP is arranged to operate the transistor at both sides of the I-V characteristics curves we have seen previously.
The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region. This means then that we can ignore the operating Q-point biasing and voltage divider circuitry required for amplification, and use the transistor as a switch by driving it back and forth between its fully-OFF (cut-off) and fully-ON (saturation) regions .
While transistors have many uses, one of the less known uses by amateurs is the ability for bipolar transistors to turn things on and off. While there are limitations as to what we can switch on and off, transistor switches offer lower cost and substantial reliability over conventional mechanical relays. In this article, we will review the basic principles for transistor switches using common bipolar transistors.
The most commonly used transistor switch is the PNP variety shown in Figure 1. The secret to making a transistor switch work properly is to get the transistor in a saturation state. For this to happen we need to know the maximum load current for the device to be turned on and the minimum HFE of the transistor. For example, if we have a load that requires 100MA of current and a transistor with a minimum HFE of 100, we can then calculate the minimum base current required to saturate the transistor.
