The equations for calculating the Base resistance, Collector current and voltages are exactly the same as for the previous NPN transistor switch. Using the transistor values from the previous tutorials of: β = 200, Ic = 4mA and Ib = 20uA, find the value of the Base resistor ( Rb) required to switch the load fully “ON” when the input terminal voltage exceeds 2.5v. The value of the Base resistor determines how much input voltage is required and corresponding Base current to switch the transistor fully “ON”. When maximum Collector current flows the transistor is said to be Saturated. By varying this Base-Emitter voltage V BE, the Base current is also altered and which in turn controls the amount of Collector current flowing through the transistor as previously discussed. In order for the Base current to flow, the Base input terminal must be made more positive than the Emitter by increasing it above the 0.7 volts needed for a silicon device. Even though the transistor is not a perfect switch, in both the cut-off and saturation regions the power dissipated by the transistor is at its minimum. In practice when the transistor is turned “OFF”, small leakage currents flow through the transistor and when fully “ON” the device has a low resistance value causing a small saturation voltage ( V CE ) across it. An ideal transistor switch would have infinite circuit resistance between the Collector and Emitter when turned “fully-OFF” resulting in zero current flowing through it and zero resistance between the Collector and Emitter when turned “fully-ON”, resulting in maximum current flow. The difference this time is that to operate the transistor as a switch the transistor needs to be turned either fully “OFF” (cut-off) or fully “ON” (saturated). The circuit resembles that of the Common Emitter circuit we looked at in the previous tutorials. If the load is of a very high current or voltage nature, such as motors, heaters etc, then the load current can be controlled via a suitable relay as shown. With inductive loads such as relays or solenoids a flywheel diode is placed across the load to dissipate the back EMF generated by the inductive load when the transistor switches “OFF” and so protect the transistor from damage. When used in this way, the transistors open collector output can thus “sink” an externally supplied voltage to ground thereby controlling any connected load.Īn example of an NPN Transistor as a switch being used to operate a relay is given below. The simplest way to switch moderate to high amounts of power is to use the transistor with an open-collector output and the transistors Emitter terminal connected directly to ground. With a positive signal applied to the Base of the transistor it turns “ON” acting like a closed switch and maximum circuit current flows through the device. With a zero signal applied to the Base of the transistor it turns “OFF” acting like an open switch and zero collector current flows. Then the transistor operates as a “single-pole single-throw” (SPST) solid state switch. For a PNP transistor, the Emitter potential must be positive with respect to the Base. Then we can define the “saturation region” or “ON mode” when using a bipolar transistor as a switch as being, both junctions forward biased, V B > 0.7v and I C = Maximum. 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 as shown below. The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region. 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. 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. 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. Solid state switches are one of the main applications for the use of transistor to switch a DC output “ON” or “OFF”. 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. 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.
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