Transistor acting as a switch

Probably you might have heard someone said that a transistor is use as a switch or signal amplifier and you thought how? Don’t worry; you have come to where you will get the answer of the “how?” This article “transistor acting as a switch” will explain and show you how transistor is use as an electronic switch.
Transistors have three (3) regions of operations and these are the saturation (FULLY ON) region; cut off region (FULLY OFF); and active region. You can read to understand the regions.

To make a transistor become an electronic switch, you either set it at its saturation region that is “FULLY ON” or cut off region that is “FULLY OFF.” The saturation region turns the switch ON while cut off region switches it OFF. I recommend that you read those links above to understand how each region works.

In this article I made use of the two types of Bipolar Junction Transistor (BJT) which are the NPN and PNP to demonstrate transistor as switch.



npn switch on

The above diagrams are the two states of NPN Transistor acting as a switch. In the first state, the transistor is bias to its cut off region that is FULLY OFF. At this state, there is no current flowing through the Light Emitting Diode (LED) or any load connected between the positive power supply and the collector terminal of the transistor; thus the system is turn OFF.

In the second state, the transistor is bias to its saturation region that is FULLY ON. At this state, current flows through the LED and illuminates the LED which means the transistor is turn ON. The resistor R1 in the circuits is to limit the current that will pass through the base (B) of the transistor. Keep this resistor as low as possible in the range of 10Ω and below to keep the transistor fully saturated. R2 is for the LED’s protection from excess current.


The below diagrams show how to use PNP as the NPN substitute of transistor acting as a switch. The difference is that; the PNP is a reverse case and the load is drive through the collector to the 0V. Another difference is; its Emitter (E) is always connected to the positive rail.

pnp switch off

pnp switch on

However, when using a transistor to switch inductive loads such as electric motors, relays, etc. a flywheel diode is place in parallel to the load in reverse direction. This helps in blocking the back electromotive force (EMF) caused by the inductive load when the transistor is at its CUT OFF region thereby protecting the transistor from damaging. Most of times switching powerful loads by transistors are achieved by using relay(s) to switch the loads.

The transistor drives the relay and then the relay does the switching of the load. This is the efficient and healthier way of switching powerful loads with a transistor. This is because; it protects the transistor from over-stressing and damaging. The diagram below explains what I meant using the NPN type of BJT. However, you can use the PNP BJT but by connecting its emitter (E) to the positive potential difference (Pd) while the collector (C) drives the relay to the 0V potential difference. The relay used is a single pole single throw (SPST) type. Its switch contacts (A and B) close or switch ON only if the relay is energized. That is; when the transistor is FULLY ON.

Transistor acting as a switch

The load must not really use the same potential as shown in the circuit above. If you want to drive the load by a different power supply, use the relay’s switch contacts (A and B) as the switch for the load.

The specifications of the transistor matters also a lot in determining the input and out power of the transistor. You can check for the transistor’s datasheet to decide which one to use. See also this relevant article for terms used in transistor specifications HERE.


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