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How to Use a Transistor as a Switch – An All-Inclusive Guide

How to Use a Transistor as a Switch? Transistors are components that amplify, control and generate electrical signals in electronic devices. Besides the mentioned applications, you can also use them in switching operations to help open and close circuit courses. A good example is NPN and PNP transistor functioning as effective switches in circuits. 

BJT power transistor symbols

(BJT power transistor symbols)

In today’s article, we will learn how transistor switches work. Furthermore, we will outline other examples, apart from BJTs, where transistor switches operate. 

Operating Characteristics of Transistor

Transistor switches usually have two areas of operation, which include;

  • Cut Off Region

When operating your transistor in the cut-off region, you’ll need to reverse bias both junctions first (base-collector junction and base-emitter junction). Also, the ground input base current is at zero potential (IB=0). 

Then, the operating conditions of your transistor should be as follows;

  • The output collector current IC = 0 A and output voltage VOUT = VCC
  • The VBE is less than cut – in voltage 0.7V
  •  VOUT = VCE = VCC = “1”

There is no current flow through the device since there’s a large depletion layer on the transistor’s junctions. Therefore, the transistor functions as an open switch/fully-OFF. 

  • Saturation Region

Here, a forward bias is the emitter and base junction of the BJT. Next, connect the input to VCC

Operating conditions; 

  • A cut-in voltage level lesser than base-emitter voltage. 
  • The collector current is at a maximum. 
  • Maximum collector current flows (IC = Vcc/RL).
  • Base-emitter voltage VBE is more significant than 0.7V. 

Forward biasing results in a small width of the depletion layer, which consequently causes a minimum collector-emitter voltage drop. As such, you’ll operate the Bipolar Junction Transistor as a closed switch/fully-ON since the current flowing through it has a maximum value. 

Transistor as a Switch Circuit Diagram 

NPN Transistor Switching Circuit

A switching operation using an NPN transistor only works when you supply voltage to the base terminal to the transistor. Moreover, on applying a voltage between the ground and emitter, the emitter-to-collector voltage becomes zero, thus causing a short circuit. 

NPN transistor

(NPN transistor)

Then, if you apply a zero-input voltage, the transistor functions in the cut-off region, becoming an open circuit. Next, you can choose a reference point to connect a load to its switching output.

Finally, turning on the NPN transistor will cause the load to flow from source to ground. However, the base input terminal has to be more favorable than the emitter by about 0.7V to enable the base current to flow. 

NPN BJT symbol

(NPN BJT symbol)

The transistor gets into saturation mode when maximum current (collector) flows through it. 

PNP Transistor Switch

A PNP transistor (a mechanical switch) often works similarly to an NPN transistor. But, the current in the PNP transistor flows through its base. Thus, it is ideal in operating configurations having hostile grounds. 

Furthermore, based on its emitter, the PNP transistor’s base terminal is often in a negative bias mode. That means that the current flow will occur with a negative base voltage. It is because you require a reference point to connect your PNP transistor to its switching output. 

In that way, switching on the transistor enables current to flow from the source to the ground. 

Digital Logic Transistor Switch

Further on, you can combine transistors to make logic gates such as NOT, OR, and NAND. However, a MOSFET integrated circuit is preferable to BJTs since they are more power-efficient. 

MOSFET symbol

(MOSFET symbol)

  • Circuit Overview

A base resistor in the circuitry limits the logic gate’s output current. 

  • Darlington Transistor Switch

Sometimes, you’ll use multiple switching transistors to switch the load voltage directly, or current since the Direct Current gain of the transistor is low. Hence, you’ll use an input transistor to turn off and on a larger current that handles the output transistor. 

NPN Darlington transistor symbol

(NPN Darlington transistor symbol)

Generally, Darlington transistors comprise two connected bipolar transistors, PNP and NPN transistors. Their connection ensures a multiplication of the first transistor current gain and the second transistor yielding a high current gain as one transistor unit. 

  • Circuit Overview

First, the input/first transistor receives an input signal at its base. Then, the signal amplifies before driving to an output/second transistor. 

Next, the signal amplifies further to generate a very high current gain. 

Since Darlington transistors have high switching speeds, you can use them in stepper motor/DC motor control, lighting circuits, and inverter circuits. 

DC motor

(DC motor)

Transistor as a Switch

We will now show you how you can practically use transistors as switches. 

  • Transistor to Switch a Light-Emitting Diode 

In our first example, we will switch on a LED bulb. 

  • Circuit Overview

When you open the switch at the base terminal, there’ll be no current flow at the base. Therefore, the transistor will be at a cut-off state. As such, the LED is off since now the transistor functions as an open circuit. 

LED bulb

(LED bulb)

On closing the switch, you’ll enable the base current to flow through the transistor. Then, saturation will increase, which will ultimately turn on the LED. 

Note; resistors in this circuit limit current flowing through the LED and base. Additionally, you can modify the LED’s intensity by varying the base current path’s resistance. 

Transistor to Operate the Relay

Secondly, you can have a small transistor circuitry to operate a relay. 

  • Circuit Overview

Here, the circuit is as follows;

If you apply an input at the base, you’ll drive the transistor into the saturation region. Consequently, the circuit experiences a short. In the process, you’ll energize the relay coil and then operate the relay contacts.

Moreover, in inductive loads (inductors and motors), suddenly removing power results in a high potential relay coil crossway. The high voltage may destroy other circuit components. Thus, a diode parallel to the load prevents damage from highly induced voltages. 

  • Transistor to Drive the Motor

Lastly, using a transistor to drive a motor unidirectionally by turning on the transistor at regular time intervals.  

  • Circuit Overview

 Applying a flywheel diode prevents damage to the DC motor (an inductive load). 

Then, switch on the transistor in saturation, and cut-off regions continually turn off and on the motor. 

Note; You can switch the transistor at variable frequencies to control the motor speed from standstill to full speed. You can also obtain the switching frequency from ICs or control devices like MCU. 

Conclusion

The article above gives concise information on the characteristics, types, operating regions, and applications of switching transistors. If you need any clarification concerning the topic, kindly reach out to us

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Hommer Zhao
Hi, I am Hommer, the founder of WellPCB. So far, we have more than 4,000 customers worldwide. If you have any questions, you can feel free to contact me. I really appreciate any help you can provide.

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