Transistor as a Switch: What It Is and How it Works

Transistor as a Switch 

Source: Wikimedia Commons

Transistors are devices generally used for generating, controlling, and amplifying electrical signals.

But did you know transistors could be what you need for switching applications? Yes, we can have a transistor operate as a switch.

Also, it’s easy to use a transistor as a switch in any circuit, and it works effectively for the closing and opening of your courses.

Plus, you can use both NPN and PNP transistors as switches.

In this article, you’ll learn everything about transistor switches and how they work.

Even if it’s complicated, we’ll break it down for you.

So, hang in there!

Why We Use Transistors as Switches

We have different types of switches, including pushbutton switches, slide switches, toggle switches, etc.

Despite the variety of controls, why do we use transistors as switches?

Since all buttons have the same functions, why do we prefer a transistor?

Pushbutton Switch

Source: Wikimedia Commons

The reason is simple.

While the other switches are mainly mechanical, transistor switches are purely electrical.

Transistors don’t need human intervention and can turn on and off based on the current supply.

Operating Regions

Transistor switches have two operating regions, including the cutoff and the saturated region.

Cutoff Region

For transistor switches operating in the cutoff region, the operating conditions are zero output collector current (I_C), zero input base current (I_g), and maximum collect voltage (V_CE).

These operating conditions cause no current to flow through the device.

Also, there’s a  large depletion layer over the circuit, which causes the transistor to switch entirely off.

Cutoff Characteristics

• The base-emitter voltage is less than 0.7v
• The base-collector junction stays in reverse bias mode
• Also, the base and input remain grounded (0v)
• Base-emitter junction also stays in reverse bias mode
• V_OUT = V_CE = V_CC = “1”
• The transistor switch is fully off
• Here, the transistors work as an open switch
• There is no collector current flow (I_C = 0)

In truth, a transistor switch operating in the cutoff region or OFF mode has two junctions working in reverse bias modes.

Additionally, if you’re using a PNP transistor, the emitter potential will harm the base.

Saturation Region

When your transistor operates in the saturation region, it stays in forwarding bias mode, allowing a series of results to generate a small depletion layer.

Also, it will enable the maximum current to flow through the transistor.

Thus, placing the transistor switch in a fully on state.

The results that lead to this effect include; applied maximum base current= maximum collector current= minimum collector-emitter voltage drop.

Saturation Characteristics

• You can connect the input and base to the V_CC
• The transistor switch is full-on
• The base-emitter voltage is more significant than 0.7v
• Base-collector junction stays in forwarding bias mode
• Base-emitter junction stays in forwarding bias mode
• The ideal saturation is V_CE= 0
• Here, the transistor works as a closed switch
• The max collector current flow = I_C= V_CC/R_L)
• V_{OUT =} V_CE = 0

Thus, a transistor working in On Mode or saturation region will have its two junctions operating in forwarding bias mode.

In contrast, you must have a positive emitter potential to the base if it’s a PNP transistor.

How Does a Transistor Switch Work?

When your transistor works as an SPST (single-pole single-throw) solid-state switch, you can apply a zero signal to its base to set it to OFF mode.

Once it’s off, it serves as an open switch and blocks the flow of the zero-collector current.

When you apply a positive signal to the base, it sets the transistor to ON mode.

Then, the transistor becomes a closed switch and allows the maximum current to flow through the circuit.

Additionally, there is an easy way to switch any amount of power from moderate to high.

All you’ve to do is connect the emitter terminal of the transistor directly to the ground and pair the transistor with an open-collector output.

Using your transistor switch this way, you can sink any excessive voltage to the ground.

Thus, allowing you to control any load you connect to your circuit.

NPN Transistor as a Switch

Interestingly, you can use both PNP and NPN transistors as switches.

Switching operations can only occur when supplying enough voltage to the base of the transistor’s terminals.

Also, when you apply enough voltage between the emitter and the ground, the emitter-to-collector voltage will equal 0.

For this reason, the transistor will serve as a short circuit.

Also, applying zero voltage at the input will make the transistor operate in the cutoff region—making it an open circuit.

You can use a reference point to connect the load to the switching output for this switching connection.

Switching on the transistor will allow the current to flow through the load from the source to the ground.

NPN Transistor as a Switch Circuit Diagram

PNP Transistor as a Switch

The operation of the PNP transistor as a switch is similar to that of the NPN transistor.

However, the difference is the current flows from the base.

Therefore, you can use this switching operation for configurations with a negative ground.

Also, in the case of a PNP transistor, the base terminal is always in negative bias mode based on the emitter.

The current will only flow with a negative base voltage for the PNP switching operation.

Why? Because you use a reference point to connect the transistor to a switching output.

Therefore, when turning on the transistor, current will flow from the source before reaching the ground.

PNP Transistor as a Switch Circuit Diagram

Transistor to Switch LED

Additionally, you can use a transistor to switch an LED. Here’s how it works.

When a base terminal switch is set as open, no current will flow through the base.

So, the transistor will operate in a cutoff region. Hence, the transistor will be an open circuit, and the LED will remain off.

In contrast, when the switch is set as closed, the base current will flow through the transistor and change its operation to the saturation region.

Therefore, the LED will switch to ON.

Moreover, you can use resistors to limit the current flowing through the base to the LED to avoid damage.

You can even adjust the LED’s intensity by varying the resistance in the base current path.

Transistor to Switch Led Circuit Diagram

Transistor to Operate the Relay

Interestingly, you can control relay operations with a transistor.

With a bit of arrangement, you’ll power up a relay’s coil with a transistor—allowing you to prevent any extra load you connect to it.

The input you’ll apply at the base must send the transistor into saturation mode for this to work.

So, you can power up the coil and operate the relay contacts.

Removing power from inductive loads can maintain a high voltage across the relay coil.

Plus, the sustained high voltage can potentially destroy your circuit.

For this reason, you’ll need to connect a diode in parallel with the inductive load.

You can use this to safeguard your course from voltages generated by the inductive load.

Transistor to Control Relay Circuit Diagram

Transistor to Drive Motors

Lastly, you can use a transistor to regulate and control the speed of a DC motor.

Plus, you can do this unidirectional by switching the transistor at frequent intervals.

Keep in mind that a DC motor is also an inductive load.

Therefore, you must pair it with a diode to protect your circuit.

Now, you can turn the DC motor on and off by switching the transistor from saturation to cutoff regions.

Additionally, you can change the transistor at variable frequencies to control the motor’s speed from low to full speed.

Transistors to Drive Motors Circuit Diagram

Applications

Indeed, the primary application of the transistor switch is controlling the flow of power from one part of the circuit to another.

Fundamentally, operating the transistor in saturation or cutoff regions will create the off/on effect of any mechanical switch.

Other applications of the transistor switch include:

• Digital logic gates

Digital Logic Gates

Source: Free SVG

• H-bridge circuits
• Oscillators

Oscillator

Source: Wikimedia Commons

• Interfacing high-voltage devices like motors, LEDs, and relays

Relay

Source: Wikimedia Commons

Final Words

In brief, transistors can serve as an electrical version of mechanical switches based on current rather than physical touch.

In truth, transistor switches can accomplish a wide variety of applications, even more than the few listed above.

Though it’s easy to use a transistor switch, ensure you use a flywheel diode when dealing with inductive loads—so you don’t damage your circuit.

If you want to make your transistor switch to a simple circuit and have some more questions, feel free to contact us, and we’ll be happy to help.