How can transistors be used as a switch

In active mode, two junctions are differently biased that means emitter-base junction is forward biased whereas collector-base junction is reverse biased. In this mode, current flows between emitter and collector and the amount of current flow is proportional to the base current. In this mode, both collector base junction and emitter base junction are reverse biased. As both the PN Junctions are reverse biased, there is almost no current flow except small leakage currents usually in the order of few nano amps or pico amps.

In this mode of operation, both the emitter-base and collector-base junctions are forward biased. Current flows freely from collector to emitter with almost zero resistance. In this mode, the transistor is fully switched ON and is essentially a close circuit. The below figure shows the output characteristics of a BJT. In the below figure, the cutoff region has the operating conditions when the output collector current is zero, zero base input current and maximum collector voltage.

Therefore, the transistor is completely in OFF condition. Similarly, in the saturation region, a transistor is biased in such a way that maximum base current is applied that results in maximum collector current and minimum collector-emitter voltage. This causes the depletion layer to become small and to allow maximum current flow through the transistor. Therefore, the transistor is fully in ON condition. This type of switching application is used for controlling LEDs, motors, lamps, solenoids, etc.

A transistor can be used for switching operation for opening or closing of a circuit. This type solid state switching offers significant reliability and lower cost when compared to conventional relays. Some of the applications use a power transistor as switching device, at that time it may necessary to use another signal level transistor to drive the high-power transistor.

Based on the voltage applied at the base terminal of a transistor switching operation is performed. Therefore, the transistor acts as a short circuit. Similarly, when no voltage or zero voltage is applied at the input, transistor operates in cutoff region and acts as an open circuit.

In a switching circuit, the idea is to simulate, as near as possible, the ideal switch having the properties of open circuit when off, short circuit when on, and an instantaneous transition between the two states.

Parameters are chosen such that the "off" output is limited to leakage currents too small to affect connected circuitry; the resistance of the transistor in the "on" state is too small to affect circuitry; and the transition between the two states is fast enough not to have a detrimental effect.

Cart 0. Back About Us Woman Owned. Whereas in practical scenarios, approximately percent of more current is necessary for proper transistor saturation. This means the base current which is necessary for the device is 1. In a few cases, the current gain of direct current in the BJT device is very minimal for direct switching of the load voltage or current.

Because of this, switching transistors are utilized. In this condition, a small transistor device is included for ON and OFF of a switch and an increased value of current for regulating the output transistor.

In this configuration, the amplification factor is the result of the product of two transistors. Darlington transistors are usually included with two bipolar PNP and NPN types of transistors where these are connected in the way that the gain value of the initial transistor is multiplied with the gain value of the second transistor device. This produces the result where the device functions as a single transistor having maximum current gain even for a minimal base current value.

The whole current gain of the Darlington switch device is the product of current gain values of both PNP and NPN transistors and this represented as:. For instance, when the input transistor has a current gain value of and the second has a gain value of 50, then the total current gain is.

The switch configuration of the Darlington transistor demonstrates that the collector terminals of the two devices are connected with the emitter terminal of the initial transistor which has a connection with the base edge of the second transistor device.

So, the current value at the emitter terminal of the first transistor will form as the input current of the second transistor thus makes it in On condition. The input transistor which is the first one gets its input signal at the base terminal. The input transistor gets amplified in a general way and this is used to drive the next output transistors. The second device enhances the signal and this results in a maximum value of current gain. One of the crucial features of the Darlington transistor is its maximum current gain when related to the single BJT device.

In addition to the ability of maximum voltage and current switching characteristics, the other added benefit is its maximum switching speeds. This switching operation allows the device to be specifically used for inverter circuits, DC motor, lighting circuits, and stepper motor regulation purposes. The variation to take into account while utilizing Darlington transistors than that of conventional single BJT types when implementing the transistor as a switch is that the input voltage at the base and emitter junction requires to be more which is nearly 1.

In a transistor, unless a current flows in the base circuit, there is no current can flow in the collector circuit. This property will allow a transistor to be used as a switch.

There are a few applications of switching circuits operated by transistors. Here, I considered NPN transistor to explain a few applications which are using transistor switch.

The circuit is designed by using a transistor as a switch, to light the bulb in a bright environment and to turn it off in the dark and a Light-Dependent Resistor LDR in the potential divider.

Then the transistor is switched OFF. When the LDR is exposed to the bright light, its resistance falls to less value resulting in more supply voltage and raising the base current of the transistor. Mountain Time:. One of the most fundamental applications of a transistor is using it to control the flow of power to another part of the circuit -- using it as an electric switch. Transistor switches are critical circuit-building blocks; they're used to make logic gates , which go on to create microcontrollers, microprocessors, and other integrated circuits.

Below are a few example circuits. Let's look at the most fundamental transistor-switch circuit: an NPN switch. Our control input flows into the base, the output is tied to the collector, and the emitter is kept at a fixed voltage.

While a normal switch would require an actuator to be physically flipped, this switch is controlled by the voltage at the base pin. When the voltage at the base is greater than 0. When the voltage at the base is less than 0. The circuit above is called a low-side switch , because the switch -- our transistor -- is on the low ground side of the circuit. Alternatively, we can use a PNP transistor to create a high-side switch:. Similar to the NPN circuit, the base is our input, and the emitter is tied to a constant voltage.

This time however, the emitter is tied high, and the load is connected to the transistor on the ground side. This circuit works just as well as the NPN-based switch, but there's one huge difference: to turn the load "on", the base must be low. This can cause complications, especially if the load's high voltage V CC being 12V connecting to the emitter V E in this picture is higher than our control input's high voltage.

For example, this circuit wouldn't work if you were trying to use a 5V-operating Arduino to switch off a 12V motor. In that case, it'd be impossible to turn the switch off because V B connecting to the control pin would always be less than V E. You'll notice that each of those circuits uses a series resistor between the control input and the base of the transistor.

Don't forget to add this resistor!