Using the Transistor as a Key

In this article we will see the use of the transistor as the key. When used as an AC signal amplifier, the basic bias voltage of transistors is applied so that it always works in the "active" zone. That is, linear part curves of output characteristics are used.

However, both NPN and PNP type bipolar transistors can be made to work as "on/off" type solid state switch by tilting the base terminal of transistors, unlike for a signal amplifier.

Solid state switches, replacing a DC output with "on" or "off", is one of the main applications that the transistor should use. Some output devices, such as LEDs, require only a few milliampers at logic-level DC voltages. Therefore, it can be driven directly by the output of a logic door. However, high-power devices such as motors, solenoids or lamps often require more power than provided by an ordinary logic gate, so transistor switches are used.

If the circuit uses the Bipolar transistor as a switch, the bias of the transistor, the NPN or PNP transistor, is arranged to work on both sides of the characteristic curves of " I-V " that we have seen before. The workspaces of a transistor switch are known as saturation zones and cutting zones.

Work Zones

Using the Transistor as a Key
Work Zones

The pink shaded area below the curves represents the "cutting" region, while the blue area on the left represents the "Saturation" region of the transistor. Both of these transistor regions are defined as follows:

  1. Cutting Zone

The operating conditions of the transistor here are zero input base current (IB), zero output collector current (IC) and maximum collector voltage (VCE). This does not cause a large depletion layer and current flowing from the device. Therefore, the transistor is changed to "completely closed".

2. Saturation Zone

Then, when using a bipolar transistor as a switch, both connections must be forward biased. VB> 0.7v and IC = We can define "saturation zone" or "ON mode" to a maximum. Emitter potential for a PNP transistor should be positive according to base.

The transistor then operates as a "unipolar single-shot" (SPST) solid state switch. When zero signal is applied to the base of the transistor, it "shuts down" and zero collector current flows, acting as an open switch. When a positive signal is applied to the base of the transistor, it "opens" acting as a closed switch and the maximum circuit current flows through the device.

The easiest way to switch from medium to high power is to use a transistor emitter terminal with an open collector output and a transistor emitter terminal directly connected to the soil. When used in this way, the open collector output of transistors can connect the externally supplied voltage to the soil, thereby controlling any connected load.

The following is an example of an NPN Transistor as a key used to operate a relay. Inductive loads such as relays or solenoids, when the transistor is "off", a flywheel diode is placed along the load to disperse the rear EMF produced by the inductive load, thereby protecting the transistor from damage. If the load is a very high current or voltage, such as engines, heaters, etc., the load current can be controlled with a suitable relay as shown.

Basic NPN Transistor Switching Circuit

Using the Transistor as a Key
Basic NPN Transistor Switching Circuit

The circuit is similar to the common emitter circuit that we looked at in the previous tutorials. The difference this time is that in order to operate the transistor as a switch, the transistor must be completely "off" (cut) or completely "open" (saturated).

In practice, when the transistor is "off", small leakage currents flow through the transistor and are completely "on", the device has a low resistance value, which leads to a small saturation voltage (VCE). Although the transistor is not an excellent switch, the power distributed by the transistor in both cutting and saturation zones is minimal.

For the base current to flow, the base inlet terminal must be made more positive than the emitter, rising above the 0.7 volt required for a silicone device. By changing this base-emittervoltage VBE, the base current is also replaced, which controls the amount of collector current flowing through the transistor, as discussed earlier.

When the maximum collector current flows, it is said that the transistor is saturated. The value of the base resistance determines how much input voltage is required to fully "turn on" the transistor and the corresponding base current.

Example of a Transistor Used as a Key

Using transistor values from previous tutorials: β = 200, Ic = 4mA and Ib = 20uA, find the value of the base resistance (Rb) required to fully "turn on" the load when the input terminal voltage exceeds 2.5 v.

Using the Transistor as a Key
Rb

The next lowest preferred value: 82kΩ guarantees that this transistor switch is always saturated.

Digital Logic Transistor Switch

Using the Transistor as a Key

Basic resistance is necessary to limit the output current through the RB logic gate.

PNP Transistor Switch

We can also use PNP Transistors as a key, this time the difference is that the load is connected to the soil (0v) and the PNP transistor passes power to it. To open the PNP transistor running as a switch as "on", the base terminal is connected to the ground or to zero volts (low), as shown.

PNP Transistor Switching Circuit

Using the Transistor as a Key
PNP Transistor Switching Circuit

The equations used to calculate base resistance, collector current and voltages are exactly the same as the previous NPN transistor switch. The difference this time is that instead of replacing the ground with an NPN transistor (sinking current), we replace power with a PNP transistor (welding current).

Darlington Transistor Switch

Sometimes the DC current gain of the bipolar transistor is too low to directly change the load current or voltage. Therefore, multiple switching transistors are used. Here a small input transistor is used to "turn on" or "turn off" a much larger current processing output transistor. To maximize signal gain, the two transistors are connected to a "complementary gain compound configuration." What is more commonly referred to as the "Darlington configuration" is the product of two separate transistors.

Darlington Transistors contain two separate bipolar NPN or PNP type transistors interconnected, multiplied only by the second transistor current gain to produce a device that acts as a single transistor with very high current gain for a much smaller base current gain. The overall current gain of a Darlington Device Beta (β) or hfe value is the product of two individual earnings of transistors and is given as follows:

Using the Transistor as a Key

Therefore, Darlington Transistors with very high β values and high collector currents are possible compared to a single transistor switch. For example, if the first input transistor has a current gain of 100 and the second switching transistor has a current gain of 50, the total current gain will be 100 * 50 = 5000. For example, if our load current from above is 200mA, the darlington base current is only 200mA / 5000 = 40ua. A greater reduction for a single transistor than the previous 1ma.

Darlington Transistor Configurations

Using the Transistor as a Key
Darlington Transistor Configurations