How to choose the right PNP or NPN transistor switch

“The two standard transistors are PNP and NPN, and their circuit symbols are different. The layers of semiconductor material used to make transistors are indicated by letters. In order to maintain the linear transmitter symbol, the arrow in the schematic diagram is always the transmitter. The emitter of PNP “emits” electrons, and the emitter of NPN “emits” holes. The difference between PNP and NPN transistors is the direction of the arrow on the emitter. Regardless of whether the P cross section is the emitter or the base, the arrow always points to the direction of hole flow.

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Author: Andrew Carter

The two standard transistors are PNP and NPN, and their circuit symbols are different. The layers of semiconductor material used to make transistors are indicated by letters. In order to maintain the linear transmitter symbol, the arrow in the schematic diagram is always the transmitter. The emitter of PNP “emits” electrons, and the emitter of NPN “emits” holes. The difference between PNP and NPN transistors is the direction of the arrow on the emitter. Regardless of whether the P cross section is the emitter or the base, the arrow always points to the direction of hole flow.

NPN and PNP BJT symbols

The process of selecting a suitable PNP transistor is exactly the same as the process of selecting an NPN transistor. When choosing the appropriate switching transistor:

• The maximum collector current of the triode must be greater than the load current

• The maximum current gain of the transistor must be at least 5 times the load current divided by the maximum output current of the IC

• Select the transistor that meets the requirements and write down its characteristics

• Calculate the approximate value of the base resistance

• If the load is a motor or a relay coil, a protective diode is required to protect the transistor from the short-term high voltage when the load is turned off by connecting the load.

PNP transistor

PNP varieties are commonly used transistor switches at cost, as shown in the figure below. Saturating the transistor is the secret to making the transistor switch work properly. To ensure that our transistor switches are always in saturation, it is best to calculate a current that is 30% more than the actual need. To ensure that the transistor switch is completely turned off, R2 is used in the circuit below. The resistor ensures that the base of the transistor does not become slightly negative, causing a very small amount of collector current to flow.

Typical PNP transistor application

NPN transistor

If a positive grounding configuration is required, NPN transistors can also be used. The basic NPN transistor switching circuit is similar to the common emitter circuit, except that the transistor is completely turned off or completely turned on. The base input terminal must be more positive than the emitter so that the base current flows.

A typical NPN transistor application

A variety of applications are using transistor switches. Some are connected to high-current or high-voltage equipment, such as motors, relays or lights. The actual transistor used as the switch is not important, as practically any general-purpose NPN or PNP transistor can be used. It only needs to know the minimum HFE and transistor power consumption. Using transistor switches is generally more reliable and cheaper than using mechanical relays. It is a good practice to always use a diode when turning on any inductive load. If you use a power transistor to turn on high-current devices, you may need to use another lower-current transistor switch.