Transistor case styles.
Plate 1

Understanding Bipolar Transistor Switches

by Lewis Loflin

This will present a broad introduction of PNP and NPN switching transistors oriented towards common 5-volt micro-controllers. Bipolar transistors consist of two semiconductor junctions (thus the bipolar) that serve a broad number of electronic uses from audio amplifiers to digital circuits.

Here we are only interested their use as electronics switches to drive loads such as relays, lamps, motors, etc. They come in a number of packages and case styles.

Schematic symbols for NPN and PNP transistors.
Plate 2

In plate 2 above we have the electronic symbols for both NPN and NPN. They work exactly the same except have opposite electrical polarities. If a set of transistors have exact electrical properties but opposite polarities they are called a complimentary pair.

Another type of transistors are known as MOSFETs or metal-oxide semiconductor field-effect transistors, which will be covered separately.

Example NPN and PNP switches.
Plate 3

Plate 3 illustrates the typical electrical connections for both PNP and NPN bipolar transistors on a negative ground micro-controller system. Note current flow in these illustrations is from negative to positive.

Note the arrows dictating current flow - with the PNP the collector current (Ic) is from collector (C) to emitter (E) while the NPN Ic is emitter to collector.

Note the position of the transistors in relation to GND, the +12-volts, and the load in this case DC motors. Operating as a switch to turn the motor ON-OFF the PNP transistor is located in the +Vcc side of the load and will source the current.

With the NPN transistor on the right the switch is in the ground side of the load and is said to sink the current.

Sink and source are important to know when connecting programmable logic controllers (PLCs) used to control machinery in industry.

Push switch Sw1 current flows from GND through R1 and forward biases the base (B) with respect to the emitter. This combines with the collector current to produce the emitter current back to the 12-volt supply.

Push Sw2 this allows current from GND through the emitter that spits to form Ib and Ic for the NPN transistor. This too forward biases the base-emitter junction. The relationship for both is as follows:


Ie = Ic + Ib;
hfe = Ic / Ib.

The values hfe represents DC gain - a small base-emitter current creates a larger emitter-collector current.

When used as switches the transistors are used in the saturation mode where additional base-emitter current will produce no additional collector emitter current.

Checking a diode.
Plate 4

Plate four illustrates how to check a PN semiconductor junction. A diode is the most basic semiconductor junction where current will flow in one direction only. The digital volt meter (DVM) has to diode check function that supplies enough voltage to forward bias the diode when the cathode side is negative and the anode side is positive.

If the DVM leads are reversed no current flow takes place. The voltage drop across a forward biased PN junction is approximately 0.6V

NPN and PNP transistors as back-to-back diodes.
Plate 5

Plate 5 illustrates how the two PN junctions in bipolar transistors act as back-to back diodes. (Note the opposite polarities!) There's no way current can flow from emitter-collector or collector-emitter. When the base-emitter junction is forward biased current flows though the reversed biased base-collector junction.

TIP41 NPN transistor switch.
Plate 6

In plate 6 we are using a TIP41 NPN transistor rated at 6 amps with a minimum hfe of 20. Always assume the lowest hfe from the transistor spec sheets!

Our load (an LED) requires 100mA to find the need base-emitter current Ib = Ic / hfe = 0.1A / 20 = 5mA. I'll assume an Ib of 10mA to make sure the TIP41 fully cuts on (saturation).

With 5V input from an Arduino, PIC, etc. subtract 0.6V for the base-emitter voltage then divide 4.4V / 10mA = 440 ohms.

Note the emitter-collector voltage across a transistor at saturation is 0.5V.

2N3055 transistor switch must have pre-driver.
Plate 7

The 2N3055 is a very high power transistor designed to supply heavy current. In this case we are driving a 10 amp motor. Divide 10 amps by an hfe of 20 we need at least 500mA. There is no way this will work because Arduino, PIC, etc. I/O pin simply can't supply that level of drive current.

2N3055 transistor switch with 2N2222a pre-driver.
Plate 8

Illustrated in plate 8 we have a what is known as a Darlington configuration where the collect-emitter current of one transistor supplies the base-emitter current of a second transistor. The values of hfe from each transistor is multiplied together to produce a massive current gain in this example 2000.

Q2 would also be known as pre-driver.

TIP42 PNP transistor switch with 2N2222a pre-driver.
Plate 9

Plate 9 illustrates how to connect a PNP transistor to an Arduino or similar micro-controller. Because of the high voltage of 11-volts on the base of Q1 will destroy the I/O pin (limited to 5-volts) we must use a NPN transistor switch (Q2) as a pre-driver.

MJ2955, TIP42, and 2N2222 Darlington transistor switch.
Plate 10

In plate 10 we use a high-power Mj2955 (the compliment to the earlier 2N3055) with a TIP42 PNP transistor to form a Darlington transistor. Once again we use an NPN pre-driver to protect the micro-controller I/O pin from the high base voltage of Q2.

I hope the series was helpful. Any corrections, suggestions etc. e-mail me at lewis@bvu.net.

See the following spec sheets: