How to Test NPN from PNP Transistors with a Digital Multimeter

A digital multimeter (or DMM) can be used to test and differentiate between PNP and NPN transistors.

It is important to understand that a bipolar transistor is a kind of transistor which is dependent on the contact of two different types of semiconductors for successful operation. Their operation involves both electrons and holes, these two types of charge carriers are typical of two kinds of doped semiconductor material, where doped means that impurities were purposefully introduced to modulate the electrical properties.

NPN happens to be one of two different kinds of bipolar transistors. The NPN bipolar transistor is composed of a stratum of one P-doped semiconductor, also known as the base. The P-doped is sandwiched between two N-doped layers. A small current which enters the base is augmented to create a considerably ample emitter and collector current. The transistor becomes live and active when a potential positive difference is measured the NPN transistor emitter to the base (in the case that the base is considered higher in relation to the emitter). The current flows freely between the emitter and collector of the transistor in the active state. In this P-type base region, the majority of the current is moved along by electrons which travel from the emitter (as minority carriers) to the collector.

PNP is the other kind of bipolar junction transistor. PNP varies from NPN in that it consists of a band of N-doped semiconductor sandwiched among two layers of P-doped material. The collector output amplifies the little current that leaves the base. In other words, a PNP transistor is considered to be active, or “on” when the base is low in relation to the emitter.

Both NPN and PNP transistors work in a similar way. The major difference is that the power supply polarities for different types are reversed. Another difference between NPN and PNP is that the frequency response of NPN transistor is higher than PNP (due to the flow of electrons having accelerated speed in comparison to hole flow).

The Digital multimeter (also known as DMM) are instruments used to measure ohms, volts, and amps. A digital multimeter can be used to test for failures like opens, leakages or shorts. To test for such issues the meter must be set to the diode test mode. The base of the transistor must be connected to the red (positive) meter lead, and the emitter must be connected to the black (negative) meter lead. An indication of a good NPN transistor is seen as a “junction drop” voltage that can range between .45v and .9v. PNP transistors in good standing will read as “Open”.

Relocate the black lead to the collector whilst leaving the red meter lead on the base. The readings should indicate the same numbers as the previous test. Repeat the test after reversing the meter leads in your hands, with the difference being that the black meter lead is connected to the base of the transistor. The red meter lead should not be connected to the emitter. In this situation, a PNP transistor in good standing will visualize a junction drop voltage of anywhere between .45v to .9v. NPN transistors will rate as open if they are considered good.

Leaving the black meter lead connected on the base, the red lead should be moved to the collector and the reading should reflect the findings in the previous test. Now one meter lead should be placed on the collector and the other meter lead on the emitter. In this situation the meter needs to read Open. Reverse the meter leads and the meter should once again read Open. This method of testing NPN and PNP works for both kinds of transistors. It is important to keep in mind that some transistors have diodes and resistors built in, which may cause confusion with these readings.

The reason that the diode test function mode is utilized is because digital multimeters have the ability to show infinite resistance for all 6 combinations of junction measurements due to the fact that their effective resistance test voltage remains less than a junction diode drop. A short circuit is indicated if your DMM reads 0 Ohms or a voltage drop of 0. It is a good idea to practice on a transistor which is working to have a constant to compare your readings to. Successful detection of PNP and NPN can be achieved using a digital multimeter as long as careful steps are taken to perform the required stages correctly.

Written by: Norman Everman took a screwdriver to the back of his boom box in the late 80s and hasn’t looked back since. Currently the co-owner of a TV and video production company, he once worked out that he had laid enough coaxial cable to stretch around the world.