How to test a transistor with a multimeter

Experienced electricians and electronics engineers know that there are special probes for a complete check of transistors. With the help of these devices, you can not only check the health of the latter, but also its gain – h21e.

The need for a probe

A probe is really a necessary device, but if you just need to check the transistor for serviceability, a multimeter is also quite suitable.

transistor device

Before proceeding with the test, it is necessary to understand what a transistor is.

It has three terminals that form diodes (semiconductors) among themselves.

Each pin has its own name: collector, emitter and base. The first two conclusions pn transitions are connected in the base.

One pn junction between base and collector forms one diode, the second pn junction between base and emitter forms a second diode.

Both diodes are connected in a circuit opposite through the base, and this whole circuit is a transistor.

We are looking for a base, emitter and collector on a transistor

How to find a collector

To immediately find the collector, you need to find out what power the transistor is in front of you, and they are of medium power, low power and powerful.

Transistors of medium power and powerful are very hot, so heat must be removed from them.

This is done using a special cooling radiator, and heat is removed through the collector terminal, which in these types of transistors is located in the middle and is connected directly to the case.

It turns out such a heat transfer scheme: collector outlet – housing – cooling radiator.

If the collector is defined, then it will not be difficult to determine other conclusions.

There are cases that greatly simplify the search, this is when the device already has the necessary designations, as shown below.

We make the necessary measurements of direct and reverse resistance.

However, all the same, the protruding three legs in the transistor can lead many novice electronics engineers into a stupor.

How do you find the base, emitter and collector?

You can’t do without a multimeter or just an ohmmeter.

So let’s start searching. First we need to find the base.

We take the device and make the necessary measurements of the resistance on the legs of the transistor.

Take the positive probe and connect it to the right terminal. Alternately, the negative probe is brought to the middle, and then to the left conclusions.

Between the right and middle, for example, we showed 1 (infinity), and between the right and left 816 Ohm.

These testimonies do not give us anything yet. We take measurements further.

Now we move to the left, we bring the positive probe to the middle terminal, and with the negative probe we successively touch the left and right terminals.

Middle again – right shows infinity (1), and middle left 807 Ohm.

It also does not tell us anything. We measure further.

Now we move even more to the left, we bring the positive probe to the leftmost conclusion, and the negative probe sequentially to the right and middle.

If in both cases the resistance will show infinity (1), then this means that the base is the left terminal.

But where the emitter and collector (middle and right conclusions) will still need to be found.

Now you need to measure the direct resistance. To do this, now we do everything the other way around, the negative probe to the base (left terminal), and we alternately connect the positive probe to the right and middle terminals.

Remember one important point, the resistance of the pn base-emitter junction is always greater than the pn base-collector junction.

As a result of measurements, it was found that the resistance base (left terminal) – right terminal is equal to 816 Ohm, and base resistance – average output 807 Ohm.

So the right pin is the emitter and the middle pin is the collector.

So, the search for the base, emitter and collector is completed.

How to check the transistor for serviceability

To check the transistor with a multimeter for serviceability, it will be sufficient to measure the reverse and forward resistance of two semiconductors (diodes), which we will do now.

There are usually two junction structures in a transistor pnp и npn.

P-n-p – this is an emitter junction, you can determine this by the arrow that points to the base.

The arrow that goes from the base indicates that this is an npn transition.

The pnp junction can be opened with a negative voltage applied to the base.

We set the multimeter operation mode switch to the resistance measurement position at the mark “200».

The black negative wire is connected to the base terminal, and the red positive wire is connected in turn to the emitter and collector terminals.

Those. we check the emitter and collector junctions for operability.

Multimeter readings ranging from 0,5 to 1,2 kΩ they will tell you that the diodes are intact.

Now we swap the contacts, connect the positive wire to the base, and connect the negative wire in turn to the emitter and collector terminals.

The multimeter settings do not need to be changed.

The last reading should be much larger than the previous one. If everything is normal, then you will see the number “1” on the display of the device.

This indicates that the resistance is very large, the device cannot display data above 2000 ohms, and the diode junctions are intact.

The advantage of this method is that the transistor can be checked directly on the device without unsoldering it from there.

Although there are still transistors where low-resistance resistors are soldered into the pn junctions, the presence of which may not allow correct resistance measurements, it can be small, both at the emitter and collector junctions.

In this case, the conclusions will need to be soldered and measurements taken again.

Signs of a Transistor Fault

As noted above, if the measurements of direct resistance (black minus on the base, and plus alternately on the collector and emitter) and reverse (red plus on the base, and black minus alternately on the collector and emitter) do not correspond to the above indicators, then the transistor is out of order .

Another symptom of a malfunction is when the resistance of pn junctions in at least one measurement is equal to or close to zero.

This indicates that the diode is broken, and the transistor itself is out of order. Using the above recommendations, you can easily check the transistor with a multimeter for serviceability.