Technology to verify the operation of the transistor

The transistor is the most popular active component that is part of the electrical circuits. Anyone who is interested in electronics, from time to time there is a need to check such an element. Especially often it is necessary to make checks for beginning amateurs, who use transistors used in their circuits, for example, which are removed from old boards. For "drowsing" you can use special test equipment, which allows you to measure the parameters of transistors, so that they can then be compared with those specified in the directory. However, for elements that are part of the amateur scheme, it is sufficient to perform a check according to the rule: "is defective, defective".This article tells how to test a transistor with a multimeter just by this method of testing.

Contents:

Preparation of tools
Testing technology
Bipolar
Field
Composite

Preparation of tools

Every modern radio amateur has a universal tool called a digital multimeter. It allows you to measure the constant and alternating currents and voltages, the resistance of the elements. It also allows you to test the operation of the circuit elements. Near the switch to the "continuity" mode, as a rule, a diode and a speaker are drawn( see the photo in Figure 1).

Figure 1 - Front panel of the

multimeter Before checking the element, you must ensure that the meter itself is working:

The battery must be charged.
When switching to the semiconductor check mode, the display should show the number 1.
Probes should be in good order, since most devices are Chinese, and breaking the wire in them is very frequent. You need to check them by leaning the ends of the probes to each other: in this case, the display will show zeros and the squeak will sound - the device and the probes are working.
The probes are connected according to the color marking: the red probe is in the red connector, the black probe is connected to the black connector with the inscription COM.

If you do not know how to use this unit, we recommend reading the detailed instructions for dummies on how to use the multimeter!

Testing technology

Bipolar

The bipolar transistor( BT) structure includes 2 p-n or 2 n-p junctions. The conclusions of these transitions are called the emitter and collector. The derivation of the middle layer is called the base. Simplified, the WT can be represented as two on-off diode as shown in Figure 2.

Figure 2 - NPN model and its diode "analog"

It is not difficult to verify the bipolar transistor with a multimeter, as you will now see. As is well known, the main property of the p-n junction is its one-sided conductivity. If a positive( red) probe is connected to the anode, and black to the cathode, the value of the forward voltage at the millivolt transition will be displayed on the multimeter display. The voltage value depends on the type of semiconductor: for germanium diodes this voltage will be of the order of 200-300 mV, and for silicon ones from 600 to 800 mV.In the opposite direction, the diode does not pass current, so if you change the probes in places, then the display will show 1, indicating an infinitely large resistance.

If the diode is "punched", then most likely there will be a sound signal, and in both directions. If the diode is "in the cliff", then on the indicator, a unit will be displayed.

Thus, the essence of checking the serviceability of the transistor is to "ping" the p-n jumps of the base-collector, emitter-base and emitter-collector in direct and reverse switching:

Collector base: The red probe connects to the base, black to the collector. The connection should function as a diode and conduct current in only one direction.
Emitter base: The red probe stays connected to the base, the black plug is connected to the emitter. Similar to the previous point, the connection should only conduct current when switched on directly.
Emitter-collector: For a good transition, the resistance of this section tends to infinity, which will be indicated by a unit on the indicator.

When checking the operation of the pnp type "diode" analog will look the same, but the diodes will be connected in reverse. In this case, the black probe is connected to the base. The emitter-collector transition is verified similarly.

Field

Field-effect transistors( PTs) or "fielders" are used in power supplies, monitors, audio and video equipment. Therefore, with the need for verification, the hardware repair technicians are more often confronted. Self-check such an element at home can also be using a conventional multimeter.

Figure 3 shows the structural diagram of the FET.The Gate, Drain, Source can be located in different ways. Very often, manufacturers label them with letters. If there is no marking, then it is necessary to consult the reference data, having first learned the name of the model.

Figure 3 - Structural diagram of

It should be borne in mind that in the repair of equipment in which PTs are installed, the problem often is to check the operability and integrity without evacuation of the element from the board. Most often out of order are powerful field-effect transistors, installed in impulse power supplies. Also it should be remembered that "fieldworkers" are extremely sensitive to static discharges. Therefore, before testing the FET without evaporating, it is necessary to wear an antistatic wrist strap and observe safety precautions.

Figure 4 - Antistatic bracelet

You can check the PT with a multimeter by analogy with the bipolar transistor bumps. A good "field" between the terminals has an infinitely large resistance, regardless of the applied test voltage. However, there are some exceptions: if you attach a positive tester probe to the gate, and a negative one to the source, then the gate capacitance will be charged, and the transition will open. When measuring the resistance between the drain and the source, the multimeter can show some resistance value. Inexperienced masters often take such a phenomenon as a symptom of a malfunction. However, this does not always correspond to reality. Before checking the drain-source channel, it is necessary to short-circuit all the terminals of the FET so that the capacity of the transitions is discharged. After that, their resistance will again become large, and you can re-check whether the transistor is working or not. If such a procedure does not help, then the element is considered non-working.

"Poloviki" standing in powerful switching power supplies often have an internal diode on the drain-source transition. Therefore, this channel behaves as a normal semiconductor diode during the test. To avoid a false error, it is necessary to check the presence of an internal diode before checking the transistor with a multimeter. It is necessary to swap the test probes. In this case, the unit should be displayed on the screen, which indicates an infinite resistance. If this does not happen, then, most likely, the PT is "punched".

Composite

A typical composite transistor or Darlington circuit is shown in Figure 5. These 2 elements are located in the same housing. Inside is also a pull-up resistor. Such a model has similar conclusions as for a bipolar one. It's easy to guess that you can check the composite transistor with a multimeter in exactly the same way as BT.It should be noted that some types of digital multimeters in test mode have a voltage of less than 1.2 V on the terminals, which is not enough to open the pn junction, in which case the device shows a break in the circuit.

Figure 5 - Darlington Diagram

If after reading the article you still do not fully understand how to test the transistor with a multimeter, the video lesson below will visually see the verification technology:

How to perform the check correctly?

Thus, the verification task of this circuit element is reduced to a serial "ringing" of p-n junctions, and if they are in good order, then the device can be considered as working. We hope that now you know how to test a transistor with a multimeter at home!

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