Checking radio elements Diodes transistors capacitors thermistors and optocouplers

Detailed testing of radio elements can be performed using both digital multimeters and analog (switch). Consider checking typical power supply elements.

Diodes

Testing of semiconductor diodes with a pointer device should be carried out by turning on the device for measuring resistance, starting from the lowest limit (set the switch to the xl position). In this measure the resistance of the diode in the forward and reverse directions. In the case of a healthy diode, the device will show a small resistance (several hundred ohms) for the forward bias of the diode, in the reverse direction - an infinitely large resistance (break). For a faulty diode, the forward and reverse directions differ little.

When checking with a digital multimetre, the device is transferred to the test mode (otherwise, in the mode of measuring the resistance in the forward and reverse directions, the diode will show a break). If the diode is in good condition, then on the digital display shows the voltage of the pn junction, in the forward direction for silicon diodes this voltage is 0.5 ... 0.8 V, for germanium 0.2 ... 0.4 V, in the opposite direction - gap.

Transistors

Given that the transistor has two pn junctions, when testing transistors, both transitions are tested, otherwise the verification is similar to that of diodes. Testing is conveniently carried out by measuring the resistance of the transitions relative to the base output, placing one of the electrodes of the device to the base of the measured transistor. For low-power transistors when measured with a pointer device, both transitions in the forward direction have fairly close values ​​(on the order of hundreds of ohms) and in the opposite direction - a discontinuity.

The collector-emitter transition, which must also have a discontinuity, is subject to additional verification. When testing powerful transistors, the resistance of the transitions in the forward direction can be several ohms. The digital device shows the voltage for the direct direction of transitions of 0.45 ... 0.9 V.

To determine the structure and outputs of an unknown transistor, it is desirable to use a pointer device. When determining the findings, you must first ensure that the transistor is in good condition. To this end, the base output is determined by approximately the same small base-emitter and base-collector resistances in the forward and larger - in the opposite direction.

The polarity of an instrument probe that biases the transitions in the forward direction will determine the structure of the transistor: if the instrument probe has a polarity of “-”, then the transistor has a pn-p structure, and if a “+” is, then np-p. To determine the emitter and collector pins of the transistor, the probes of the device are connected to the yet unknown pins of the transistor. Found the output of the base through a 1 kΩ resistor is alternately connected to each of the remaining pins. In this case, the collector-emitter transition resistance is measured alternately. The pin to which the resistor is connected, having the smallest resistance value of the junction, will determine the collector of the transistor, the remaining electrode will be an emitter.

Optocouplers

To test the optocouplers, the input part (light emitting) is energized from an external power source. In this case, the transition resistance is controlled, as a rule, the collector-emitter in the receiving part. A healthy optocoupler has a much lower collector-emitter junction resistance when the power is on (several hundred ohms) than when it is turned off. The constant resistance of the collector-emitter junction indicates a fault in the optocoupler.

Capacitors

Faulty capacitors may be detected during an external inspection of a faulty power supply. Attention should be paid to cracks in the housing, electrolyte leakage, corrosion at the terminals, heating of the capacitor body during operation. A good test can be a parallel connection to a previously known good capacitor. The absence of such information indicates the need to water a suspicious condenser. The device included in the resistance measurement mode is set to the upper limit. When testing, check the ability of the capacitor to the processes of charge and recharge. Testing is convenient to carry out the pointer device. In the process of charging, the needle of the device deviates to the zero mark, and then returns to the initial state (infinitely large resistance). The greater the capacitance of the capacitor, the longer the charging process. In the "leak" capacitor, the charge process continues with the discharge process, i.e. the subsequent process of reducing resistance. Digital multimeter when checking capacitors beeps. If there is no signal, the capacitor is defective.

Thermistors

In these resistors, the resistance varies considerably with temperature. Thermistors are tested at normal temperature and at elevated. Increased temperature can be achieved by heating the thermistor case, for example, using a soldering iron. Thermistors are usually used in power supplies, the resistance of which at normal temperature is unity Ohms, with a negative temperature coefficient of resistance, therefore the resistance of a functioning thermistor should decrease when heated.