One multimeter to detect all components

A digital multimeter is a relatively simple measuring instrument and an essential tool for electronic engineers. This article will teach you how to use a digital multimeter to detect whether components are normal. A digital multimeter can be used to detect the characteristics of components such as resistors, capacitors, current, diodes, transistors, MOS field effect transistors, etc.

Digital multimeter function introduction:

1. Measure the resistance value

a. First, adjust the multimeter to the Ohm range (Ohm is the unit of resistance) and select a suitable range (usually 10K or 20K).

b. Place the red and black test leads of the multimeter on both ends of the resistor (resistance does not matter whether it is positive or negative), and then observe the reading of the multimeter. If there is no reading, it may be because the range is too small. Select a large range and measure again.

2. Detection of the quality of photoresistors

When testing, set the multimeter to the R×1kΩ position, and keep the light-receiving surface of the photoresistor perpendicular to the incident light. The resistance directly measured on the multimeter is the light resistance. Then place the photoresistor in a completely dark place, and the resistance measured by the multimeter is the dark resistance. If the light resistance is several thousand ohms to tens of thousand ohms, and the dark resistance is several to tens of megohms, it means that the photoresistor is good.

3. Measure the capacitance

a. First adjust the multimeter to the capacitance position. Generally, there is only one range for measuring capacitance.

b. Place the red and black test leads of the multimeter on both ends of the capacitor, and then observe the readings of the multimeter. Note that some capacitors have positive and negative poles (for example, electrolytic capacitors, which usually have the long leg as positive and the short leg as negative), so when measuring capacitors with positive and negative poles, the red test lead should be connected to the positive pole and the black test lead to the negative pole.

4. Determine the quality of the crystal oscillator

First use a multimeter (R×10k block) to measure the resistance value at both ends of the crystal oscillator. If it is infinite, it means that the crystal oscillator has no short circuit or leakage. Then insert the test pen into the AC power jack, pinch any pin of the crystal oscillator with your fingers, and touch the other pin to the metal part of the top of the test pen. If the neon bulb of the test pen turns red, it means that the crystal oscillator is good; if the neon bulb is not bright, it means that the crystal oscillator is damaged.

5. Measure the polarity of each leg of the rectifier bridge

Set the multimeter to R×1k, connect the black probe to any pin of the bridge stack, and use the red probe to measure the other three pins in turn. If the readings are all infinite, the black probe is connected to the positive output of the bridge stack. If the readings are 4~10kΩ, the pin connected to the black probe is the negative output of the bridge stack, and the other two pins are the AC input terminals of the bridge stack.

6. Detect line breakpoints

First adjust the multimeter to the AC 2V position.

7. Unidirectional thyristor detection

The forward and reverse resistance between any two poles can be measured with the R×1k or R×100 range of the multimeter. If a pair of poles is found to have a low resistance value (100Ω～1kΩ), the black test lead is connected to the control pole, the red test lead is connected to the cathode, and the other pole is the anode. Thyristors have a total of three PN junctions. We can determine whether it is good or bad by measuring the size of the forward and reverse resistance of the PN junction. When measuring the resistance between the control pole (G) and the cathode [C), if the forward and reverse resistances are both zero or infinite, it indicates that the control pole is short-circuited or open-circuited; when measuring the resistance between the control pole (G) and the anode (A), the forward and reverse resistance readings should be very large;

When measuring the resistance between the anode (A) and the cathode (C), both the forward and reverse resistances should be large.

8. Polarity identification of bidirectional thyristor

The bidirectional thyristor has main electrode 1, main electrode 2 and control electrode. If the resistance between the two main electrodes is measured with a multimeter R×1k, the reading should be close to infinity, while the forward and reverse resistance readings between the control electrode and any main electrode are only tens of ohms. Based on this characteristic, we can easily identify the control electrode of the bidirectional thyristor by measuring the resistance between the electrodes. When the black test lead is connected to the main electrode 1 and the red test lead is connected to the control electrode, the forward resistance measured is always smaller than the reverse resistance. Based on this, we can easily identify the main electrode 1 and the main electrode 2 by measuring the resistance.

9. Identification of transistor electrodes

If you want to distinguish the three electrodes of a transistor with unclear or no model number, you can also use a multimeter to test it. First, turn the multimeter range switch to the R×100 or R×1k resistance block. The red test lead touches any electrode of the transistor, and the black test lead touches the other two electrodes in turn, and measure the resistance between them respectively. If the measured resistance is a few hundred ohms, the electrode touched by the red test lead is the base b, and this tube is a PNP tube. If the measured resistance is a few tens to hundreds of kiloohms, the electrode touched by the red test lead is also the base b, and this tube is an NPN tube.

On the basis of identifying the tube type and base b, the collector is determined by using the principle that the forward current amplification factor of the transistor is greater than the reverse current amplification factor. Arbitrarily assume that one electrode is the c pole and the other electrode is the e pole. Set the multimeter range switch to the R×1k resistance block. For: PNP tube, connect the red test lead to the c pole and the black test lead to the e pole, then pinch the b and c poles of the tube at the same time with your hand, but do not let the b and c poles touch directly, and measure a certain resistance value. Then swap the two test leads for a second measurement, and compare the resistances measured twice. For: PNP tube, the electrode connected to the red test lead is the collector when the resistance value is small. For the NPN tube with a small resistance value, the electrode connected to the black test lead is the collector.

10. Measure the leakage resistance of large-capacity capacitors

Use a 500-type multimeter and set it to the R×10 or R×100 range. When the pointer points to the maximum value, immediately switch to the R×1k range for measurement. The pointer will stabilize in a relatively short time, and the leakage resistance value can be read out.

11. Check the quality of the LED

First set the multimeter to R×10k or R×l00k, then connect the red test lead to the "ground" terminal of the digital tube (taking the common cathode digital tube as an example), and connect the black test lead to the other terminals of the digital tube in turn. All seven segments should light up separately, otherwise it means that the digital tube is damaged.

12. Identify the electrodes of junction field effect transistors

Set the multimeter to the R×1k position, touch the pin assumed to be the gate G with the black test lead, and then touch the other two pins with the red test lead. If the resistance values ​​are relatively small (5-10 Ω), swap the red and black test leads and measure again. If the resistance values ​​are both large (∞), it means that they are all reverse resistances (PN junction reverse), which belongs to N-channel tubes, and the pin touched by the black test lead is the gate G, and it means that the original assumption is correct. If the resistance values ​​measured again are all very small, it means that it is a forward resistance, which belongs to a P-channel field effect tube, and the black test lead is also connected to the gate G. If the above situation does not occur, you can swap the red and black test leads and test according to the above method until the gate is determined. Generally, the source and drain of the junction field effect tube are symmetrical during manufacturing, so when the gate G is determined, it is not necessary to distinguish the source S and drain D, because these two poles can be used interchangeably. The resistance between the source and the drain is several thousand ohms.

13. Determine the polarity of unsigned electrolytic capacitors

First, short-circuit the capacitor to discharge it, then mark the two leads as A and B, set the multimeter to R×100 or R×1k, connect the black test lead to lead A and the red test lead to lead B, and read the value after the pointer stops moving. After the measurement, short-circuit and discharge it; then connect the black test lead to lead B and the red test lead to lead A, compare the two readings, the one with a larger resistance value is connected to the positive electrode, and the red test lead is connected to the negative electrode.

14. How to judge whether a potentiometer is good or bad

First measure the nominal resistance of the potentiometer. Use the ohmmeter of the multimeter to measure the two ends of "1" and "3" (assuming that the "2" end is the active contact). The reading should be the nominal value of the potentiometer. If the pointer of the multimeter does not move, the resistance value does not move, or the resistance value is very different, it indicates that the potentiometer is damaged. Then check whether the active arm of the potentiometer is in good contact with the resistor. Use the ohmmeter of the multimeter to measure the two ends of "1", "2" or "2", "3", and rotate the shaft of the potentiometer counterclockwise to a position close to "off". At this time, the resistance should be as small as possible. Then slowly rotate the shaft handle clockwise, and the resistance should gradually increase. When it is rotated to the extreme position, the resistance should be close to the nominal value of the potentiometer. If the pointer of the multimeter jumps during the rotation of the shaft handle of the potentiometer, it means that the active contact is in poor contact.