Detection Method of Common Components

I. Testing methods and experience of resistors:

1. Detection of fixed resistors.

A The actual resistance can be measured by connecting the two pens (positive and negative) with the pins at the two ends of the resistance respectively. In order to improve the measurement accuracy, the measurement range should be selected according to the nominal value of the measured resistance. Because of the nonlinearity of the ohmic block scale, the middle part of the scale is more precise. Therefore, the pointer indication value should fall as far as possible to the middle part of the scale, i.e. within the range of 20%-80% radian from the beginning of the full scale, in order to make the measurement more accurate. It varies according to the error level of resistance. The errors between the reading and the nominal resistance are allowed to be (+5%), (+10%) or (+20%) respectively. If not, beyond the error range, it means that the resistance value has changed.

B Note: When testing, especially when measuring resistance above tens of k_, don't touch the conductive part of the pen and resistance; when the tested resistance is welded down from the circuit, at least one head should be welded in order to avoid other components in the circuit affecting the test and causing measurement errors; although the resistance value of the color ring resistance can be determined by the color ring mark, it is better to use it. Use a multimeter to test its actual resistance.

Detection of cement resistance. The method and precautions for testing cement resistance are exactly the same as that for testing ordinary fixed resistance.

3. Detection of fuse resistor. In the circuit, when the fuse resistor breaks the circuit, it can be judged by experience: if the surface of the fuse resistor is found to be blackened or burnt, it can be concluded that the fuse resistor is overloaded and the current through it exceeds many times the rated value.

If the circuit is opened without any trace on its surface, it indicates that the current flowing through it is just equal to or slightly larger than its rated fuse value. For judging the quality of the fuse resistor without any trace on the surface, it can be measured by the multimeter R * 1 block. In order to ensure the accuracy of the measurement, one end of the fuse resistor should be welded off the circuit. If the measured resistance value is infinite, it indicates that the fuse resistor has failed to open the circuit. If the measured resistance value is far from the nominal value, it indicates that the resistance variable value should not be used again. In the maintenance practice, it is found that a few fuse resistors are short-circuit breakdown in the circuit, so attention should be paid to the detection.

4 potentiometer detection. When checking potentiometer, we should first rotate the handle to see if it rotates smoothly, whether the switch is flexible, whether the "click" sound is clear and crisp when the switch is on or off, and listen to the sound of contact points and resistance friction inside the potentiometer, such as "sand" sound, which indicates that the quality is not good. When testing with a multimeter, the appropriate resistance of the multimeter is selected according to the resistance value of the potentiometer under test, and then the multimeter can be tested by the following method.

The Ohm block of A multimeter is used to measure the "1" and "2" ends. Its reading should be the nominal resistance value of the potentiometer. If the pointer of the multimeter is fixed or the resistance value differs greatly, it indicates that the potentiometer has been damaged.

B detects whether the movable arm of the potentiometer is in good contact with the resistor. Measure the ends of "1", "2" (or "2", "3") with the ohmmeter. Rotate the axis of the potentiometer counterclockwise to the position close to "off". The smaller the resistance, the better.

Then slowly rotate the shank clockwise, the resistance value should gradually increase, and the pointer in the head should move smoothly. When the shank rotates to the extreme position "3", the resistance value should be close to the nominal value of the potentiometer. If the pointer of the multimeter runs out during the rotation of the shaft handle of the potentiometer, it indicates that the movable contacts have bad contact.

Detection of Positive Temperature Coefficient Thermistor (PTC). When testing, the multimeter R * 1 gear is used, which can be operated in two steps.

A normal temperature test (indoor temperature is close to 25 C); the actual resistance value of the two pins contacting PTC thermistor of two meter pens is measured and compared with the nominal resistance value. The difference between the two pins is normal within (+2_). If the difference between the actual resistance and the nominal resistance is too large, it indicates that the performance is poor or damaged.

B heating detection; on the basis of normal room temperature test, the second step test-heating detection can be carried out. A heat source (e.g. soldering iron) is heated near PTC thermistor. At the same time, the multimeter is used to monitor whether its resistance value increases with the increase of temperature. If so, it shows that the thermistor is normal, and if the resistance value does not change, it shows that its performance is deteriorating and can not continue to use. Be careful not to make the heat source close to the PTC thermistor or directly contact the thermistor to prevent it from scalding.

Detection of negative temperature coefficient thermistor (NTC).

(1) The method of measuring nominal resistance value Rt by multimeter is the same as that of measuring ordinary fixed resistance. That is to say, selecting appropriate resistance according to nominal resistance value of NTC thermistor can directly measure the actual value of Rt. However, because NTC thermistor is sensitive to temperature, the following points should be paid attention to when testing: ART is measured by the manufacturer when the ambient temperature is 25 C, so when measuring Rt with multimeter, it should also be carried out when the ambient temperature is close to 25 C to ensure the reliability of the test. The measurement power of B must not exceed the specified value in order to avoid the measurement error caused by the thermal effect of current. C. Pay attention to correct operation.

When testing, do not pinch the thermistor body with your hand to prevent the human body temperature from affecting the test.

(2) Estimate the temperature coefficient alpha t by measuring the resistance value Rt1 at room temperature t1, then using electric soldering iron as heat source, near the thermistor Rt, measuring the resistance value RT2, and then calculating the average temperature T2 of the thermistor RT surface at this time with a thermometer.

7. Detection of varistors. Measuring between two pins of varistor with R * 1K block of multimeter