What justifies testing the ohm range of household appliance plugs with a multimeter when some of them are connected and some are not?
If you are measuring electrical appliances such as kettles or electric kettles, or desk lamps that use the original incandescent lamp, when the switch is turned on, the resistance at both ends of the measuring plug should be about 100 ohms. When measuring the two ends of the plugs of TVs, refrigerators, washing machines, etc., the resistance values are different, because the plugs of each electrical appliance are different from their internal power circuits, and display different resistance values.
After personally experiencing the washing machine when the switch is turned on, the resistance measured at both ends of the plug is about 200 ohms for the motor in the washing machine.
Some household appliances such as LCD TVs and refrigerators have a resistance value close to infinity under normal use.
Based on the above, using a multimeter to measure the on-off of the electrical plug to judge whether the electrical appliance is good or bad is not suitable for all electrical appliances, and the multimeter is not a panacea. Only suitable for some electrical appliances, such as electric kettles, kettles, rice cookers, electric soldering irons, motor coils, incandescent light bulbs, etc.
Before measuring household appliances, you must understand the circuit diagram of the appliance, especially the power input circuit. Some use a transformer to step down and rectify the pointer will change when measuring with a multimeter, and some use a switch circuit or other circuits, and the pointer may not necessarily change. In addition, the circuit breaker multimeter will have no indication. Therefore, before measuring, compare the drawings or open the electrical appliances to check the circuit structure, so as to avoid misjudgment.
The power supply methods of various household appliances are different, and the power level is also different, so the measured resistance value is also different. If it is a switching power supply, it has a diode protection start inside, so the resistance cannot be measured.
Detection of fixed small capacitors
1. Detection of small capacitors below 10pF Because the capacity of fixed capacitors below 10pF is too small, measuring with a multimeter can only qualitatively check whether there is leakage, internal short circuit or breakdown. When measuring, you can choose the resistance file of the multimeter, and use two test leads to connect the two pins of the capacitor arbitrarily, and the resistance value should be infinite. If the measured resistance value (the pointer swings to the right) is zero, it means that the capacitor is damaged by leakage or internal breakdown.
2. Detect whether the 10PF～0.01μF fixed capacitor is charged, and then judge whether it is good or bad. The multimeter uses electric barriers. The β values of the two triodes are both above 100, and if the penetration current is more important, 3DG6 and other types of silicon triodes can be used to form a composite tube. The red and black test leads of the multimeter are respectively connected to the emitter e and collector c of the composite tube. Due to the amplification effect of the composite triode, the charging and discharging process of the measured capacitor is amplified, and the pendulum of the pointer of the multimeter is increased, which is convenient for observation. It should be noted that during the test operation, especially when measuring a capacitor with a small capacity, it is necessary to repeatedly switch the pins of the capacitor under test to touch points A and B, so that the swing of the pointer of the multimeter can be clearly seen.
3. For fixed capacitance above 0.01μF, use the R×10k block of the multimeter to directly test whether the capacitor has a charging process and whether there is an internal short circuit or leakage, and the capacity of the capacitor can be estimated according to the amplitude of the pointer swinging to the right.
