Resistor detection method and experience in inverter repair - Database & Sql Blog Articles

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1. Testing Fixed Resistors. A. To measure the actual resistance value, connect the two test leads (without polarity) to both ends of the resistor. For better accuracy, choose the appropriate range based on the nominal resistance value. Due to the nonlinear nature of the ohm scale, the middle section is more precise. Therefore, aim for the pointer to be in the middle of the scale, ideally between 20% and 80% of the full scale, for a more accurate reading. Depending on the tolerance, a deviation of ±5%, ±10%, or ±20% may be acceptable. If the measured value exceeds these limits, the resistor may have changed its resistance value. B. Note: When testing resistances above tens of kΩ, avoid touching the test leads or the resistor itself. Ensure the resistor is removed from the circuit, at least one end should be desoldered, to prevent interference from other components that could affect the measurement. Although color bands can indicate resistance values, it's best to use a multimeter to confirm the actual value when in use.

2. Testing Cement Resistors. The process for testing cement resistors is identical to that of standard fixed resistors.

3. Testing Fuse Resistors. In a circuit, if a fuse resistor is blown, you can identify it by visual inspection. If the surface is black or burnt, it likely indicates an overload, with current exceeding the rated value. If there are no visible signs, it may have been triggered by a slightly higher current than the rated value. To test, use the R×1 setting on a multimeter. Remove one end from the circuit for accurate results. If the resistance is infinite, the resistor is open. If the measured value is far from the nominal value, it may not be suitable for reuse. During maintenance, some fuse resistors may appear intact but still be damaged, so careful testing is essential.

4. Testing Potentiometers. First, turn the knob to check for smooth rotation and proper switch operation. Listen for a clear "click" sound when turning on or off, and pay attention to any internal noise like "sand" sounds, which may indicate poor quality. When using a multimeter, select the appropriate range based on the potentiometer’s resistance. Then perform the following tests: A. Measure between terminals 1 and 2; the reading should match the nominal resistance. If the pointer doesn’t move or the value differs, the potentiometer may be damaged. B. Check the contact between the wiper and the resistor. Measure between terminals 1 and 2 (or 2 and 3), then slowly rotate the shaft. The resistance should increase smoothly. If the pointer jumps, it suggests poor contact.

5. Testing PTC Thermistors. Use the R × 1 setting on a multimeter. Perform two steps: A. At room temperature (~25°C), measure the resistance. It should be close to the nominal value, within ±2 Ω. If it’s significantly different, the thermistor may be faulty. B. Heat the thermistor with a soldering iron and monitor the resistance. It should increase with temperature. If no change occurs, the thermistor is damaged. Avoid direct contact with heat sources to prevent damage.

6. Testing NTC Thermistors. A. Measure the nominal resistance Rt using the same method as for fixed resistors. Ensure the ambient temperature is around 25°C for accurate results. Do not exceed the maximum power rating to avoid thermal effects. Avoid touching the thermistor during testing. B. Estimate the temperature coefficient αt by measuring resistance at two temperatures, t1 and t2, then calculate the change.

7. Testing Varistors. Use the R × 1k setting on a multimeter. The insulation resistance between the two pins should be infinite. If it’s low, the varistor is damaged and cannot be used.

8. Testing Photoresistors. A. Cover the light window with black paper. The resistance should be close to infinity. If it’s low, the photoresistor is damaged. B. Expose it to light. The resistance should drop significantly. If it remains high, the photoresistor is open. C. Move a piece of black paper over the light window. The multimeter pointer should swing as the light is intermittently blocked, indicating proper function.