1. Testing Fixed Resistors: To measure the actual resistance value of a fixed resistor, simply connect the two test leads (without considering polarity) to both ends of the resistor. For better accuracy, choose the appropriate range based on the nominal resistance value. The ohmic scale is nonlinear, so the pointer should ideally fall in the middle of the scale—between 20% and 80% of the full scale—for more precise readings. Depending on the tolerance, a deviation of ±5%, ±10%, or ±20% from the nominal value is acceptable. If the measured value exceeds this range, the resistor may be faulty. When testing resistors with values over tens of kΩ, avoid touching the test leads or the resistor itself to prevent body resistance from affecting the reading. Also, remove at least one end of the resistor from the circuit to avoid interference from other components. Even though color bands can indicate resistance, it's always safer to use a multimeter for an accurate measurement.
2. Testing Cement Resistors: The procedure for testing cement resistors is identical to that of standard fixed resistors. No special tools or techniques are required.
3. Testing Fuse Resistors: In a circuit, if a fuse resistor has burned out, you can often tell by its appearance. A blackened or burnt surface indicates excessive current flow, while a clean surface suggests the current was just above or slightly higher than the rated value. To check a fuse resistor without visible damage, use the R×1 setting on a multimeter after removing one end from the circuit. If the resistance reads as infinite, the resistor is open. If the resistance is significantly different from the nominal value, it’s not suitable for reuse. Some fuse resistors may break internally, so careful inspection is necessary during testing.
4. Testing Potentiometers: Begin by rotating the shaft to ensure smooth movement and check for any "click" sounds when switching. Listen for any unusual friction or noise inside the potentiometer, which may indicate poor quality. When using a multimeter, set it to the correct resistance range. Measure between terminals 1 and 2; the reading should match the nominal resistance. If the pointer doesn’t move or shows a different value, the potentiometer is damaged. Check the contact between the wiper and the resistor by turning the shaft and observing the resistance change smoothly. Any jumping or irregularity indicates a faulty connection.
5. Testing PTC Thermistors: Use the R×1 setting on a multimeter. First, test at room temperature (around 25°C). The measured resistance should be close to the nominal value, within ±2 Ω. If it’s significantly off, the thermistor may be faulty. Next, apply heat using a soldering iron near the thermistor and monitor the resistance. It should increase with temperature. If there’s no change, the thermistor is damaged. Be cautious not to touch the thermistor directly with the heat source to avoid overheating.
6. Testing NTC Thermistors: Measuring NTCs is similar to measuring regular resistors. Choose the right range and measure the resistance. However, note that NTCs are temperature-sensitive. Ensure the ambient temperature is around 25°C for accurate results. Avoid applying too much power to prevent self-heating effects. Do not hold the thermistor with your hands during testing, as body heat can affect the reading. To estimate the temperature coefficient, measure the resistance at two different temperatures and calculate the change.
7. Testing Varistors: Use the R×1k setting on a multimeter to check the insulation resistance between the two pins. It should read infinity. If the resistance is low, the varistor is damaged and should not be used.
8. Testing Photoresistors: Cover the light-sensitive window with a piece of paper. The resistance should be close to infinity. If it’s low, the photoresistor is damaged. Expose it to a light source, and the resistance should drop significantly. If it doesn’t change, the photoresistor is faulty. Shake a small piece of paper over the window to simulate intermittent light. The multimeter pointer should swing accordingly, indicating proper function.
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