Resistance is one of the most fundamental components in electronic circuits. Testing resistors is a basic skill that every electronics enthusiast or technician should master. The following sections describe various methods and practical experiences for testing different types of resistors.
**1. Fixed Resistor**
**Testing Methods:**
To measure the actual resistance value, simply connect the two test leads (without polarity) to both ends of the resistor. To ensure accuracy, choose an appropriate range on your multimeter based on the nominal resistance value of the resistor.
**Testing Experience:**
(1) Due to the nonlinear nature of the ohmmeter scale, the middle section is more accurate. Therefore, aim to have the pointer fall within the 20% to 80% range of the full scale. A tolerance of ±5%, ±10%, or ±20% may be acceptable depending on the resistor's precision. If the measured value exceeds this range, the resistor may be faulty.
(2) When measuring resistances above tens of kilohms, avoid touching the test leads or the resistor itself to prevent body resistance from affecting the reading. If the resistor is soldered into a circuit, at least one end should be desoldered to isolate it from other components and reduce measurement errors. Although color codes can indicate the resistance value, it's always best to use a multimeter for an actual measurement.
Cement resistors, which are typically fixed resistors, are tested using the same method as standard fixed resistors.
**2. Fuse Resistor**
**Testing Methods:**
(1) If the fuse resistor is open, you can often tell by visual inspection—if it’s blackened or burned, it likely had excessive current passing through it. If there are no visible signs, the current may have been just slightly over its rated value.
(2) To check for internal damage, remove one end of the resistor from the circuit and use the multimeter’s R×1 block. If the resistance reads infinite, the resistor is open. If the resistance is significantly different from the nominal value, it should not be reused.
**Testing Experience:**
In some cases, a blown resistor may appear intact but could be shorted or damaged internally. Always double-check with a multimeter.
**3. Potentiometer**
**Testing Methods:**
(1) First, turn the shaft to check if it moves smoothly and the switch operates properly. A crisp "click" sound indicates good contact. If you hear a "sand-like" noise, the potentiometer may be worn out.
(2) Use the multimeter’s resistance setting to measure between terminals 1 and 3. The reading should match the potentiometer’s nominal resistance. Then, check between terminals 1 and 2 while rotating the shaft. As you turn the shaft, the resistance should increase gradually. At the extreme position, the resistance should be close to the nominal value.
**Testing Experience:**
If the meter pointer jumps during rotation, it suggests poor contact between the wiper and the resistor track.
**4. Positive Temperature Coefficient Thermistor (PTC)**
**Testing Methods:**
Use the R×1 block on the multimeter. First, measure the resistance at room temperature (around 25°C). The value should be close to the nominal resistance (within ±2Ω). Next, apply heat using a soldering iron near the PTC. Its resistance should increase as the temperature rises. If it doesn’t, the thermistor is likely damaged.
**Testing Experience:**
Avoid placing the heat source too close to the PTC to prevent overheating and damage.
**5. Negative Temperature Coefficient Thermistor (NTC)**
**Testing Methods:**
(1) Measure the nominal resistance using the same method as a regular resistor. Select the appropriate range on the multimeter.
(2) To estimate the temperature coefficient, measure the resistance at room temperature, then again after heating the thermistor. Compare the values to determine if the NTC behaves correctly.
**Testing Experience:**
(1) NTCs are sensitive to temperature, so ensure the ambient temperature is around 25°C when measuring.
(2) Avoid applying too much power during testing, and do not touch the thermistor with your hands to prevent body heat from affecting the results.
**6. Varistor**
**Testing Methods:**
Use the R×1k block on the multimeter to measure the insulation resistance between the two terminals. It should read infinite. If it shows a low value, the varistor is leaking or damaged.
**Testing Experience:**
A low resistance reading means the varistor is no longer functional and should be replaced.
**7. Photoresistor**
**Testing Methods:**
(1) Cover the light-sensitive window with a black film. The resistance should be close to infinity.
(2) Shine a light on the window. The resistance should drop significantly.
(3) Use a piece of black paper to cover and uncover the light window intermittently. The multimeter pointer should swing back and forth. If it remains still, the photoresistor is likely damaged.
**Testing Experience:**
For method (1), a higher resistance is better. For method (2), a lower resistance indicates better performance. If the resistance is high or zero, the photoresistor is damaged and cannot be used.
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