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. This article outlines various methods and practical tips for testing common 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 based on the nominal resistance value of the resistor.
**Testing Experience:**
(1) Due to the nonlinear scale of the ohmmeter, it's best to aim for the middle portion of the scale, ideally between 20% and 80% of the full scale, for more accurate readings. A deviation of ±5%, ±10%, or ±20% may be acceptable depending on the tolerance of the resistor. If the measured value falls outside this range, the resistor may be faulty.
(2) When measuring high-value resistors (tens of kilohms), avoid touching the test leads or the resistor itself to prevent body resistance from affecting the reading. It’s also important to desolder at least one end of the resistor from the circuit to avoid interference from other components. Even though color codes can help identify resistor values, it's always better to use a multimeter for an accurate 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 its appearance—blackened or burnt surface indicates excessive current. If there are no visible signs, it might have failed due to a current slightly above its rated value.
(2) Use the R×1 block on a multimeter to test. Ensure one end is disconnected from the circuit. If the resistance reads infinity, the resistor is open. If the resistance is significantly different from the nominal value, it may not be suitable for reuse.
**Testing Experience:**
In some cases, a blown resistor may appear intact but still be damaged internally.
**3. Potentiometer**
**Testing Methods:**
(1) First, turn the shaft to check if it moves smoothly and if the switch feels responsive. A crisp "click" sound when turning on/off is a good sign. If you hear a "sand" noise, it may indicate poor quality or internal wear.
(2) Set the multimeter to the appropriate resistance range. Measure between terminals 1 and 3—the reading should match the potentiometer’s nominal resistance. Then, move the wiper between terminals 1 and 2, and then 2 and 3. The resistance should change smoothly as the shaft rotates.
**Testing Experience:**
If the meter pointer jumps during rotation, it may indicate 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 (~25°C). The value should be close to the nominal value, within ±2Ω. Next, apply heat (e.g., with a soldering iron) and observe if the resistance increases. If it doesn’t, the thermistor may be faulty.
**Testing Experience:**
Avoid placing the heat source too close to the PTC to prevent damage.
**5. Negative Temperature Coefficient Thermistor (NTC)**
**Testing Methods:**
(1) Measure the nominal resistance using the same method as for a fixed resistor.
(2) Estimate the temperature coefficient by measuring resistance at two different temperatures and comparing the results.
**Testing Experience:**
(1) NTCs are sensitive to temperature, so measurements should be taken at around 25°C for consistency.
(2) Avoid applying too much power during testing, and don't touch the component directly with your hands.
**6. Varistor**
**Testing Methods:**
Use the R×1k block on the multimeter to measure the insulation resistance between the two pins. It should read infinity. If it shows a low resistance, the varistor is likely damaged.
**Testing Experience:**
A low or zero reading indicates leakage or damage, making the varistor unsuitable for use.
**7. Photoresistor**
**Testing Methods:**
(1) Cover the light-sensitive window with a black film. The multimeter should show a high resistance, close to infinity.
(2) Expose the window to a light source. The resistance should drop significantly, and the needle should swing.
(3) Move a small piece of black paper over the window to simulate intermittent light. The needle should swing back and forth. If it stays still, the photoresistor may be damaged.
**Testing Experience:**
For method (1), a higher resistance means better performance. For method (2), a lower resistance is better. If the resistance is too high or too low, the photoresistor may be damaged.
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