What Is a Varistor and Its Working Principle?
A varistor is a type of electronic component used to protect circuits against excessive transient voltages by acting as a voltage-dependent resistor. Its resistance changes significantly with the applied voltage. Varistors are commonly made from materials like zinc oxide (ZnO) or silicon carbide (SiC) and are often found in power strips, surge protectors, and various electronic devices to safeguard against power surges.For example, the BTS50085-1TMA is a commonly referenced model in discussions about advanced microcontroller applications, which often require robust varistor protection to ensure system reliability.
The working principle of a varistor is based on its non-linear current-voltage characteristic. Under normal operating conditions, the varistor exhibits high resistance, allowing only a small leakage current to pass through.
However, when a voltage surge occurs and exceeds a certain threshold, the varistor's resistance drops dramatically, allowing it to conduct a large amount of current and clamp the voltage to a safer level. This rapid response to overvoltage conditions helps to protect sensitive electronic components from damage. Once the surge subsides, the varistor returns to its high-resistance state, ready to respond to future voltage spikes.
This self-restoring capability makes varistors an essential component in enhancing the durability and reliability of electronic systems.
Common Failures of Varistors
Overvoltage Damage
Thermal Damage
Short Circuit
Open Circuit
Physical Damage
Aging and Wear
Harsh Environmental Conditions
Incorrect Specification
Symptoms of Varistor Failure:
Visible Damage: Burn marks, cracks, or discoloration.
Functional Issues: The circuit may fail to power up, exhibit erratic behavior, or have increased susceptibility to voltage spikes.
Electrical Testing: Abnormal resistance readings, such as low resistance (indicating a short) or infinite resistance (indicating an open circuit).
How to Test a Varistor? - Step by Step
You can use a digital multimeter to test a varistor effectively. Here are the step-by-step instructions to test a varistor:
Step 1
First, disconnect the varistor from any power source completely. This means removing it from the circuit or making the circuit power down and capacitors are discharged to avoid any residual voltage.
Step 2
Next, you can set your digital multimeter to the resistance measurement mode (ohmmeter setting). Before testing the varistor, check your multimeter by shorting its probes together. The reading should be close to zero ohms, indicating the meter is functioning properly.
Step 3
After that, place the multimeter probes across the varistor terminals. Then observe the resistance reading. A good varistor typically shows very high resistance (usually in the megaohm range) at low voltages.
Step 4
If the multimeter shows a low resistance reading (close to zero ohms), the varistor is likely shorted and needs to be replaced.
If the multimeter shows infinite resistance (or very high resistance beyond the expected range), the varistor may be open or damaged, and it should be replaced.
Step 5
Moreover, for a more thorough test, you can apply a known voltage higher than the rated voltage of the varistor and measure the current. This requires a variable power supply and proper safety precautions, as the procedure can be risky and is typically not necessary for basic testing.
Additional Tips:
Before testing with a multimeter, visually inspect the varistor for any signs of physical damage, such as burns, cracks, or discoloration.
Refer to the varistor’s data sheet for its specific voltage and resistance characteristics, which can help you understand the expected range of resistance values.
Handle electronic components carefully, especially when dealing with potentially high-voltage circuits. Wear appropriate protective gear and follow all safety guidelines.
FAQs about Varistor Testing
What resistance value indicates a good varistor?
A good varistor typically shows very high resistance (in the megaohm range) when no high voltage is applied.
Can you test a varistor in-circuit?
It's best to test a varistor out of the circuit to get an accurate measurement, as other components in the circuit can affect the readings.
Why is my varistor showing different resistance readings at different times?
Varistors can exhibit different resistance readings due to temperature changes or prior exposure to voltage spikes. The varistor should be at a stable temperature and has not recently been subjected to high voltage when testing.
Conclusion
In conclusion, varistors should be tested regularly as part of preventive maintenance, especially in environments prone to frequent voltage surges. This blog provides you with a detailed guide to do a varistor testing efficiently above, you can follow the steps above to know the specific situation of your varistor.