How to control the protection voltage of varistor? Design of varistor protection circuit

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How to control the protection voltage of varistor? Design of varistor protection circuit

Posted Date: 2024-02-05

A varistor is a resistive device with nonlinear volt-ampere characteristics, usually made of semiconductor materials. When the voltage applied to a varistor reaches or exceeds its nominal voltage (also called varistor voltage or base current voltage), its resistance decreases sharply and the current through it increases sharply. This allows the varistor to play the role of over-voltage protection in the circuit, absorbing excess current to protect sensitive devices. Additionally, varistors can withstand sustained high voltages without damage and can return to a high-resistance state when the voltage is removed.

The English name of the varistor is "Varistor", abbreviated as "VDR". In Taiwan, China, varistor is called "surge absorber" or "electrical shock suppressor". Varistors have the advantages of strong resistance to impulse current, strong ability to withstand overvoltage, fast response speed, and small size. They are widely used in electronic circuits to protect circuits from possible damage due to transient voltage surges in power supply systems. .

How to control the protection voltage of varistor

To control the protection voltage of the varistor, the following methods can be adopted:

1. Select the appropriate varistor: Different types of varistor have different protection voltage ranges. Based on specific application needs, select a varistor model that can meet circuit protection requirements. Check the specifications or technical information of the varistor to understand its rated voltage range to ensure that it can effectively protect the overvoltage condition in the circuit.

2. Connect multiple varistors in series: When the expected protection voltage exceeds the rated voltage of a single varistor, you can consider connecting multiple varistors in series to increase the protection voltage. When connected in series, the rated voltages of each varistor should be added to determine the protection voltage of the entire circuit. However, be aware that varistors connected in series may increase the overall resistance value and affect the normal operation of the circuit.

3. Connect other components in parallel: In addition to using a varistor, it can also be connected in parallel with other components (such as capacitors, Zener diodes, etc.) to provide a more comprehensive protection circuit. These components can increase circuit voltage regulation, absorb overvoltage pulses, or provide voltage reduction protection.

4. Control the current limiter: In the circuit protected by the varistor, adding a current limiter can control the current flow during overvoltage. This can help limit the rate of voltage rise and prevent the varistor from being damaged by excessive current.

Varistor protection circuit design

Varistors are often used for lightning protection and anti-surge design at the power inlet of instruments and equipment. In such applications, varistors are usually used together with gas discharge tubes, fuses, thermistors and other devices to interact with each other. Combine to function better.

The figure below shows a typical lightning protection and anti-surge circuit structure. RV1~RV3 are varistor, GAS is gas discharge tube, F1 and F2 are fuse tubes. RV1 is used to protect differential mode surge voltage, and RV2, RV3, and GAS are combined to protect common mode surge voltage. The fuse is used for short circuit protection to prevent the varistor from breaking down and causing a short circuit.

The common mode surge protection circuit composed of RV2, RV3, and GAS in the figure combines the advantages of varistor and gas discharge tube to compensate for their respective shortcomings. It is a commonly used surge protection circuit. The gas discharge tube has a large flow capacity, but its operating time is long and the leakage current is large, while the varistor has a fast response speed and a small leakage current.

How to replace the varistor if it is broken?

When a varistor is damaged, you may consider the following alternatives:

Replace the varistor with the same model: The preferred method is to replace the varistor with the same model. Look for specifications or technical information provided by the original manufacturer to find alternative models with the same rated voltage, resistance value, and dimensions. This ensures that the new varistor will work properly and provide the same protection.

Select a varistor with similar parameters: If you cannot find a varistor of the same model, you can select an alternative model with similar parameters such as rated voltage, resistance value, and size. Make sure the replacement model selected meets the protection needs of the circuit and perform testing to ensure performance meets requirements.

Use other protection components: If you cannot find a suitable varistor replacement, you can also consider using other types of protection components, such as metal oxide varistor (MOV), diodes, overvoltage protection chips, etc. Depending on the specific application and protection needs, select appropriate protection components for replacement.

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