Basic knowledge of varistors: working principle, function, application and selection principle

Basic knowledge of varistors: working principle, function, application and selection principle

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What is a varistor?

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The full name is "voltage-sensitive resistor", and

the English name is "Voltage Dependent Resistor", abbreviated as "VDR". It is a special resistor whose resistance is not constant, but changes with the change of the voltage applied to its two ends. In other words, the resistance of the varistor is in a certain relationship with the applied voltage. This characteristic makes the varistor widely used in circuits.

How does a varistor work?

The basic principle of the varistor is to use the nonlinear characteristics of semiconductor materials. When the voltage is low, its resistance is large and it is in a high-resistance state; when the voltage reaches a certain value, the resistance drops rapidly and it is in a low-resistance state. This characteristic makes the varistor have important applications in overvoltage protection, lightning protection, power supply filtering, signal modulation and other fields.

This process is easy to understand: when the voltage across the varistor is lower than its threshold voltage, the current flowing through it is extremely small, which is equivalent to a resistor with infinite resistance. This means that when the voltage applied to it is lower than its threshold, it is equivalent to an open switch. When the voltage applied to a varistor is higher than its threshold voltage, the current flowing through it surges, equivalent to a resistor with infinitely small resistance. This means that when the voltage applied to it is higher than its threshold, it is equivalent to a closed switch.

What are varistors used for?

1. Overvoltage protection:
Power supply system: Prevents transient overvoltages in power lines, such as lightning and other forms of electrical interference.
Electronic equipment: Protects electronic equipment such as computers, phones, televisions, and other household appliances from damage.

2. Lightning protection:
In the power supply system of a building, varistors can help absorb overvoltages caused by lightning.

3. Power supply filtering:
In power supply circuits, varistors help reduce electromagnetic interference (EMI) and radio frequency interference (RFI) and improve the quality of power supply.

4. Signal conditioning:
In sensors and measurement circuits, varistors can be used as signal conditioning elements to amplify, filter, or adjust signals.

5. Control and regulation:
In some special applications, such as motor control, battery management system and power factor correction circuit, varistors are used to control the size of current and voltage.

6. Sensors:
Varistor can be used to make sensors of physical quantities such as pressure, flow, humidity, etc.

7. Discharge channel:
In high voltage systems, varistors can be used as a safe discharge channel to avoid the danger caused by charge accumulation.

8. Pulse load:
In circuits that need to withstand short-term high current pulses, such as switching power supplies and motor starting, varistors can provide the necessary damping effect.

The selection and application of varistors need to consider factors such as their rated voltage, current capacity, response time, and voltage resistance to ensure that they can provide effective protection and control in practical applications.

What are the selection principles of varistors?

When selecting a varistor, the following principles need to be considered to ensure that it can perform optimally in the circuit:

1. Rated voltage:
When selecting a varistor, its rated voltage must be higher than the highest possible operating voltage of the circuit to ensure that the resistor can operate safely under various operating conditions.

2. Maximum continuous operating voltage:
In addition to the rated voltage, it is also necessary to consider the peak voltage that may appear in the circuit and select a varistor that can withstand the maximum continuous operating voltage.

3. Current carrying capacity:
According to the surge current or short-circuit current that may appear in the circuit, select a varistor with sufficient current carrying capacity to ensure that it can work safely under overcurrent conditions.

4. Response time:
For circuits that require fast response, such as high-frequency circuits or real-time control circuits, a varistor with a fast response time should be selected.

5. Energy absorption capacity:
In circuits with high energy density, a varistor that can absorb large amounts of energy should be selected to prevent damage to the circuit.

6. Insulation resistance and withstand voltage:
In high-voltage environments, the insulation resistance and withstand voltage of varistors are particularly important and should meet the corresponding safety standards and operating requirements.

7. Temperature stability:
In environments with large temperature changes, varistors with good temperature stability should be selected to ensure that their performance does not change significantly with temperature fluctuations.

8. Size and packaging:
According to the space limitations of the circuit, select a varistor with appropriate size and packaging.

9. Cost and availability:
On the premise of meeting the circuit performance requirements, consider the cost and availability and select a cost-effective product.

10. Certification and standard compliance:
Confirm that the selected varistor complies with relevant industry standards and safety certifications, such as UL, CE, RoHS, etc.

Combining the above factors, a reasonable selection of varistors can ensure the safe and reliable operation of the circuit, and can cope with overvoltage and high current shocks under abnormal conditions.

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