What is a Varistor?
A varistor (voltage-dependent resistor or VDR) is an electronic component used to protect circuits from voltage surges. Its resistance decreases with increasing voltage, meaning it can absorb and dissipate energy to protect sensitive components from damage.
How Varistors Work
Varistors are made of semiconductor materials, often metal oxide, and they exhibit non-linear resistance properties. When the voltage is below a certain threshold, the varistor acts like an insulator. If the voltage exceeds this threshold, its resistance drops significantly, allowing it to shunt the excess current away from the protected components.
- Normal Operation: During normal voltage levels, the varistor remains inactive, maintaining a high resistance.
- Overvoltage Protection: When a voltage spike occurs, the varistor’s resistance decreases, allowing it to conduct and dissipate the excessive energy.
Types of Varistors
- Metal Oxide Varistor (MOV): The most common type of varistor, widely used in power systems to protect from surges, transients, and lightning strikes.
- Silicon Carbide Varistor: These were used earlier but have been mostly replaced by MOVs due to their better performance and lower cost.
Applications of Varistors
Varistors are commonly used in electrical and electronic circuits to protect against voltage surges and spikes, such as:
- Power supply circuits
- AC line protection
- Communication equipment
- Lightning protection
How to Choose the Right Varistor
When selecting a varistor, you need to consider several factors:
- Maximum Clamping Voltage: This should be slightly higher than the normal operating voltage of the circuit.
- Energy Absorption Capability: The varistor should be able to absorb the maximum expected energy from a surge.
- Response Time: Varistors are typically very fast in responding to overvoltage conditions, with response times in the nanosecond range.
Varistor Circuit Example
Here’s a simple example of how a varistor is connected in a circuit:
+-------+-------+
| | |
| | |
| Varistor |
| | |
| | |
+-------+-------+
| |
| Load |
+---------------+
In this configuration, the varistor is placed in parallel with the load. It will only conduct when the voltage exceeds its threshold, protecting the load from surges.