A varistor is a part that helps keep electronics safe from sudden high voltages. It stays off when the voltage is normal. It turns on when the voltage spikes. This way, it stops damage to other parts.
Definition of a Varistor
A varistor (short for "variable resistor") has two ends. At normal voltage, its resistance is very high. When voltage goes above a set level, its resistance drops. You may see its symbol (varistor symbol or MOV symbol) in circuit diagrams.
Function of Varistor in Circuit Protection
The main job of a varistor is to guard circuits from extra voltage. If there is a spike due to lightning or switching, the varistor lowers its resistance. It sends the extra current away from the sensitive parts. After the spike ends, it goes back to high resistance.
Types of Varistors
There are a few kinds, but the metal oxide varistor (MOV) is the most common. You can also find a capacitor varistor in some designs.
Metal Oxide Varistor (MOV)
An MOV is made of zinc oxide mixed with other metal oxides. This mix sits between two metal plates. It gives the MOV its special voltage behavior.
MOV Symbol and Characteristics
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MOV Symbol: Looks like a diode curve with an arrow.
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Clamping Voltage: The voltage where it starts to conduct.
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Energy Rating: How much surge energy it can take.
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Response Time: How fast it reacts (in nanoseconds).
MOV Electrical Properties
At normal voltage, an MOV lets almost no current pass. Once the voltage passes its clamping level, its resistance drops a lot. It then handles the surge by letting the current flow safely to ground.
Property | Typical Range |
---|---|
Clamping Voltage (V₍cl₎) | 120–1100 V |
Energy Capacity (Joules) | 10–200 J |
Leakage Current (µA) | 0.5–20 µA (at Vr) |
Capacitor Varistor and Its Uses
A capacitor varistor combines an MOV and a small capacitor. It protects against surges and cuts noise. Engineers use it when they need both in one part.
Working Principle of Varistors
How Varistors Respond to Voltage Changes
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If voltage is normal, the varistor is off.
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If voltage spikes, it turns on and lowers resistance.
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When voltage is back to normal, it turns off again.
Varistor Characteristics Curve
On a plot of voltage vs. current, you see a steep rise at the clamping point. Below that, current is small. Above it, current grows fast. This is what makes a varistor clamp surges.
Applications of Varistors
Varistor Surge Protection in Electronics
Varistors are in many surge protectors and power supplies:
- Power strips and surge protectors use MOVs to guard devices.
- Switch-mode power supplies (SMPS) use them to protect from mains spikes.
- Telecom and data lines use special varistors for surge and noise.
Common Uses in Circuit Designs
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Consumer electronics like TVs and computers.
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Industrial controls such as PLCs and motor drives.
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Automotive electronics to handle load-dump surges.
Conclusion
Summary of Varistor Functions
Varistors help keep electronics safe by changing their resistance when voltage spikes. They absorb and divert extra energy, so parts last longer.
Future Trends in Varistor Electronics
As electronics get smaller and work at higher voltages, varistors will get better:
- Lower-leakage MOVs for low-power modes.
- Nano-engineered varistors for more energy in less space.
- Hybrid parts that mix varistor, capacitor, and inductor for compact designs.
Knowing the basics of varistors helps you pick the right part for a safe and reliable circuit.
FAQ
What does a varistor do?
A varistor clamps high voltage spikes by changing its resistance, protecting electronic components.
How do I choose the right MOV?
Select based on clamping voltage, energy rating, and expected surge currents in your circuit.
What is the difference between an MOV and a capacitor varistor?
An MOV only clamps surges, while a capacitor varistor also provides noise suppression thanks to its capacitance.
Can varistors wear out?
Yes. Repeated high-energy surges can degrade a varistor’s clamping ability over time.
Where are varistors commonly used?
They are found in power strips, SMPS units, telecom lines, industrial controls, and automotive electronics.