Selecting a Varistor for Transient Voltage Suppression

I’m thankful I’ve never been on a plane or in a car that was struck by lightning. If this were to happen, overvoltage protection circuits would kick into gear and help suppress harmful currents from damaging sensitive electronics. If you’re deploying a new product in a high voltage environment, in an aerospace system, or in other heavy-duty areas, you’ll need to design overvoltage protection into your new product.

There is a range of options for overvoltage protection in new devices. Components are available for overvoltage protection ranging from diodes to fuses and relays. One component that doesn’t get the attention it deserves is a varistor. These components have a small form factor, low cost, and comparable overvoltage protection compared to other components. Here’s what you need to know about different overvoltage protection components and how they stack up to varistors.

Varistor vs. Other Overvoltage Protection Components

A varistor has nonlinear behavior that is similar to that seen in a TVS diode, although it does not exhibit rectification. The response of this component, meaning its DC resistance/AC impedance, is nonlinear and decreases monotonically as the input surge strength increases. These components are bi-directional, i.e., they can be driven in forward or reverse. This type of device exhibits similar characteristics as a back-to-back Zener diode configuration.

Varistors are most commonly made from zinc oxide, although they are also made from silicon carbide. The material used to build a varistor will determine the subthreshold swing, clamping voltage, and durability of the device. Zinc oxide has much higher resistance than silicon carbide, thus it has a lower leakage current at low voltage. These devices are normally available as through-hole components, although SMD components are also available.

Another type of zinc oxide varistor is a multilayer varistor (MLV). These varistors are designed to operate with AC signals typically found in PCBs for power electronics systems (moderately voltage, relatively low frequency). When placed as shunt elements in protected circuits, they are ideal for suppressing transients from inductive load switching, ESD, and lightning-surge remnants that can damage ICs.

Important Varistor Specifications

The wide range of varistors on the market makes it difficult to determine the best option for your next system. The transient voltage threshold and peak voltage/current values are important to consider, but there is more to choosing a varistor than these values. Here are some important specifications to consider in varistor datasheets:

• Clamping voltage: This is the voltage that will be dropped across the varistor when pulsed with a specific transient pulse shape and peak current.

• Maximum energy: This is the maximum energy the MOV can dissipate for a specific transient pulse shape. Suppressing this much energy will degrade the varistor and it may not function properly in subsequent transient voltage suppression events.

• Maximum DC vs. AC voltage: The AC overvoltage protection value is different from the DC value. AC overvoltages are normally specified as RMS values, and these values are less than the rated DC values. These values can be chosen slightly above the desired line voltage as the varistor needs to suppress large transients.

• Peak current vs. peak voltage curve: These two voltage values depend on the clamp voltage. In general, the clamp voltage increases as the peak voltage and current protection values increase. 

• Response time: An ideal varistor has zero response time, but real varistors have response times on the order of microseconds or nanoseconds. The response time is related to the load capacitance and resistance, which in turn are related to the package geometry and material composition. Zinc oxide varistors provide a shorter response time when clamping transients.

• Temperature stability: Above a certain temperature, the peak power suppression ratings of a varistor will tend to decrease quite rapidly. This rating is quite important if your varistor will be deployed in a warm environment.

• Maximum current vs. transient pulse time: The rated maximum current a varistor can withstand will roll off as the transient pulse time increases.

The varistors shown below include SMD and through-hole components over a range of voltage, current, and power ratings. Through-hole devices are ideal for industrial systems or light aircraft, while SMD components may be better for embedded devices that must be deployed in a high voltage environment.

Littelfuse, V10E275P

The V10E275P from Littelfuse is part of the UltraMOV line of varistors. This component provides clamping up to 350 V with up to 3.5 kA peak current with 8/20 transients. This through-hole component has low-temperature sensitivity up to ~85 °C. The other components in this line of varistors have reproducible specifications for a variety of model sizes, allowing designers to swap for a smaller varistor without compromising voltage protection. Larger packages have higher peak current suppression values at various transient surge times, as shown on pages 42 and 43 of the datasheet.

EPCOS, B72220S151K101

The B72220S151K101 varistor from EPCOS provides line voltage protection in an AC system with a fast-rated response time of ~25 ns. The clamping voltage is rated at 395 V with 8 kA maximum surge current values. The maximum rated current has a slow roll-off as transient time increases, as shown in the graph below.

Eaton, MLVB06V18C003

The MLVB06V18C003 varistor from Eaton is a lower voltage varistor, but it has a very low capacitance of 3 pF, giving a short response time of 1 ns. This varistor is only rated up to 18 V, so it is not ideal for deployment in high voltage environments. This is a surface-mount component, making it ideal for transient voltage suppression in high-density systems. This varistor comes in 0603 or 0402 SMD packages.

Any PCB that runs at high voltage requires transient suppression circuitry to protect sensitive circuitry. You can find the varistors shown here and many other components for transient voltage suppression in our Part Selector guide.

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