There is a vast array of circuit protection devices, however this post covers the common MOV, or Metal Oxide Varistor which is typically used in high-voltage products such as household electronics & surge protectors. It is important to note that not every Outlet Strip will feature MOV surge protection, and many that do will have various energy (Joule) ratings. Purchasing a higher Joule rating will provide increased lifespan of the unit from failing prematurely since it will be able to handle a larger electrical surge or higher number of smaller surges.
The varistor is a non-linear bi-directional voltage dependent protection device with a relatively high transient current and energy rating (in the nanosecond to millisecond time frame). The fast response-time is used for protecting electronic circuits from voltage transients, surges, spikes, overvoltage events, and ESD. These are typically across the input lines before a circuit and sometimes across the output lines after a circuit. These are normally open devices (very high resistance) until an over-voltage event occurs of which the varistor will then clamp the voltage by decreasing its resistance exponentially.
Since most overvoltage conditions have an unknown value, it is hard knowing if the varistor has been utilized within its limits. However, if operated within spec, the varistor will typically not show any signs of degradation over time, but will become “more resistive” over time after voltage event(s) occur. Varistors initially fail in an almost shorted state but can burn up, start a fire, or simply open from excessive current or heat if they do not have a thermistor or fuse to limit current flow or heat. It is usually wise to have some form of additional circuit protection for when an MOV fails.
Thermally Protected MOVs help prevent the part from burning up or causing excess damage and already have a thermal element built inside of them for faster detection of excessive heat while saving board space. These come in 2 or 3 leaded packages per varistor of which the 3rd lead is usually an “output indicator” showing the state of the MOV inside, usually by outputting to some sort of external indicator circuitry.
Pulse Rating Curves or Repetitive Surge Capability charts are also included in most datasheets to show what type of events they can handle. Exceeding these event specifications may result in the device not meeting original published specifications. If the specifications are not properly selected, it can result in the varistor not functioning, lifecycle degradation, or complete failure all together. Since each varistor may have different test methods on how varistor values are determined, it is important to look at the datasheet before determining if the correct varistor is selected.
Terms & Definitions
Varistor Voltage (Min): Approximate minimum voltage to see a resistance change within the MOV, or “start to conduct” voltage, this value is typically determined while running within a specified controlled circuit or electrical values.
Varistor Voltage (Typ): The typical surge voltage or approximate “middle-point” voltage between the minimum and maximum varistor voltage. This is often a generic value given derived from the middle point of the min and max voltages.
Varistor Voltage (Max): Sometimes called clamping voltage (MAX), is the approximate max voltage the varistor will conduct (or pass through to the circuit) for a specified peak pulse duration without causing device failure if operated within spec. This value is typically determined while running within a specified controlled circuit or electrical values such as max clamping voltage @ class current.
Current – Surge: The maximum peak current for a specified peak pulse duration of a given waveform which may be applied without causing device failure. Although the component will handle this surge, most manufacturer’s regard the surge current as a one-time occurrence before the part is recommended for replacement.
Energy: The max amount of Joules (or watt-second) the MOV can dissipate for a specified peak pulse duration of a specified waveform during an event. This value is typically determined while running within a specified controlled circuit and electrical values. The MOV can react to this maximum level only one time, and then is recommended for replacement. Although the component may appear to be working after this event, it may not function properly.
Maximum AC Volts: Maximum RMS line voltage that can be applied across the varistor on a constant basis. This value can be chosen to be slightly above the actual RMS line voltage. The peak voltage of the sine wave should not overlap the minimum varistor voltage, which would reduce the lifetime of the component. Thankfully this is a regular practice for manufacturer’s to figure this within their specifications.
Maximum DC Volts: Maximum DC line voltage that can be applied across the varistor on a constant basis. This value can be chosen to be slightly above the actual DC line voltage, but many times the manufacturer will include this margin built within their specifications.
Process for Determining the Correct Varistor
- Determine the continuous working voltage that will be normally across the varistor and select a varistor with maximum AC or DC voltage to match this or slightly higher. 10-15% higher max rated voltage than the actual line voltage is common as supply lines often have a voltage variance tolerance. Often, a varistor will already have this ratio figured into their voltage values. When an extremely low leakage current is more important than the lowest protection level possible, a varistor of even higher operating voltage may be considered.
- Establish the energy (in J or Joules) absorbed by the varistor during an event. Although this can sometimes be an unknown value, this is determined by using all the absolute max load values of the varistor during an event within environmental and datasheet specifications. It is important to select a varistor with an energy dissipation rating that at minimal is equivalent or ideally exceeds the energy dissipation required by the event the circuit may produce. The MOV can react to its rated maximum energy level only one time, and then is recommended for replacement. Although the component may appear to be working after this event, it may not function or protect properly. It is usually safe to select a physically larger, higher rated Joule option as long as you can afford the physically larger size and a slightly higher price. There may be a capacitance difference with a larger Joule option, but this is typically negligible.
- Calculate the peak transient current through the varistor, often known as the surge current. The surge current, is the maximum current that can pass through the MOV within a specified duration and waveform. It is important to select a varistor with a surge current rating that is equivalent or ideally exceeds the current rating required by an event the circuit may produce to ensure proper function. Although the component will handle this surge, most manufacturer’s regard the surge current as a one-time occurrence before the part is recommended for replacement.
- Determine power dissipation requirements. It is important to select a varistor with a power rating that is equivalent or ideally exceeds the power handling required by the event the circuit may produce. It is common to have the power, surge current, and energy rating to be much higher than the event anticipated. These de-rating factors are commonly at least 50x greater than the required many-time stress tolerance. If unsure about the factors of the event, it is more safe to pick a device of higher power, surge current, and energy ratings.
- Select a model to provide the required Varistor Voltage MAX, sometimes known as clamping voltage (MAX). Clamping voltage must be selected based on the approximate max voltage value you will allow your circuits input or output to see during an event. You will have to make sure your circuit will be able to handle this voltage. In short, this is approximately the highest voltage your circuit down-line should experience. However, the varistor will begin conduction from approximately the min varistor voltage and this may have some smaller clamping effect prior to the actual clamping voltage.
Additional Resources
DigiKey Thermally Protected MOVs
All Digi-Key TVS - Varistors, MOVs
Metal Oxide Varistor PTM (product training module)