New Industry Products

TDK Debuts Two New Series of ThermoFuse Varistors

November 03, 2020 by Gary Elinoff

The new devices are designed for overvoltage protection and are equipped with monitor outputs.

The MT25 and MT30 series from TDK is designed for voltage ranges from 150 to 385 VRMS and for 150 to 750 VRMS, respectively. The units are integrated with internal fuses, which are construed to cut off power in the event that the varistor reacts to a surge, cutting off the circuit being protected from the effects of the surge.


Image courtesy of TDK


As defined by IEC 61643-11, the MT25 has a maximum surge current of 20 kA at a pulse of 8/20 µs, and the corresponding figure for the MT30 series is 25 kA.

What’s all this 8/20µs Stuff, Anyway?

An 8/20 μs pulse rises to its maximum in 8 microseconds. By the time 20 μs have passed, its intensity has receded to 50% of its maximum. The often quoted, but a little-understood definition is often used as a proxy for a lightning pulse.

What is a Varistor?

As TDK defines it, a varistor, or variable resistor, is “Put simply, a resistor where the resistance decreases when the more voltage is applied to it.” A varistor will fail when it is subjected to too much voltage for too long a time period. The time periods involved are typically in the range of the 8/20 pulse defined above. An important point is that varistors are bidirectional devices.


What is the Difference Between Zener/TVS Diodes and Varistors?

This is a question posed and addressed by a TDK Tech Note. A few highlights:

Structure — Varistors are ceramic semiconductor devices based largely on zinc oxide, while TVS diodes are based on P-type and N-type semiconductors.

Speed — The biggest misconception is that varistors are still slow to respond to spikes, which, as illustrated in the graph below, is no longer the case.


Image courtesy of TDK


Capacitance — Capacitances across varistors are higher than those across comparable TVS diodes. This is a critical point for high-frequency circuitry.

MT25 and MT30 Device Operations

A look inside reveals SiOVs (silicon oxide varistors) connected in series with an internal fuse.

Image courtesy of MT25 datasheet

Normally, the MOV is at a very high resistance. An external LED, when connected to the monitor lead, will indicate that all is well by emitting light. If a surge is strong enough to short out the MOV, the same surge will blow the fuse, decoupling the device under protection from the power source, preventing damage.

The monitor outputs are available with or without electrical isolation.


Device Variations

The MT25 Series

Modified image courtesy of MT25 datasheet


MT30 Series

Modified image courtesy of MT30 datasheet


  • Inverters
  • Lighting system power supplies
  • Industrial power supplies
  • Photovoltaic systems
  • Telecommunications


Physical Considerations

Both series of ThermoFuse varistors are available in completely encapsulated packages.

They operate over a temperature range of -40 to +85 ℃

  • Members of the MT25 series feature dimensions of 25 x 28 x 14 mm.
  • The MT30 series units measure 34 x 28 x 14 mm.

Regulatory and Safety Considerations

  • UL1449 approved, types 4CA and 1CA, MT25 and MT30 respectively. File number E321126.
  • Coating: epoxy molding, flame-retardant to UL 94 V-0
  • Climatic category to IEC 60068-1: 40 / 85 / 56