ThermoFuse varistors offer intrinsically-safe over-voltage protection

TDK-Epcos ThermoFuse varistors offer intrinsically safe over-voltage protection. The components are based on disk varistors with diameters of 14mm (T14 series) and 20mm (T20 series) that are connected in series with a thermally-coupled fuse. The T14 series (B72214T) covers the voltage range from 130VRMS through 420VRMS. Its surge current capability is 6000A for a pulse of 8/20µs in accordance with IEC-61000-4-5, while the permissible power dissipation is 0.6W. The dimensions are 17mm x 21.6mm x 13mm. The T20 components are suitable for voltages between 130Vrms and 1000Vrms. Their surge current capability is up to 10,000A (8/20 µs) and their power dissipation is 1W. Depending on the voltage, the dimensions range from 22mm x 26mm x 12mm to 27.5mm x 27.5mm x 18.5mm. If the varistor of a ThermoFuse overheats, the fuse trips and isolates the varistor from the grid. This prevents fires on the printed circuit board or damage to components located near the varistor. Fuse and varistor are housed together in a plastic enclosure that, like the coating of the varistor, is made from flame-retardant material. The components have three connections - two for the power cable - and one as a monitor output, which enables the display of the status of the component with an LED, for example. The main applications for ThermoFuse varistors are home appliances, power supplies, photovoltaic inverters, frequency converters, ballasts for lighting systems, surge voltage protectors and electronic meters, says the company.