PTC Thermistors

Thermal cut-outs are generally used in appliances that expected to frequently reach high temperatures, where the temperature needs to be carefully controlled up to a specific limit. The re-settable nature of thermal cut-outs make them ideal for repeated use in devices with cyclic thermal loading.

A prime example of this is kettles, but they are also frequently used air conditioners, motors, and an array of other kitchen appliances.

As they rely on bimetal elements, their thermal cut out temperature can vary significantly depending on the metals used and shape of the element. This results in thermal protection of devices over significant temperature ranges. The ASAHI US-622, for example, has an impressive operational temperature range of 0°C to +185°C.

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There are 2 types of thermal cut-outs that we sell here at ATC Semitec:

Snap Action Disc Thermal Cut-Outs, supplied by either ASAHI or EAW, which use a bimetallic disc.

Probe Thermal Cut-Outs, supplied by PEPI, which contain a bimetallic strip element.

PEPI’s JS Self-Hold Thermal Cutout includes an integral “self-hold” thermal and current sensitive cut-out allowing the protector latches to open after tripping and then reset remotely. This offers the same benefit as fitting a manual-reset cut-out but without the disadvantage of having to access it.

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Bimetallic elements (or Bimetals) are comprised of 2 metals with differing thermal expansion coefficients, that are fixed together. As the element is heated, one side will expand at a faster rate than the other, resulting in unidirectional bending.

By altering the shape of the element at ambient temperature, the shape change can be made instantaneous, hence bimetals are often found as either curved discs or strips, which will ‘snap’ into shape.

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Thermal cut-outs (or thermal switches) are a re-settable, temperature-sensitive device, which open at high temperatures, stopping current flow, and the re-close as the temperature decreases.

The opening and closing of the device is due to a bi-metallic element, which changes shape instantaneously with a change in temperature. They can either reset automatically, or manual reset options are available when an automatic and unattended restart could create a hazardous condition.

Thermal cut-outs are therefore used as fail-safe devices, preventing temperature from rising to dangerous levels, whilst their ability to reset makes them ideal for applications where high temperatures are reached frequently, without the need to replace the appliance.

We offer a range of products from world leading manufacturers, PEPI and Asahi in either probe or disc formats

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SEFUSE SM series solder-based thermal fuses are housed in a ceramic case.  The solder alloy inside responds to external high temperatures and at its set-point, will start to melt.  As the solder melts, it separates to either end of the case (helped by the surrounding flux) and thus breaks the electrical circuit.  See diagram below;

In the SM type, leads are connected by a fusible alloy. The current flows directly from one lead to the other. The fusible alloy is coated with a special flux.

When ambient temperature rises to the SEFUSE operating temperature, the fusible allow melts and condenses into a drop around the end of each lead because of the surface tension and the coating of special flux. The electrical circuit then opens.

To view our full range of Eutectic Solder Alloy Thermal Fuses click here

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SEFUSE SF  series pellet thermal fuses are housed in a metal casing.  The organic thermal pellet inside responds to external high temperatures and close to its set-point, will start to melt.  As the pellet melts, it triggers a spring-loaded contact mechanism which breaks the electrical circuit as shown in the diagram below;

The SF-R thermal fuse contains a sliding contact, springs, and a thermal pellet inside a metal case. When spring B is compressed, contact between lead A and the sliding contact is maintained. At normal temperatures, current flows from lead A to the sliding contact and then through the metal case to lead B.

When the ambient temperature rises to the SEFUSE operating temperature, the heat transferred through the metal case melts the thermal pellet. When the thermal pellet melts, spring A and B expand, moving the sliding contact away from lead A. The electrical circuit is opened by breaking contact between the sliding contact and lead A.

To view our full range of Organic Pellet Thermal Fuses click here

Have a question or need a sample ? Call one of our engineers today on 01606 871680

ATC Semitec stock a number of different types of thermal fuse:

Organic pellet – see SEFUSE SF/R types for details

Eutectic solder alloy –  see SEFUSE SM/A types for details

Bimetal element –  see PEPI H types for details

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Thermal fuses are generally used to protect electric heaters, household electrical appliances and electrical products used in hazardous areas.

Specific examples include

• • washing machines
  • tumble dryers
  • convector heaters
  • coffee makers
  • hair dryers
  • laser printers
  • rechargeable battery packs..

When designing with a thermal fuse, you need to consider the operating current, applied voltage and the duty hours of the appliance that it will protect.  The normal running temperature needs to be a minimum of 25°C below the fuse opening temperature not including any self-heating due to I²R/ current self-heating. The Th (holding temperature) and Tm (maximum temperature) values stated for the fuse should also be taken into account. This is to ensure reliable operation over the required product life, and that any IEC/EN standards they are approved to, are complied with.

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A thermal fuse (or thermal cut-off or “tco”) is a one-shot/one-time, non- re settable, temperature-sensitive device that provides ultimate overheat temperature protection in small appliances, electric heaters and electronic circuits.

Thermal fuses are fail-safe devices which activate when other safety measures fail to operate. They help  prevent temperatures rising to dangerous levels, and  thus stop product fires from occurring. The original fault in the appliance could have been caused by either control malfunction or customer misuse.      

Thermal fuses therefore offer the ultimate thermal protection.

They are single use protection devices that cannot be reset or repaired. They have to be replaced once the original fault which caused the product failure has been identified and rectified. In the case of many consumer products, this would mean replacing the complete appliance.

Have a question or need a sample ? Call one of our engineers today on 01606 871680