Difference Between a Thermal Breaker and a Thermal Fuse

DigiKey is located in northern Minnesota.

It gets cold! Tomorrow’s weather forecast suggests an overnight low of -14 °F (-26 °C).

This is the time of year when people break out the space heaters to stay warm. For example, I have a 1500W oil-filled space heater next to me as my home office is a bit cold. All such devices including space heaters, coffee makers, and hair dryers feature redundant thermal safety devices to prevent excessive temperatures and to reduce the possibility of burning down your house.

On a minute-by-minute basis, a primary thermostat regulates the device’s temperature. Unfortunately, these devices have a limited lifetime and will eventually fail. The worst-case situation is a failure where thermostat contacts are welded shut. This is a very dangerous situation. For example, suppose a damaged space heater is smothered under a blanket. Without additional protection, this is a fire waiting to happen.

There are two ways to prevent this situation including thermal fuses and thermal circuit breakers both devices are included in DigiKey’s Thermal Cutoffs (Thermal Fuses) category. Like their electrical fuse and circuit breaker analogs, the fuse is a one-and-done device while the circuit break may be reset.

As an example, consider the first thermal fuse in DigiKey’s listings. The Cantherm SDF DF128S is shown here provides a one-time protection by opening at 262 °F (128 °C). From the consumers perspective, opening such a fuse renders the product inoperative.

Figure 1: Cantherm brand thermal fuse with a one-time protection at 262 °F (128 °C).

A representative thermal breaker is shown in Figure 2. This Bourns SDF0WBB is the last item to appear in DigiKey’s listing. Unlike the thermal fuse, this thermal breaker will self-reset. Note that there are two thermal specifications including the 302°F (150°C) trip and a 104°F (40°C) reset point.

Figure 2: Bourns self-resetting thermal breaker with a 302°F (150°C) trip and a 104°F (40°C) reset.

Tech Tip: The thermal circuit breaker is not a thermostat. For example, the Bourns’ datasheet is very clear “The breaker cannot be used as a repetitive ON-OFF thermostat.

Parting Thoughts

Before we close, let’s return to that defective space heater smothered under a blanket, consider how the two safety devices would respond. As the temperature rises, the fuse will open rendering the heater inoperative. On the other hand, the breaker would continuously cycle between the high thermal trip and the low thermal reset points. Note that some thermal circuit breakers feature a manual reset to prevent this thermal cycling.

Stay Warm, stay safe,


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