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issue: July 2005 APPLIANCE Magazine

Electronics Report Engineering
Self-Hold Thermal Protectors

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The new UB8P and UB81P self-hold thermal cut-out and thermal protectors from Selco Products Company are designed to provide overheat protection in high-voltage applications.

The new UB8P and UB81P self-hold thermal cut-out and thermal protectors from Selco Products Company are designed to provide overheat protection in high-voltage applications.

With a high 16-A capability, the new models are designed for thermal overload protection in a broad range of applications, including space heaters, dishwashers, mixers, blenders, grinders, and other electronic appliances. According to Mark Koeckritz, product manager for Selco (Anaheim, CA, U.S.), the devices are ideal for protecting critical devices where automatic reset could cause eventual application failure.

The body of the surface-mount devices are manufactured of phenolic resin and feature a patent-pending partition wall design that is said to isolate the switch from the main body. Mr. Koeckritz says this design effectively controls arcing that can cause a break in voltage contact (particularly in 250-V applications), ensuring continuous operation to set point.

The protectors feature a bimetallic element that allows for quick thermal response to temperature rise and an internal positive temperature coefficient (PTC) resistance heater for both 125- and 250-V applications. Selco says this design provides efficient insulation and fast thermal response time compared to units where the heater is mounted externally to the thermostat.

A “self-hold” non-self resetting function also provides a high level of operational safety, especially in appliances with moving parts. “By adding an internal PTC self-regulating resistance heater to a standard automatic reset thermostat, we were able to offer a secondary safety feature to help protect the end user from bodily injury,” Mr. Koeckritz says. “Typical thermal protectors
and thermostats are automatic reset or manually resettable. The UB8P/UB81P’s self-hold feature is unique due to the fact that the power must be removed in order to reset the temperature switch.”

One possible application where this safety feature would be especially beneficial is an industrial food blender with large, moving blades. “When the switch is opened due to the bimetal element reaching its pre-set open temperature, its contacts will remain open until the blender motor is switched off by the user,” Mr. Koeckritz says.

Measuring 31.5 by 6.5 by 4.2 mm, the UB8P and UB81P feature an operating temperature set range of 75°C to 145°C. Temperature tolerances are ± -5°C, ± -7°C, and ± -10°C with a standard differential of 30°C ± 15°C. Contact ratings are 16 A at 125 V and 10 A at 250 V a.c. to 6,000 cycles.

Suppliers mentioned in this article:
Selco Products Co.

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