Appliance Design - March 2008 - (Page 35)

SWITCHES & RELAYS Fig. 2. Protected relay. Fig. 3. Unprotected relay. In a typical bi-metal controlled frypan, heat is conducted by the temperature probe from the underside of the frypan into the control housing, where the temperature sensitive bi-metal strip is located. The time it takes for a change in pan temperature to be conducted back to the bi-metal strip is the primary cause of an electric skillet’s 20 DegC+ temperature hysteresis, and inability to maintain a constant simmer. Replacing the thermostat with an electronic temperature control provides accurate, consistent and constant cooking temperature in the pan. A small, low-mass independent temperature sensor, such as a Negative Temperature Coefficient resistor, (NTC), can all but eliminate the time delay between pan temperature and control response. A power-switching device works with the NTC or sensor to control the heating element. There are two main power-switch options for high-power heating applications, the solid-state device and the relay. The TRIAC is the logical choice for a solid-state device solution in this type of application. Reliable, simple and cheap, it would seem to be the ideal solution. However, the electric skillet is a highpower device. The TRIAC’s internal junction resistance can generate up to 20 W of constant power (heat) in this application, heat which must be removed from the TRIAC to maintain its integrity and lifespan. A heat-sink capable of dissipating at least 3 DegC/W or more would be required. A heat-sink of that capacity would not physically fit inside the controller housing. The lack of free air-flow www.applianceDESIGN.com within the housing also represents a major design issue. The second option, the relay, is also reliable, simple and cheap. It has no such junction temperature issues. Once the relay is closed, the contact resistance is negligible, and there is no significant power dissipation or heat-sinking required. However, there is one limitation of the relay, its limited switching cycles. It is the reason that an electronic frypan control does not currently exist. In order to achieve accurate and constant pan temperature, the relay must switch the element up to seven times per minute for high dissipation loads at high temperatures. Relays typically have a lifetime of 100,000 switching cycles. This would give the frypan a lifetime in the order of 250 hours, possibly not even lasting a full year in the kitchen. So, if a TRIAC is unsuitable on its own, and a relay won’t last long enough, how can the load be controlled so that an elec- tronic skillet control can be realised? Relay protection The primary cause of relay failure is the degradation of the relay contacts caused by electrical arcing across the relay contacts during contact closing and opening operations, accounting for 75 percent of relay failures. Hendon Semiconductors Pty. Ltd. has engineered a solution that greatly minimizes relay contact arcing by using a TRIAC in parallel with the relay contacts. The Hendon Semiconductors’ IES5541A is an 8-pin integrated circuit in bipolar silicon that controls the switching of a relay and parallel TRIAC to implement relay contact protection. The TRIAC is only used to conduct the load current and protect the relay contacts on contact opening and closing operations. The short conduction time means the TRIAC does not require a heat-sink. Fig. 1. Bi-metal thermostat controlled frypan temperature profile. applianceDESIGN March 2008 35 http://www.appliancedesign.com

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Appliance Design - March 2008 Contents Editorial Shipments/Forecasts News Watch A New Appliance is Set to Break into Homes Soon - the Micro CHP Unit, which Generate both Heat and Power. Thermally Enhanced Varistors Help Protect Low-Power Systems Against Damage Caused by Over-Current, Over-Temperature and Over-Voltage Faults. Hybrid Controller Reduces Standby Power Consumption and Improves Active-Mode Efficiency. Battery-Management ICs Solve Design Challenges for Cordless Appliances Using High-Voltage, Lithium-Ion Battery Technology. A Semiconductor Solution Protects the Relay in a Temperature Controller for a Cooking Appliance. New Polyurethane Foam Insulation System Optimizes both Insulation Performance and Productivity. Innovations in Decorative, Pre-Finished Metals Expand Range of Design Options for Appliance Designers. Design Marts Association Report: AHAM Advertiser's Index

Appliance Design - March 2008

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