Appliance Design - March 2008 - (Page 36)

SWITCHES & RELAYS Fig. 4. Circuit diagram. Fig. 5. Temperature profile of IES5541A electric skillet. Fig. 6. Melting chocolate temperature profile. Testing of the IES5541A has shown that relays rated for 100,000 switching cycles can achieve in excess of 1 million cycles at rated load with TRIAC contact protection. Whenever the IES5541A drives the relay open or closed, it fires the TRIAC for four mains cycles, 66 mS (milliseconds) for 60 Hz AC mains or 80 mS for 50 Hz AC mains. The result is that only the TRIAC junction voltage, ~1 V, is applied across the relay contacts on closing operations, eliminating arcing on the initial contact closure. If contact bounce occurs it is sensed by the IES5541A, which refires the TRIAC, limiting arcing to a mere 5 s to 6 s (microseconds). On contact opening, the IES5541A senses any arcing and fires the TRIAC, limiting arcing to 5 s to 6 s. The potential 1,000-fold reduction in arcing time, over lifetime of the relay, eliminates more than 80 minutes of arcing. The IES5541A senses AC mains zero-crossings and controls the relay and TRIAC accordingly to implement zero-crossing switching, which significantly reduces radiated electromagnetic interference. The photo in Fig. 2 shows a typical relay after 100,000 cycles using the IES5541A. The contacts are like new, still bright and clean and with a very small amount of pitting. By contrast, the photo in Fig. 3 shows a typical relay after 100,000 cycles without relay protection. As can be seen, the contacts are corroded, pitted and exhibit increased contact resistance from oxidation and material removal. The IES5541A incorporates a sensor input so that it can be interfaced directly with an NTC specifically for applications such as the electric skillet. www.applianceDESIGN.com 36 applianceDESIGN March 2008 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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