The electric skillet, as we know it, was first designed more than 54 years ago using a bi-metal thermostat and heating coil or element embedded in the pan. The low input power heater, combined with the thermal mass of the pan and contents, was an excellent design that provided good temperature control and simmering.

Over time, two things have degraded the thermal design and performance of the electric skillet. Lower density aluminium has been introduced to lower the cost and weight of the electric skillet, resulting in a lower thermal mass. Also, the input power has been increased, mainly due to marketing direction; more power implies improved product and performance. However, these two changes have worked against the bi-metal thermostat.

Enlarge this picture Fig. 1. Bi-metal thermostat controlled frypan temperature profile.

The bi-metal thermostat responds slowly as it struggles to maintain a constant temperature in the face of high power and low thermal mass. The result is a wide range of cooking temperatures for a given setting; the temperature swing can be more than  20 DegC (36 DegF) depending on brand and configuration. The wide temperature swing reduces the cooking cycle time as low as only one cycle every 10 minutes at low temperatures. (See Fig 1.)

Modern electric skillets are harder to use and control than their earlier counterparts, and have less application in today’s kitchens. The most common complaint is that the pan will not produce a constant simmer. The contents boil during the power heating cycle and cool during the non-power cycle.

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.

Fig. 2. Protected relay.

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 high-power 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 within the housing also represents a major design issue.

Fig. 3. Unprotected relay.

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 electronic skillet control can be realized?

Relay protection

Enlarge this picture Fig. 4. Circuit diagram.

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.

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.

Enlarge this picture Fig. 5. Temperature profile of IES5541A electric skillet.

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 ms to 6 ms (microseconds). On contact opening, the IES5541A senses any arcing and fires the TRIAC, limiting arcing to 5 ms to 6 ms. 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.

Enlarge this picture Fig. 6. Melting chocolate temperature profile.

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. It requires no external power supply and can also be microcontroller driven.

Skillet solution

Fig. 7. Melting chocolate in a frypan.

The extended relay life and temperature control via NTC sensor provided by the IES5541A make it the ideal solution for the electric skillet. The electronic control and faster cycle rate possible with the IES5541A allows very accurate and constant temperature to be maintained in the pan. Fig. 4 shows a circuit diagram of the IES5541A in a typical heater control application.

A very flat temperature profile with negligible overshoot can be achieved using high power elements and low thermal mass pans. Fig. 5 shows the temperature performance characteristic of the IES5541A when used to control a typical electric skillet.

The temperature varies by less than 5 DegC, even at a control temperature of 50 DegC. Improving the temperature control to a point where applications not previously possible, for example, melting chocolate directly, can now be achieved, without burning, overcooking or overheating. Fig. 6 shows the temperature profile for melting chocolate.

The chocolate has melted.

The IES5541A gives appliance designers an innovative and highly effective option for temperature and load control in cooking appliances, particularly electric skillets. The designer can replace bi-metal thermostats in temperature-control applications with protected relays, to ensure more accurate and constant temperature control, resulting in a more usable cooking utensil. The constant pan temperature provided by the IES5541A gives faster cooking times, uniform cooking throughout the food, extended cooking surface lifetime, and extends the application of the electric skillet to preparation of confectionary, crepes and other delicate cooking uses not previously possible.

For more information, email: hendon.info@ies-sa.com.au