Bodas

AN958
Low-Cost Electric Range Control Using a Triac
Author: Reston Condit Microchip Technology Inc.

MECHANICAL THERMOSTAT OPERATION
Electric ranges create heat by applying the AC line voltage across a resistive heating element. An adjustable mechanical thermostat in series with the heating element has a rotary dial which sets the amount of current to the element. The mechanical thermostatshown in Figure 1 has a complex set of metal tabs, spacers and contacts that work together to connect and disconnect power depending on the setting of the rotary dial. The following sequence of events describes the operation of a mechanical thermostat when the dial is turned halfway between off and full on. 1. 2. 3. 4. Contact is made between the two terminals of the switch. Resistive materials inthe switch cause parts of the switch to heat and expand. The expansive materials push the contact apart and the switch stops conducting current. The element then cools until contact is made again.

INTRODUCTION
Most countertop cooking appliances like electric ranges, skillets and fryers have an adjustable mechanical thermostat to vary the heat output of the range. This solution is inexpensive,but there are several drawbacks to mechanical thermostats: • Mechanical thermostats have to be calibrated at the factory. • They have poor simmer performance (control is not precise at low temperatures). • The accuracy of these devices is poor. • Mechanical components wear out over time. This application note will describe a low-cost microcontroller-based replacement for the mechanical thermostatwhich eliminates these drawbacks. The PIC10F204, Microchip's 6-pin PICmicro® microcontroller (SOT-23 package), is used to implement this solution. The PICmicro gathers user inputs from a potentiometer and controls current to the heating element via a triac. This application note will discuss triac theory, so it is also a good resource for other applications that interface to AC lines (i.e., lightswitches, vacuum cleaners and various other household appliances). Power to the PICmicro is supplied directly from the AC lines via a resistive power supply. Compared to the mechanical thermostat, the PIC10F204 solution offers design flexibility, including the addition of user-friendly features. Two such features are incorporated into the PICmicro solution detailed here. These features are: (1) astatus LED indicating the range is on or off, and (2) an automatic shutdown. The automatic shutdown provides added safety by shutting the range off after 2 hours if it is left unattended.

Based on the position of the dial, the switch repeats this sequence more or less frequently. The switch allows infinite control, but without a clear reference. As a result, the switch is not very accurate. Theswitch is constantly subjected to thermal changes and arching occurs frequently across the contacts when the dial is not in an absolute position (off or full on). These stresses affect the reliability of the switch.

 2004 Microchip Technology Inc.

DS00958A-page 1

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FIGURE 1: ADJUSTABLE MECHANICAL THERMOSTAT

TRIAC OPERATION
A traic will be used to control current flow to theheating element in the microcontroller-based design. A triac is a three-terminal bidirectional AC switch that is triggered by a low-energy signal applied to the gate. When this signal is applied, the triac goes from a high-impedance state to a conductive state, allowing current to flow to the load. A positive or negative gate signal will trigger a triac, though one signal is more efficient. Figure 2shows the four trigger modes for a triac. Each mode is referred to as a quadrant. Note that everything is referenced to terminal MT1.

FIGURE 2:

TRIAC QUADRANTS
MT2 Positive

MT2 (-) IGT G MT1 REF IGT MT2 (-) IGT G MT1 REF (+) IGT G QII QI QIII QIV (+) IGT G

MT2

MT1 REF + IGT MT2

MT1 REF

MT2 Negative

DS00958A-page 2

 2004 Microchip Technology Inc.

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Triacs...