electromagnet. This electromagnet will hold open an automatic pilot valve (opened manually by pressing a metal part of the valve against the electromagnet) as long as the pilot flame heats the hot junction adequately. If the pilot flame is extinguished, the hot junction of the thermocouple cools; the voltage produced by the thermocouple falls off to zero; the electromagnet becomes deenergized; and the spring-loaded automatic pilot valve closes, shutting off the gas supply to the appliance. Using a number of pairs of thermocouples in a single device, called a thermopile, which is shown in Figure 3, can substantially increase thermoelectric voltage output. The increased voltage can be used to create a more powerful electromagnet with a single thermocouple. Thermostatically controlled automatic gas valves can be powered by thermoelectricity generated voltages and not require AC voltage from the house circuit. Thermopiles also called Powerpile (Honeywell) will be discussed further in later sections. The powerpile generator can produce as many as 750 millivolts, enough to operate a powerpile gas valve without the use of household electricity. Thermocouples A thermocouple generates a small amount of electrical power that is used to hold open a safety shutoff valve. • The presence of the electric current is proof that the pilot is burning. • If the pilot goes out, the current stops and the safety shut-off valve closes, preventing operation of the main burner. A thermocouple pilot safety system has several major advantages: • Very safe—Anything that degrades the safety keeps the burner from working. It’s as close to being perfectly “fail safe” as anything we see in everyday life. • Very reliable—Systems work for years with no attention. • Simple—There’s not much to it. Most of us can troubleshoot a thermocouple system blindfolded with one hand tied behind our back. It does have limitations though: • Efficiency—It uses energy and if we leave it on all year long, it wastes energy. • Slow—Since it depends on heat to generate power, it takes some time to operate, both on and off. Definition & Purpose A thermocouple is a device that, when heated, generates a small electrical voltage, measured in millivolts (thousandths of a volt; 1,000 MV = 1 volt). This device is commonly used to operate a gas safety cutoff device in a gas appliance control system. Figure 3. A thermopile is composed of many thermocouples connected together so that the electrical effect is additive. Figure 4. Thermocouple ICM/July/August 2016 15
ICM July-August 2016
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