Electrical Projects The circuit of another device that can be used for checking furnace-thermostat contacts and operation. It consists of a No. 44 pilot lamp (6.3V, 0.25A), SPST switch, and a 6V lantern battery. The test leads are connected across the thermostat terminals. By raising and

lowering the temperature dial, the contacts of the thermostat should open and close. When the contacts are open, the lamp should not glow. When the contacts are closed, the lamp should glow at full brilliance. By slowly turning the temperature dial around to the point where the contacts close, erratic closure and opening of the contacts can be noted by any variation in the brilliance of the lamp.

The continuity checker can also be used for checking the operation of an aquarium heater. An aquarium heater contains a thermostat which automatically connects and disconnects the heater element as water temperature changes. These thermostats are often erratic in operation, and sparking at the thermostat contacts often is a cause of radio interference. Consider a circuit of a specialized tester for checking the operation of an aquarium-heater thermostat. It consists of a 9V transistor battery, a 100-ohm resistor, a 1.6V, 16 mA LED (light-emitting diode), and a female ac receptacle. The plug at the end of the line cord of the aquarium heater is plugged into receptacle P. Thermostat operation is checked by placing the aquarium heater under a water tap and running water at various temperatures on the heater. As the thermostat contacts open and close, the light emitted by the LED should not flicker. If it does, the thermostat probably causes radio interference, and might stick and overheat the water in the aquarium sufficiently to kill the fish. Since it is difficult to get at the thermostat of an aquarium heater, the most practical repair is to replace the unit.




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August 20, 2008