spst

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1. Motor Controls
The simplest method of controlling a motor is to interrupt the flow of current through it. This is done by means of a switch. The symbol is for an SPST (single-pole, single-throw) switch, and this may physically be a toggle switch or a light switch such as is found in most homes. Switches come in many sizes and configurations, but basically they all perform the same function; switches interrupt or apply power to a circuit or series of circuits. Consider the speed control of a motor using a multiposition rotary switch and a series of field windings. As the rotary switch is turned towards the position marked "MAX," the number of field windings is increased, the magnetic field around the armature becomes greater, and the speed of the motor increases. The switch used is a five-position rotary switch; hence, there will be five different speed positions from "MIN" to "MAX." Another way of controlling motor speed is by means of a resistor in series with the field or armature or both. A resistor is a device which adds resistance to the line and hence lessens the flow of current. Many small appliances use a fixed resistor to control the flow of current, but this does not permit speed control. Consider the use of a variable resistor, called rheostat, to control the speed of a motor. The resistance of the rheostat is variable, according to the position of a movable wiper along a coil of wire. When the rheostat is at a high resistance, less current flows through the motor field and armature, and motor speed drops. Sometimes speed control of motors is achieved using a variable transformer. This device can either be a transformer with several taps or a continuously variable transformer. Unlike a rheostat, the variable transformer cannot be used with dc, but it is better than a rheostat in high-power applications.
http://www.fun-home-projects.com/electrical_projects/​tools/​motor_controls.html

2. Other Devices
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. Consider the circuit of another very simple continuity tester. It consists of a L5V flashlight cell or a No. 6 dry cell and a 1.6V, 16 mA LED. When the test leads are shorted, the LED will glow at full brilliance. It will glow at less brilliance when checking the continuity of a circuit that has a resistance of 300 ohms or so. The effective resistance of the LED at rated current is approximately 100 ohms.
http://www.fun-home-projects.com/electrical_projects/​tools/​other_devices.html

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