Control Devices/Electrical Conditioners Unit 8/10

Objectives Upon completion of this unit the student will be able to: Describe the operation of electrical control devices. Operate control devices using AC and DC current sources. Use control devices to turn output devices on and off. Explain the function and use of resistors.

Review/Background In unit 7 you should have noticed that when the batteries were used to produce current, as soon as you completed the circuit, the lamp, motor or speaker came on and stayed on until you unplugged the connecting wires. You could not control the output operation. The same thing happened when you used the transformer. As soon as power was supplied to the whole kit, the lamp came on and stayed on. You could not control the lamp itself.

Homes, schools, businesses and the kit (with power on) have electrical current available for use at any time. In homes, schools and businesses, separate switches control the lamps and the other output devices. The switches are called control devices. The MB100 kit has four type of control devices: a switch, a relay, a potentiometer, and a rheostat. The slide switch is very much like a wall switch in a building. Its turns output devices on and off. When you turn on a switch like this, it moves to complete an electrical circuit. When you turn the switch off, it breaks the circuit. With position can be on or off, depending on how you connect the power to the switch.

Slide Switch

Relay A relay is very similar to a switch. With the switch, you pushed a button to complete the circuit. In a relay, an electrical signal makes the relay complete the circuit. A relay uses two sources of electrical current. One source is used to operate the output device, and the second source is used to open and close the relay contacts (switch). When the second power source is supplied to the relay, a coil around an iron bar forms a magnet, just like in an electric motor. The magnet attracts an iron bar (armature), closing the contacts. The closed contacts allow electricity to flow to the output device. When the second power source is off, a spring opens the contacts and the output device turns off. Like a switch, both sets (top and bottom) of relay connectors in the kit are the same.

Relay

Advantages of using a relay The use of a relay makes it possible to control a circuit from a distance, since the primary or energizing circuit need not be located near the device controlled by the relay. The circuit controlled by the relay often draws large amounts of current. The relay makes it possible to control a circuit using a high current with a circuit using low current. This is used widely in automobile starting systems. The starting current in an automobile is high. If the ignition switch were connected directly to the starter motor, both the switch and the wires would have to withstand the extreme large current surge produced by the starting system. The switch would have to be very rugged in its construction. In addition, extra heavy wires would be required from the starter motor to the dashboard. However, if the relay is used instead, and placed near the starter motor, a low-current circuit may be used to energize the relay that starts the motor. As the exciter circuit is a low-current system, neither the switch nor the connecting leads need to withstand large currents. The relay controlled system makes it possible to position the ignition switch directly on the steering column without the danger of being a safety hazard to the driver.

Figure 14-6 illustrates the principal of the relay controlled automobile ignition system.

Variable Resistors Variable Resistor: Any resistor whose resistance can easily be changed without disconnecting the circuit. There are two main types of variable resistors: Potentiometer Rheostat

Potentiometer A potentiometer (“pot”) is a special kind of switch. In a regular switch, the power is either on or off. When it is on, the amount of electric current is always the same. A potentiometer is adjustable, meaning that you can set it to send more or less electric current, depending on what you want to do. A potentiometer consists of a resistor, which is supplied with power at its ends, and a moveable contact, which can be placed at any point along the surface of the resistance material causing the voltage to vary. Nearly all electronic instruments use potentiometers in some way. In radios and televisions sets, potentiometers are used directly to control sound volume.

Potentiometers A potentiometer in the MB100 Kit has two sets of connectors. The top set makes the potentiometer a simple on/off switch, just like the circuit switch. The bottom set of connectors allows you to adjust the amount of voltage in the circuit. You will use both sets in the experiments.

In the Figure to the you will see a potentiometer basically used as a voltage divider. The two end terminals are connected across a voltage, and the movable slider is adjusted for the amount of voltage needed for some particular use. If the input is 10 volts and the slider arm (point C) is adjusted midway between the end terminal (A and B), the output voltage is 5 volts. If the slider arm is moved upward, there is more voltage across the lamp, and it burns brighter. If the slider arm is moved downward, there is less voltage across the lamp, and the brightness diminishes.

Internal Potentiometer Rotating Wiper Shaft Resistive Material Terminals

Rheostat A rheostat is a device that regulates the strength of an electrical current by varying the amount of resistance in an electrical circuit. A rheostat is usually in the form of a tightly wound coil of wire over which a contact may be moved. The resistance is determined by the length of coil through which current must pass before entering the contact. A light-dimming switch is an example of the use of a rheostat.

Figure 8-9 is a schematic representation of a rheostat in a circuit Figure 8-9 is a schematic representation of a rheostat in a circuit. A rheostat is actually circular in shape, but the symbol is usually drawn as shown. Notice the rheostat has only two terminals for connections. The rheostat is being used to control current through the light bulb.

In Figure 8-9, a coil of wire having a total resistance of “R” ohms is wound between terminals A and the other end (B). Terminal C is connected to the movable contact. When C coincided with A, there is practically no resistance in the series circuit, and the lamp lights at its brightest, as shown in Figure 8-10.

When C coincides with B, there is maximum resistance in the circuit and the intensity of the lamp is minimum. This condition is shown in figure 8- 11. If resistance is high enough, the lamp may not glow at all. When C is at come point between A and B, as in Figure 8-9, the resistance in the circuit is that introduced by the coil between terminals A and C. the lamp glows with medium intensity.

Review Four type’s of control devices: a switch, a relay, a potentiometer, and a rheostat. Switch is a simple on/off operation. Relay an electrical signal completes the circuit. Variable resistors are those whose effective (or useful) values of resistance can be varied without disconnecting the circuit. Rheostats are variable resistors which have two terminals. They are used mainly for control of current. Potentiometers have three terminals and are used mainly for control of voltage. A potentiometer can be used as a rheostat by using the center terminal and one of the end terminals.