10v

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1. The Volt - Ohm - Milliammeter
The most widely used of all test instruments is the volt-ohm-milliammeter, more commonly known as the VOM. You won't find a repair shop without one. Of the VOM's three measurement functions, the ohmmeter circuit is most often used, since this is the one which can give an indication of continuity. The two selector switches on the face of the meter must be set properly to achieve accurate measurement. The left-hand selector switch designates the function (voltage, current, or resistance measurement), while the right-hand switch is used to select the range appropriate for the quantity being measured. When checking continuity, the function switch must be set in the ohms position, and the range should be set for one of the lower scales (R X 10). Since you are not after a precise resistance reading at this point, it is important only to have the range setting low enough to indicate when there is some internal resistance within motor-powered appliances (caused by the windings on the armature). Although you are not concerned with an accurate resistance measurement at this point, it is important that you "zero" the meter before making any continuity checks so that a shorted circuit will be apparent. To zero the meter, connect the test leads to their appropriate jacks on the face of the meter—plus is red and minus (common) is black—and while touching the leads together, adjust the zero control for a 0 reading on the ohms scale. If touching the leads together doesn't cause deflection of the blade indicator, or if the indicator won't adjust to a zero position, the meter battery is weak or making poor contact. Once you have zeroed the meter, you can begin checking for continuity. Since you want each check to be as informative as possible so that the problem can be quickly isolated, start by checking continuity through the power cord of the appliance. With the appliance power switch in the on position, apply the test leads to the prongs of the power plug (if the plug is three-prong, disregard the ground lug) and observe the meter indication. A good circuit should give a low reading on the ohms scale. A zero reading—maximum deflection—inicates that the circuit is shorted at some point. No deflection at all means that the circuit is open and will not permit current flow. Make sure that your appliance switch is on, however, before you make further checks. By gradually working your way through the circuitry you can easily locate the defective component or connection. The ohmmeter circuitry of the VOM is nothing more than a simple milliammeter circuit with a variable (zero control) resistor (Rz) and test-lead connecting points (X and Y) to introduce a resistor of unknown value into the circuit. When 10V is applied to the circuit, Rz should be of a value which can restrict current flow to 1 mA, or maximum meter deflection. We can determine this value by Ohm's law: R = E /1 = 10 / 0.001 = 10,000 ohms Allowing for internal resistance of the meter movement (roughly 1000 ohms), the formula indicates that Rz must be adjustable within the range of 9K ohms. Thus, any additional resistance introduced into the circuit will impede current flow even more and result in less than full deflection of the meter. Hence, what is basically a current indication reflects resistance values on the ohms scale.
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2. Hand - Held Dryer
The newest and smallest of the many types of hair dryers is hand-held dryer consisting of a simple circuit employing power switch, safety thermostat, heating elements, and rectifier unit to operate the dc motor. Most problems are in the thermostat or in the blower motor. The motor operates on approximately 9V dc. The heater is used as a resistor to drop the voltage to approximately 9V ac and feeds the rectifier to produce the 9V, dc. Since motor operation is dependent upon the heater, there is no cool position, only on or off. The air flow is provided by a tangential fan which is quiet and self-cleaning. A safety thermostat is mounted adjacent to the heater which will open when a higher than normal temperature is reached, such as caused by blocking the inlet or outlet with the hand. Troubleshooting When testing through the circuits to discover a defective part, open the soldered connections to isolate the part and obtain a true test. All parts can be cold tested with an ohmmeter. The heater element can be checked visually. However, with the motor disconnected the safety thermostat will quickly open; a resistance test is more reliable/Since the feed divides the heater element, the two common wires should be disconnected from the cord terminal and each side of the element tested. A resistance reading of 50 ohms or more indicates a good element. Also check the ac feed from the element to the rectifier for a good connection. The voltage feeding the rectifier should be tested by a parallel voltage test across the opposite line connections of the rectifier. This will verify a power supply from the heater element of approximately 19V ac. If the motor does not run, a quick reading must be taken as the safety thermostat will open. A voltage test on the output terminals of the rectifier should read 7—10V dc. A resistance test on the output terminals of the rectifier should read slight resistance one way and full resistance when polarity is reversed If both readings are the same, the rectifier is defective. The rectifier should always be suspected in case of a voltage drop which betrays itself as a loss of motor rpm. Therefore, if a motor is slow, always aheck the dc voltage output of the rectif i r. The dc motor should have a resistance reading of 4 to 6 ohms—disconnected from the rectifier. A very high resistance or zero resistance indicates a defective motor. Also check the wiring connections to the motor. A hot-line voltage of 10V dc is required to test the motor. A "dead" unit can be quickly tested with the upper body off and without further disassembly, by checking: 1. Cord terminals 2. Switch 3. Ac rectifier terminals 4. Safety thermostat 5. Dc rectifier terminals Disassembly Remove the front trim, two screws from the upper body, and one screw from the lower body. Lift the upper body off to expose the component parts fastened to the lower body. All electrical parts can now be tested. An electric soldering pen should be used on the solder connections. The cord wires go under the switch panel and secure to the cord terminals. By removing the screws in the switch, the switch panel with rectifier attached can be- lifted out. The rectifier is secured to the switch panel by a clip. The heater element and safety thermostat can be lifted out as an assembly. The safety thermostat is a normally closed switch. A spring clip secures the motor. The motor shaft has a friction fit into the fan. Do not drop any small object into the fan chamber as the fan may throw it through the heater element and damage it.
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3. Cordless Appliances
With the development of sealed-cell rechargeable batteries and miniaturized chargers, a large number of appliances have come to be known as "cordless." When these appliances malfunction, the problem is usually low battery voltage. See Chapter 2 for a discussion of chargers and charging techniques. We will use Disston's grass shears as a representative example of this class of appliances. Troubleshooting The shear runs below normal speed. Check the charger output with a VOM. The reading should be between 2.3 and 3.6V. If not, replace the charger since repairs are impractical. Next, check the battery pack by putting it on charge overnight. If the shear still does not develop normal speed and power, replace the battery pack. Remove the terminal assembly from the shear and inspect for bad or intermittent connections. There should be a potential of 10V between the positive and negative terminals. Wiggle the terminals as you make the reading. No voltage means that the wiring is broken and needs replacement. The shear operates for only a short period (less than 45 minutes) between charges. The first thing to look for is binding in the mechanism which would cause the motor to draw more current. Remove the blades and check for any deflection or obstruction in the gear teeth. Check to make certain the blades are not set too close together. The upper blade teeth should barely contact the lower blade teeth. With the blade mechanism installed, the motor should draw between 1.4 and 2A. Replace the blade assembly if the mesh is off, the blades are dull, or the meter reading fails to stay within specifications. Next, check the motor for faulty operation. Remove the motor from the shear. Check with a known-good battery pack. The current reading ought to fall between 0.9 and 1.2A. Should the reading be higher, replace the motor. Noisy operation may be traced to wear on the gear train, cam, or ring insert. If the gears are still good and the blades sharp, replace the ring. The armature side-to-side play should be no more than 1 / 32" and preferably less. Individual spares are not available; replace the motor as a unit. If the blades jmain motionless while the motor runs, check for stripped gear teeth and for a loose drive pinion. A loose pinion means that the motor and pinion must be replaced. No blade movement and no motor sound usually means a failure in the electrical circuit. Check the battery pack, wiring harness, switch, and motor. The switch contacts should arc slighdy as they are opened. Disassembly Disassembly is not difficult. Take out the three housing screws from the right side of the housing. Separate the parts. Next, remove the lock-button assembly by lifting the lock button out of its slot in the switch plate. This frees the switch button from its slot in the switch-button plate. Remove the contact switch from the shear by disconnecting the positive (red) battery wire from the top leaf of the switch and the positive motor wire from the bottom leaf. Now remove the terminal-board assembly by disconnecting the battery and motor leads. Take out four blade screws from the blade-base housing and remove the blade assembly from the base. To separate the blades, turn the top one 90 degrees to the bottom one and push the blade insert out from between them. Remove the large cam gear and the small combination gear from the blade-base housing . Then remove the motor-mounting screws and free the motor.
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