ma

Found 4 Results for ma.

Showing 1 to 4.

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.
http://www.fun-home-projects.com/electrical_projects/​tools/​the_volt_ohm_milliammeter.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

3. Appliance Leakage Tester
A very small leakage current normally flows from the electrical circuit through the insulation material to the exposed metal parts of electric appliances and power tools. Since the insulation material can break down due to age, wear, moisture content, or other reasons, it can allow higher leakage current to flow. This leakage current can become great enough to be lethal to anyone touching the metal surface of the appliance while at the same time contacting a grounded object such as a water pipe, radiator, etc., or when standing on a wet floor. Therefore, it is extremely important that the leakage current of ac appliances be measured before and after servicing or repairing. The leakage hazard is minimized when a three-way power cord is used with the appliance and is plugged into a polarized outlet whose third connector is properly grounded. The effectiveness of this arrangement dep>ends upon the wiring and upon the appliance having a good ground connection. The Underwriters Laboratories and the American National Standards Institute recommend that the leakage current of 120V ac appliances be limited as follows: appliances with two-wire power cords, 0.5 mA; portable appliances with three-wire power cords and third wire grounded, 0.5 mA; nonportable appliances with three-wire power cords and third wire grounded, 0.75 mA. Leakage current can be measured with an instrument such as the RCA WT-540A ac leakage tester. This instrument has no switches, dials, or other operating controls, its black test lead, which is terminated in a clip, is connected to a ground point. The red test probe has a sharp point to enable it to pierce dirt, grease, etc., and is touched to the metal parts of the appliance being tested The meter scale has only one range and is color coded red and green to indicate safe and unsafe leakage levels. If connected to an appliance that has extremely high leakage current or to one that is shorted, an internal fuse will blow. When this happens, a fuse-indicator lamp on the front panel will glow.
http://www.fun-home-projects.com/electrical_projects/​tools/​appliance_leakage_tester.html

4. Electric Food Grinder
Like the old-time hand-crank grinder, the food to be ground is fed into a hopper in the grinder head. It is ground by a large worm gear, emerging through the sieve at the lower left of the head. Power is supplied to the grinder by an electric motor in the power unit. Although the unit is electrically simple all components used must be heavy-duty because of the torque required to grind such foods as meat and gristle. If there is continuity from one of the male plugs to the other with the power switch closed, but the motor will not turn, check to see if there are bent or binding parts in the food-grinder head. Do this by removing the head and then attempting to run the power unit. With the head disconnected, the power unit should turn 90-140 rpm (revolutions per minute). If the motor still binds with the grinder head removed, the problem is in the power unit. Check within the power unit for bent or jammed components—try rotating the armature slowly by hand to see that it turns easily, without binding or strain. If continuity checks are good, and the armature turns freely, then the problem is probably in the motor. Check for burned, shorted, or open windings in the armature and the field coils. To check for leakage, with the switch closed apply 500V ac between both plugs connected together and exposed metal parts. Leakage current should not exceed 3 mA. With rated voltage applied, and the unit running, leakage current must not exceed 0.2 mA measured between all exposed metal parts and either side of the line.
http://www.fun-home-projects.com/electrical_projects/​repairs/​electric_food_grinder.html

Search Here:

Menu:
Home
Collecting
Crafts
Electrical Projects
Energy Savings
Hobbies
Home Ownership
Household Repairs