Electrical Projects 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:




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Home > Electrical Projects > Tools And Components > The Volt - Ohm - Milliammeter

November 20, 2008