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Your VOM will have one jack labeled (-) COM, into which the Black lead will be inserted. Insert the Red lead into the jack marked (+) V - - A. this same setup is used to measure AC volts, DC volts, and resistance.

Taking measurements

AC volts - Set the dial to the appropriate VAC scale. For example, if 117 volts is expected to be measured, the dial must be set to a scale above that. WARNING - if the dial is set to the 50 ACV range and 117 volts is measured, damage to the VOM is likely. If the voltage is unknown, start with the dial on the highest available setting.

DC volts - As above, the dial must be set to the appropriate scale. in addition be sure that the polarity is correct. DC voltage has a positive (+) and a negative (-) terminal. The red lead is (+), and the black is (-), connecting the leads in reverse can cause damage to the VOM. If, from the rest position, the needle moves to the left, the polarity is incorrect; reverse the leads on the circuit that is being tested and take the measurement again.

Resistance - Important, to prevent VOM damage the circuit power must be turned off. There are several scales on the VOM, usually Rx1, Rx10, Rx100, etc. When changing the dial settings, it is necessary to multiply the uppermost scale by the dial setting. For example, if the needle is pointing to 15 on the ohms () scale, and the dial is set at Rx1, then there are 15 of resistance (15 x 1 = 15). If the dial is set on Rx10 then there are 150 of resistance (15 x 10 = 150). If the dial is set on Rx100, then there are 1500 of resistance (15 z 100 = 1500).

Whenever resistance is measured, the VOM must be calibrated. This is a very simple procedure. Set the VOM to the correct scale (usual Rx1). Touch the two probes together and use the ohms adjust wheel to set the needle to zero. Keep in mind that zero ohms is on the right hand side of the scale (this would indicate a short) and the left side is infinite resistance (indicating an open). Start with the dial on the lowest setting (Rx1). If the reading shows infinite, change the dial to the next higher setting. Keep increasing the setting until an accurate reading is obtained. If the needle stays to the left side of the scale then the circuit is open.

Knowing how to operate a VOM will save considerable time when testing the wiring on a job site. It is possible to inspect the solenoid and field wiring condition from the controller. The procedure is as follows:


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