Measuring Current

4-10 5. Turn on the power in the test circuit. Read the output voltage on the appropriate ac voltage scale. For the 2.5V range, read the value directly on the scale marked 2.5. For the 10V, 50V, or 250V range, use the red scale marked AC and read the black figures immediately above the scale. Measuring Current The multimeter can function as an ammeter to measure current flow. CAUTION When using the multimeter as a current-indicating instrument, NEVER connect the test leads directly across a voltage. ALWAYS connect the instrument in series with the load. To use the multimeter as an ammeter, you must take the following steps: 1. Set the function switch at +DC (assuming the current to be positive). 2. Plug the black test lead in the COMMON jack and the red test lead into the + jack. 3. Set the range switch at one of the five ampere-range positions. 4. Ensure the equipment is OFF and then physically open the circuit in which the current is being measured. 5. Connect the VOM in series with the circuit, ensuring that proper polarity is observed when making this connection. 6. Turn the equipment ON and then read the current on the DC scale. (This is the same scale used to measure dc voltages.) The setting of the range switch determines the maximum value represented by the DC scale. Always use the range scale that corresponds to the range switch setting. CAUTION Never attempt to measure currents greater than the setting of the range switch. Increase the range with a shunt, if necessary, but do not exceed the marked current. When measuring unknown currents, follow the same procedures as when measuring voltages. Always start with the highest range available and work down. Use the range that gives approximately half-scale deflection. If this procedure isn’t followed, the meter could be burned out. Figure 4-7 is a functional block diagram of the dc current circuits in a multimeter.