Chapter 21 Electrical Test and Measurement Equipment 405 Before taking a measurement with a DMM, select the appropriate type of measurement and range using the mode selector switch. On many DMMs, the mode selector switch either lacks or has few range values. See Figure 21-11. Th e DMM automatically determines the correct range without operator intervention. Th is auto- matic action is referred to as auto-ranging. As a result, you must be careful when reading the value in the dis- play. At fi rst glance, 100 mV might appear to be 100 V. Always check the suffi x on the reading to verify that you are interpreting the displayed value correctly. When using a DMM, fi rst set the mode selector switch to the correct setting for the type of measurement. Note that voltage and current normally have diff erent mode set- tings for AC and DC. With the mode set, plug the leads into the corresponding input jacks. As mentioned earlier in this chapter, always check the voltage rating, current rat- ing, and CAT level listed on the DMM to ensure that the device is appropriate for the expected measurement. When measuring current, position the test leads so the DMM is connected in series with the load. See Figure 21-13. S A F E T Y N O T E DMM Input Jacks A common and dangerous mistake is to attempt a voltage measurement with the test leads plugged into the current input jacks. When confi gured for measuring current, the DMM creates a direct short to the voltage source. Always check to ensure that the test leads are connected correctly for the type of measurement being made, Figure 21-12. Maximum current ratings Voltage ratings and CAT levels Input jack for voltage, resistance, temperature, and diode testing Current in microamps Input jack for current measurements Current in amps or milliamps Diode test Resistance or capacitor test DC millivolts or temperature DC voltage AC voltage Goodheart-Willcox Publisher Figure 21-11. Most digital multimeters (DMMs) have features similar to those shown here. Goodheart-Willcox Publisher Figure 21-12. A digital multimeter with the test leads appropriately connected to measure voltage or resistance. Note the two current jacks to the left of the test leads. ES = 48 V R1 = 25 Ω 1.92 A OFF A COM V Ω A V Ω Goodheart-Willcox Publisher Figure 21-13. A DMM connected for measuring current. When measuring current, the DMM must be connected in series and set to the proper range. Never attempt to place the meter probes across the voltage source. Copyright Goodheart-Willcox Co., Inc.