Copyright Goodheart-Willcox Co., Inc. Chapter 17 Electrical Troubleshooting Fundamentals 379 6 circuit’s power supply, and the red lead is placed on the side of the load closest to the positive terminal of the cir- cuit’s power supply. Procedure Using an In-Line Ammeter A circuit must be opened before a technician can take a measurement with an in-line ammeter. Find a location in the circuit that has a wire that is in series with the electrical load to be measured and that can be easily disconnected. 1. To avoid shock or arcing, turn off power to the circuit. Test for voltage across the electrical load and any switches between the shutoff switch and the load. There should be no voltage in the circuit. If there is voltage, there may be a short in the circuit. 2. If there is no voltage, disconnect the previously located wire in series with the load to be tested. 3. Turn on the ammeter and adjust it to the setting for ac or dc amperes, depending on the type of circuit. Make sure the meter leads are plugged into the correct jacks on the meter. 4. Review any circuit diagrams to determine the expected current reading and set the range switch for the ammeter to the correct setting for the expected current level. 5. Connect one of the ammeter leads to the disconnected wire. If the ammeter only has probe leads, attach suitably sized, insulated alligator clips. 6. Connect the other ammeter lead to the terminal to which the disconnected wire was previously connected. 7. Gently wiggle the clips to ensure that both are securely attached. 8. Restore power to the circuit and read the measurement on the ammeter. 9. After taking any measurements, shut off the power, remove the ammeter leads, reconnect the disconnected wire to its terminal, and restore power to the circuit if Us A circ a mea in the ci electrical disconnect 1. To avoid circuit. Tes and any sw the load. The there is voltage 2. If there is no vol located wire in se 3. Turn on the ammete ac or dc amperes, de Make sure the meter le correct jacks on the met 4. Review any circuit diagra expected current reading a for the ammeter to the correc expected current level. 5. Connect one of the ammeter le disconnected wire. If the ammete leads, attach suitably sized, insula 6. Connect the other ammeter lead to t to which the disconnected wire was p connected. 7 . Gently wiggle the clips to ensure that bot securely attached. 8 8 . Restore p ower to the circuit and read the measurement m on th e ammeter. 9 . After f te taking any measurements, shut off the power, e r, remove the ammeter leads, reconnect the disconnected d ne w ire to its terminal, and restore pow t o t t he h circuit u it if aappropriate.riate.porpp Blowing an Ammeter’s Fuse When an ammeter is connected in series, current through the meter is limited by the total resistance in the circuit. If an in-line ammeter is accidentally connected in parallel, it may blow a fuse. This occurs because current flow is limited only by the resistance in the meter, which is a lot lower than the circuit’s resistance, resulting in higher current flow. Caution 17.1.4 Multimeters A multimeter is a single electrical meter that can measure multiple variables, such as voltage, current, and resistance. A multimeter is also known as a volt-ohm-milliammeter (VOM). Figure 17-7 shows a typical digital multimeter used by HVACR technicians. Most multimeters are digital instruments however, analog meters that use a needle for display are still in use. Some service technicians prefer analog meters, as they can quickly see the needle move as opposed to having to read a digital number. Beyond measuring voltage, current, and resistance, digital multimeters (DMMs) often come with a variety of functions, such as a diode check and continu- ity check, along with the ability to measure capacitance (in farads), and frequency (in hertz). In some cases, special attachments allow even more variables to be measured, such as temperature, wind speed, relative humidity, lumi- nance (light level), and acidity/alkalinity (pH), Figure 17-8. Diode Check A multimeter’s diode check function allows a technician to determine if a diode is functioning properly. This func- tion selection is usually indicated by the schematic sym- bol for a diode. The meter tests the diode by generating a potential difference between the leads. A properly operat- ing diode will conduct current in only one direction, which means the meter will register a voltage for only one of the two meter lead configurations. To check a diode, start by turning off power to the circuit and disconnecting at least one of the diode leads from the circuit. Leaving both diode leads in the circuit may result in false readings. When a meter registers a voltage on a properly Connecting Leads to Multimeter Terminals When taking various measurements with multimeters, it is extremely important to make sure the leads are plugged into the correct terminals on the multimeter. Refer to Figure 17-7. The red lead is always connected to one of the positive (+) terminals on the meter, and the black lead is connected to the common (–) terminal. Caution Diode Polarity A technician must be able to determine which end of a diode is the cathode and which is the anode. Generally, one end of a diode is marked with a line, stripe, ring, or other marking that indicates the side closest to the cathode. Pro Tip