7.2.3

1. 7.2.3 Identifying Procedures for Measuring Resistance Between Two Given Points in an Electrical Circuit Continuity tests of coils and similar components can help identify a faulty appliance part. Other tests for resistance are useful tools for troubleshooting individual appliance components. In this module, you will learn to: • Apply safety precautions while measuring electrical resistance • Identify electrical resistance • Identify variable resistors and their functions • Connect the meter to measure resistance values

2. In this module, you will learn to: • Identify the operating principle of an ohmmeter circuit • Zero Ohms adjust: analog meters • Zero Ohms check: digital meters • Measure resistance values • Perform a circuit continuity test

3. Applying safety precautions while measuring electrical resistance Measuring resistance in circuit components calls for great care and is an exacting operation. If fundamental rules are violated, damage can be done to the customer’s equipment, your multimeter, and possibly to yourself.

4. Identifying electrical resistance • Copper offers little opposition to electrical current flow, while rubber practically blocks electrical current flow. This opposition to current flow is referred to as resistance (R). • Sometimes it is necessary to include a resistance in a circuit to limit electric current flow. Devices used for this purpose are called resistors. • There are two basic types of resistors according to their control function: • (1) Fixed resistor • (2) Variable resistor • The unit of measurement for resistance is the ohm, with the symbol: Ω. • The schematic symbol for a fixed resistor is:

5. Identifying variable resistors and their functions The resistance value of a variable may be varied from zero to some fixed maximum value. The schematic symbol for a variable resistor is: Variable resistors are useful in control circuits where a variance in output of an electrical device is desired. Some variable resistors specifically designed for this purpose are called potentiometers. A familiar application is a volume control for audio speakers on older radios and televisions. The heat anticipator in electro-mechanical appliance thermostats is a combination of a fixed resistor with a variable resistor.

6. Connecting the meter to measure resistance values Both analog and digital meters can be used to measure resistance value. Unlike the other functions of the multimeter, the ohmmeter uses voltage source a built into the meter, a battery. The ohmmeter should never be connected to any outside electrical source.

7. Figure 1. Basic Ohmmeter Identifying the operating principle of an ohmmeter circuit The operating principle of the ohmmeter is to force a current to flow through an unknown resistance, then measure the current. Applying Ohms Law for a given voltage, the current flow will be determined by the resistance in the circuit. The purpose of the battery is to force current through the unknown resistance. The fixed resistor R1 limits the current through the meter to a safe level. Variable resistor R2 is called the ZERO OHMS adjustment, and is part of the multimeter. The purpose for the ZERO OHMS adjustment is to compensate for voltage changes because of battery aging.

8. Figure 2a. Initial Reading Figure 2b. Zero Adjustment Reading Zero Ohms adjusting: analog meters Each time the ohmmeter is used, check the zero indication on the meter before measuring resistance. Check and adjust it each time the switch is positioned to a different range. To set the ZERO OHMS controls on an analog meter, proceed as follows: • Set the range switch at one of the resistance range positions, switch at either ‑ DC or + DC. • Connect the black test lead in the COMMON ‑ jack and the red test lead in the + jack.

9. Clip the contact end of the test leads together to short out the resistance circuit. • Observe the meter indication. It should read zero on the right hand end of the Ohms arc, which is at the top of the dial. • If the pointer does not read zero, rotate the ZERO OHMS knob until it does. If the pointer does not move far enough to read zero, one or more batteries need to be replaced. • When the pointer shows zero, unclip the shorted test leads. The ohmmeter circuit is now ready to measure resistance.

10. Zero Ohms check: Digital Meters Digital meters will typically not display a zero reading when the test leads are shorted out. Check the manufacturer’s instructions to determine the specified readings to expect. Figure 3. Digital Meter “0” Ohms Display According to the manufacturer’s specifications for the meter shown in Figure 3, the display should show a reading  0.2 with test leads touching (shorted).

11. Measuring Resistance Values Remember: Always follow equipment and meter manufacturer's instructions. To measure resistance with an analog meter proceed as follows: • Turn off the power to the circuit. • Set the range switch in one of the resistance range positions. • Use R X 1 for resistance readings from 0 to 200 ohms. • Use R X 100 for resistance readings from 200 to 20,000 ohms. • Use R X 10,000 for resistance readings above 20,000 ohms. • Set the function switch at either ‑ DC or + DC. • Connect the black test lead in the COMMON ‑ jack and the red test lead in the + jack. • Short the test leads together and adjust for zero ohms.

12. To measure resistance with an analog meter proceed as follows: • Separate the test leads; then connect them across the resistance to be measured. If there is a "forward" and "backward" resistance such as in rectifiers, switch back and forth between the two DC positions of the function switch to reverse the polarity. • Read the indication on the OHMS arc at the top of the dial. Note this arc reads from right to left for increasing values. • Multiply the readings by the multiplier factor at the switch position for resistance value in ohms. "K" on the dial stands for "thousandths".

13. To measure resistance with an digital meter proceed as follows: • Turn off the power to the circuit. • Select resistance on the meter dial (). • Plug the black test probe into the COM input jack. Plug the red test probe into the  input jack. • Connect the probe tips across the component or portion of the circuit where you want to determine resistance. • View the reading, being sure to note the unit of measurement—ohms (), kilohms (k), or megohms (M). NOTE: 1,000 = 1 k 1,000,000 = 1 M

14. Performing a circuit or component continuity test A continuity test is a quick go/no-go resistance test that distinguishes between an open and a closed circuit. As with all resistance measurements, a continuity test must be made with the power disconnected. When you suspect a particular component to be faulty and which should have low resistance (such as a normally closed switch, or a motor winding or solenoid coil)—a continuity test can be a very useful diagnostic method. Likewise if you suspect a problem in connecting wiring or a connector, a continuity test may be a great time saver. A continuity test should produce a zero or near-zero ohms reading when the meter probes are connect to each side of the component or circuit wiring if the component or circuit allows continuous electrical flow, (the normal condition for a coil winding). A break in the coil winding or circuit wiring will give the opposite reading (infinite resistance).

15. Time to See If You Got the Key Points of This Module… • Complete the Review on pages 7 - 9. • See if you are ready for the Certification Exam by checking off the performance criteria on page 10.