You have got studied voltage and present measurements, however you will discover resistance measurements totally different in a number of ways. Resistance is measured with the circuit’s power turned off. The ohmmeter sends its own current through the unknown resistance after which measures that current to provide a resistance worth readout.
ROLE OF THE BATTERY
Even though it reads out resistance, the ohmmeter is still a current-measuring gadget at heart. The ohmmeter is created from a DC current meter by the addition of a gaggle of resistors (called multiplier resistors) and an inner battery. The battery provides the present flow that’s eventually measured by the meter. For this reason, use an ohmmeter only on de-energized circuits.
In the process of measuring resistance, the test leads are inserted in the meter jacks. The leads are then hooked up to the ends of no matter resistance is to be measured. Since current can flow either way through a pure resistance, there isn’t any polarity requirement for attaching the meter leads. The meter’s battery sends a current flow by the unknown resistance, the meter’s internal resistors, and the current meter.
The ohmmeter is designed in order that it will display 0 Ω when the test leads are clipped together (zero external resistance). The meter reads infinite (I) resistance or over limit (OL) resistance when the leads are left open. When a resistance is placed between the leads, the readout will increase in response to how a lot present that resistance allows to flow.
To conserve its battery, an ohmmeter ought to by no means be left on the ohms operate when not in use. Since the present available from the meter will depend on the state of charge of the battery, the DMM needs to be zero adjusted to start. This may require no more than a test of touching the two probes together.
Turn off energy to circuit.
Plug the black test lead into the widespread input jack. Plug the red or yellow lead into the resistance input jack.
Choose the resistance setting.
Touch the probe suggestions throughout the element or portion of the circuit.
View the reading and note the unit of measurement, ohms, kilohms, or megohms.
RESISTANCE MEASUREMENT PROCEDURES
Comply with the steps under to measure resistance:
Earlier than testing begins the technician ought to always know what reading to expect based mostly on the manufactures specifications, name plate rating, Ohms’s law and Kirchhoffs law. Testing blindly is harmful and counterproductive.
Turn off the ability and prove the circuit to be measured is «dead» utilizing the T3 testing technique and the voltage measuring procedures. Remember to wear your PPE as we always assume a circuit is «live» until proven in any other case
Remove or isolate the component to be tested.
Plug the test probes into the appropriate probe jacks, Common and Ω. Note that the jacks used often is the similar ones used to measure volts.
Select the ohms perform by turning the function switch to ohms. Start with the lowest setting.
Touch the probes together to check the leads, connections and battery life. The meter should display zero or a very small amount of resistance for the test leads. With the leads apart, the meter ought to display OL or I, relying on the manufacturer.
Join the information of the probes throughout the break within the component or portion of the circuit for which you wish to decide resistance. If you happen to get an OL (over limit), switch to the subsequent highest setting.
View the reading on the display unit. You’ll want to note the unit of measurement.
Turn the meter off when testing is full to prolengthy battery life.
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