You have studied voltage and present measurements, but you will discover resistance measurements different in several ways. Resistance is measured with the circuit’s power turned off. The ohmmeter sends its own current via the unknown resistance after which measures that present to provide a resistance value readout.
ROLE OF THE BATTERY
Though it reads out resistance, the ohmmeter is still a current-measuring machine at heart. The ohmmeter is created from a DC present meter by the addition of a gaggle of resistors (called multiplier resistors) and an inner battery. The battery provides the current flow that is ultimately measured by the meter. For this reason, use an ohmmeter only on de-energized circuits.
Within 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 present can flow either way by a pure resistance, there is no polarity requirement for attaching the meter leads. The meter’s battery sends a present flow via the unknown resistance, the meter’s inner resistors, and the present meter.
The ohmmeter is designed in order that it will display zero Ω when the test leads are clipped collectively (zero exterior resistance). The meter reads infinite (I) resistance or over limit (OL) resistance when the leads are left open. When a resistance is positioned between the leads, the readout increases in response to how a lot current that resistance permits to flow.
To preserve its battery, an ohmmeter ought to by no means be left on the ohms function when not in use. For the reason that present available from the meter relies on the state of charge of the battery, the DMM ought 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 common input jack. Plug the red or yellow lead into the resistance input jack.
Choose the resistance setting.
Contact the probe suggestions across the element or portion of the circuit.
View the reading and note the unit of measurement, ohms, kilohms, or megohms.
RESISTANCE MEASUREMENT PROCEDURES
Follow the steps below to measure resistance:
Earlier than testing begins the technician should always know what reading to count on based mostly on the manufactures specifications, name plate ranking, 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“ using the T3 testing methodology and the voltage measuring procedures. Make sure you 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, Widespread and Ω. Note that the jacks used may be the same ones used to measure volts.
Select the ohms operate by turning the perform switch to ohms. Start with the bottom setting.
Touch the probes collectively to check the leads, connections and battery life. The meter should display zero or a really small quantity of resistance for the test leads. With the leads apart, the meter ought to display OL or I, relying on the manufacturer.
Connect the information of the probes across the break within the element or portion of the circuit for which you need to decide resistance. For those who get an OL (over limit), switch to the subsequent highest setting.
View the reading on the display unit. Be sure to note the unit of measurement.
Turn the meter off when testing is full to prolengthy battery life.
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