Introduction to Resistance Measurements

You may have studied voltage and current measurements, however you will discover resistance measurements totally different in several ways. Resistance is measured with the circuit’s power turned off. The ohmmeter sends its own present by way of the unknown resistance and then measures that present to provide a resistance value readout.


Regardless that it reads out resistance, the ohmmeter is still a present-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 internal battery. The battery provides the current flow that’s ultimately 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 attached to the ends of no matter resistance is to be measured. Since present can flow either way through a pure resistance, there is no polarity requirement for attaching the meter leads. The meter’s battery sends a current flow through the unknown resistance, the meter’s inside resistors, and the current 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 present that resistance allows to flow.

To preserve its battery, an ohmmeter ought to never be left on the ohms operate when not in use. Since the current available from the meter depends upon the state of cost of the battery, the DMM ought to be zero adjusted to start. This might require no more than a test of touching the two probes together.

Turn off power to circuit.

Plug the black test lead into the widespread enter jack. Plug the red or yellow lead into the resistance input jack.

Choose the resistance setting.

Contact the probe ideas across the part or portion of the circuit.

View the reading and note the unit of measurement, ohms, kilohms, or megohms.


Follow the steps below to measure resistance:

Before testing begins the technician should always know what reading to expect based on the manufactures specs, name plate score, Ohms’s law and Kirchhoffs law. Testing blindly is harmful and counterproductive.

Turn off the power and prove the circuit to be measured is “dead” utilizing the T3 testing methodology and the voltage measuring procedures. You’ll want to wear your PPE as we always assume a circuit is “live” till proven in any other case

Remove or isolate the part to be tested.

Plug the test probes into the appropriate probe jacks, Widespread and Ω. Note that the jacks used could be the same ones used to measure volts.

Choose the ohms perform by turning the operate switch to ohms. Start with the lowest setting.

Contact the probes together to check the leads, connections and battery life. The meter should display zero or a really small amount of resistance for the test leads. With the leads apart, the meter ought to display OL or I, relying on the manufacturer.

Connect the ideas of the probes throughout the break within the component or portion of the circuit for which you need to decide resistance. Should you get an OL (over limit), switch to the subsequent highest setting.

View the reading on the display unit. You should definitely note the unit of measurement.

Turn the meter off when testing is full to prolengthy battery life.

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