You’ve got studied voltage and present measurements, however you will find resistance measurements totally different in several ways. Resistance is measured with the circuit’s power turned off. The ohmmeter sends its own present through the unknown resistance and then measures that current to provide a resistance worth readout.
ROLE OF THE BATTERY
Regardless that it reads out resistance, the ohmmeter is still a current-measuring device at heart. The ohmmeter is created from a DC current meter by the addition of a gaggle of resistors (called multiplier resistors) and an internal battery. The battery provides the present flow that is 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 within the meter jacks. The leads are then connected to the ends of no matter resistance is to be measured. Since present can flow either way via 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 inside resistors, and the current meter.
The ohmmeter is designed in order that it will display 0 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 placed between the leads, the readout will increase in keeping with how much present that resistance permits to flow.
To preserve its battery, an ohmmeter should by no means be left on the ohms operate when not in use. For the reason that present available from the meter is determined by the state of cost of the battery, the DMM must be zero adjusted to start. This may require no more than a test of touching the 2 probes together.
Turn off power to circuit.
Plug the black test lead into the frequent enter jack. Plug the red or yellow lead into the resistance enter jack.
Select the resistance setting.
Contact the probe ideas across the component 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 ought to always know what reading to anticipate primarily based on the manufactures specs, name plate rating, Ohms’s law and Kirchhoffs law. Testing blindly is dangerous and counterproductive.
Turn off the power and prove the circuit to be measured is «dead» using the T3 testing method and the voltage measuring procedures. Be sure to wear your PPE as we always assume a circuit is «live» till proven otherwise
Remove or isolate the part to be tested.
Plug the test probes into the appropriate probe jacks, Common and Ω. Note that the jacks used may be the same ones used to measure volts.
Choose the ohms function by turning the function 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 very small quantity of resistance for the test leads. With the leads apart, the meter ought to display OL or I, relying on the manufacturer.
Join the ideas of the probes across the break within the element or portion of the circuit for which you need to determine resistance. In the event 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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