The Virtual Repairman's Technical Reference

 

 

 

 

 

 

How to check resistance (continuity) in a component-

A Digital Multi-meter  (Ohmmeter) is needed for testing components:

Circuit diagram of an ohmmeter connected across a resistance, R.An ohmmeter can be used to directly measure an unknown resistance R.

This is the simplest way to measure resistance.  It is important to realize

that an ohmmeter can not be used when the resistance is connected in a circuit and a current is flowing through it!

 

 

Typical Digital Multimeter

Diagram of a typical digital multimeter for measuring voltage, current and resistance

 

Scale Measurement Range
in words ohms (Ω)
2M two megohm
two million ohms
0-2,000,000
200K two hundred kilohm
two hundred thousand ohms
0-200,000
20K twenty kilohm
twenty thousand ohms
0-20,000
2K two kilohm
two thousand ohms
0-2,000
200 two hundred ohm 0-200

 

 

Setting up the meter

To use the multi-meter as an ohmmeter turn the selector dial to point to the ohms range. On some models (usually not on the cheaper ones) the meter will have a single ohms setting and will auto select the best range for you.

To measure resistance you must connect the test leads to the sockets labeled 'COM' and 'Ω'. These may be in different positions on your meter but the labeling should be the same.

Without anything connected between the test leads the display will give an out of range indication (-1 in this case) to show that the resistance is too large to measure. In effect, you are trying to measure the resistance of the air between the two inputs, which is very large.

 

Taking measurements

To check the meter is working touch the leads together and the display should read 0.0 Ω. It may read slightly higher, e.g. 0.3 Ω. This is the resistance of the test leads themselves.

Ohm meter and resistance wire

Turn the selector dial to the lowest resistance range that gives a valid reading.

In this case the resistance wire has a resistance of 4.2 Ω.

 

Accuracy

The overall accuracy of the reading will depend upon the multi-meter model. The more expensive the meter the better the accuracy will be. For a cheap meter the accuracy is typically in the region of +/- 1% of the displayed value. Check in the manual for the accuracy of your model.

It is important to select the correct measurement range. In the above example if the 2 kΩ range is selected the display will read .004 kΩ and the decimal place will be lost. If the dial is set to 20 kΩ the display may read 0.00 kΩ! Always turn the dial down to the lowest range to get the best accuracy.

 

OHM's Law- How to calculate resistance and voltage using meter readings:

 

 

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