How to measure low resistances?

There is a special case of a resistance measurement where the value of the component to be measured is small and the direct measurement with a multimeter / tester is impractical. The cases mentioned include:

• measuring the resistance of a track on a printed circuit board
• measurement of the resistance of a winding (resistance with all the turns of a coil or inductor)
• measuring the resistance of a fuse
• measuring the resistance of a relay
• measuring the resistance of a switch
• etc.

On this case, we will perform an indirect measurement using of a voltage source, an ammeter and a voltmeter.

Method to measure low resistances

We connect the power source Vb and the current limiting resistor R, as shown in the above figure. Thus, a relatively large current flows through the component to be measured (the track circuit, winding, etc.)

To achieve this, we use a voltage source of 10 volts (Vb) and a 20 ohms / 10 watts limiting resistor (R). In series with the circuit we put an ammeter.

This ammeter deliver the current that will flow trought the circuit. We measure the voltage drop across the component with the voltmeter and, using the Ohm’s law, we obtain the component resistance value (Rx). Rx = Vx/I

For example:

See the diagram above. The current value measured with the ammeter is 450mA and the voltage measured on the component is 10 mV, then the resistance of the component is:

Rx = 10mV/450mA = 0.0222 ohms

Note: This measurement method gives very low resistance values. Make sure that the tips to be used with the voltmeter have a very low resistance.

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