How to measure the resistance of a delicate or sensitive component?
There is a special way of resistance measurement when the component to be measured is very sensitive or delicate. The component to be measured could be the coil that is used in analog multimeters and that moves the needle on the front of the instrument.
There are more components that have delicate resistors, such as analog VU meters, among others. In our case, we will use the example of an analog multimeter coil.
Directly measuring the resistance of this type of coil with a digital multimeter or an analog multimeter may cause the coil to become unusable. In this case, an indirect measurement is made with a voltage source, an ammeter and a voltmeter.
How to measure the resistance?
To carry out this measurement, it is necessary to have two multimeters available, one working as an ammeter and the other as a voltmeter. Better if they are digital multimeters, because of the accuracy they have.
Connect the power supply and a high value current limiting resistor, as shown in the image. This will pass a small current through the coil. To achieve this, a 5 volt voltage source and a 100K limiting resistor can be used.
- In series with the circuit the ammeter is placed, from where the value of the current that circulates throughout the circuit will be obtained.
- With the voltmeter, the voltage drop between the ends of the component is measured.
- With the measured voltage and using Ohm’s law, the resistance of the component (Rm) is calculated. Rm = V/I.
Example of calculating the value of a sensitive resistor
Looking at the graph above,
- The value of the current measured with the ammeter is: 60 uA.
- The value of the voltage measured on the component is: 70 mV
Using the formula: Rm = V/I
The resistance of the component is: Rm = 70 mV / 60 uA = 1166.67 ohms = 1.167 Kilo ohms.