Resistance Measurement — Common Mistakes

Resistance Measurement — Common Mistakes

Published by Arun Isaac on

Tags: electronics

A few common and silly mistakes people might make when they are learning to use a multimeter to measure resistances…

A bunch of resistors of different colors and sizes

Figure 1: Colorful resistors

First of all, the most common mistake is attempting to use the multimeter/ohmmeter to measure the resistance of a circuit element when the circuit element is connected as part of a larger circuit. The resistance reading you get corresponds to not simply the resistance of the circuit element you intended to measure, but rather the equivalent resistance after considering every other circuit element in parallel to it. So, while measuring resistance, one should always disconnect the circuit element of interest from the rest of the circuit and measure the resistance.

One common place where one might make this mistake is when measuring the resistance of potentiometers. One might have adjusted the potentiometer in a circuit and might want to know the new resistance. In such a case, one must not forget to disconnect the potentiometer before attempting to measure its resistance.

And, another common mistake is to measure the resistance with your hands touching the leads of the circuit element. For small resistances, this shouldn't matter much as your body's resistance would be too high to affect the reading. But, for higher resistances (in the tens of kohms and above), the resistance of your body will no longer be negligible, and the overall resistance reading will be that of the parallel combination of the circuit element and your body.

In fact, if the resistance of the circuit element is known, one might even use this method to estimate the resistance of one's body. Also, it is in general not advisable to touch any part of a powered up circuit in operation, especially when it involves high valued resistances. One might inadvertently end up reducing the overall resistance and thus affecting the operation of the circuit.

Two frames side by side -- one showing a wrong way to measure resistance using a multimeter, and the other showing the correct way. The wrong way shows a person's fingers touching the resistor leads when measuring the resistance. The correct way shows the resistor inserted into a breadboard with only the multimeter probes making contact with the resistor leads. The first wrong measurement technique shows a multimeter reading of 45.4 kohm, while the second correct measurement technique shows a multimeter reading of 55.7 kohm.

Figure 2: Do Not Touch Resistor Leads!

And, perhaps the final more or less obvious thing would be that you cannot measure the "resistance" of a non-linear element (such as a diode or a transistor). Non-linear elements do not obey Ohm's law, and there exists no constant value which corresponds to the "resistance" of a non-linear element. Any measure of resistance is obtained as a ratio of the voltage across the element and the current through the element.

Speaking of the "resistance" of non-linear elements, ohmmeters can be used to identify the P and N sides of a diode. When the ohmmeter forward biases the diode, it reads a small value of resistance. When reversed, the ohmmeter reads a large value of resistance, possibly open circuit. Similar techniques can also be used for transistors.

Two frames side by side -- one showing an ohmmeter reading of a forward biased diode and the other that for a reverse biased diode. In the first frame showing the forward biased diode, the ohmmeter reads 1539 ohms. In the second frame showing the reverse biased diode, the ohmmeter reads an open circuit.

Figure 3: Diode Polarity Check!

Thanks to Prabhu for helping me with the photos!

Image Credits