The role of multimeter in measuring resistance

There are two functions of using a multimeter to measure resistance. One is to measure the parameters of an unknown resistor, and the other is to verify whether the resistance value of a resistor or circuit meets the requirements.

1. Measuring unknown resistance

In daily work, we often need to know the resistance value of a resistor. For example, if the printed words on the resistor in hand are not clear, or if you are not familiar with the reading of the color ring resistor, you need to use the ohmmeter to measure the specific resistance value to avoid using it incorrectly.

Another case is to know the exact value of an adjustable resistor. A typical example is the adjustment of the transistor operating point.

Some circuits have more precise requirements for the collector current of transistors, especially the early germanium tubes have poor consistency, so most of them use potentiometers to adjust the bias resistance. When LC reaches the requirement, measure the resistance of the potentiometer and then replace it with a fixed resistor of the same resistance.

2. Judge the condition of electrical appliances or circuits by measuring resistance

This requires some understanding of the resistance of the object being measured, and to measure the resistance to see if it meets our expectations. For example, for the measurement of coils. For common 220V power transformers below 100VA, the primary coil DC resistance is generally between tens and thousands of ohms, and the lower the wattage, the higher the resistance. If the measured resistance is only a few Ω or more than tens of KΩ, it means there is a short circuit or open circuit fault.

The power can also be estimated by measuring the resistance of the electric heating appliance according to P = U² / R. For example, the DC resistance of an electric kettle is 27.5Ω, so the power P is 220²/27.5Ω = 1760W. Considering the power drop caused by the error and thermal resistance, the nominal power can be determined to be 1700W.

In addition, the resistance can be measured to determine whether the circuit or wire is conductive.

However, there are some components that do not have exact resistance values, typical examples of which are semiconductor devices and thermistors and photosensors.

Because semiconductors are nonlinear, the resistance values ​​measured by different instruments or different gears of the same multimeter vary greatly, so only qualitative measurements can be made, and it is difficult to obtain accurate quantitative indicators. For example, the greater the difference between the forward and reverse resistances of a diode, the better. The resistance of a thermistor or photosensitive device can change with temperature or light, etc.