The role of multimeter to measure resistance
The multimeter has two functions to measure the resistance, one is to measure the parameters of the unknown resistance, and the other is to verify whether the resistance value of a certain resistance or circuit meets the requirements.
First, measure the unknown resistance
In daily work, we often encounter the need to know the resistance value of a certain resistor. For example, the words printed on the resistor in your hand are not clear, or you are not familiar with the reading of color ring resistors, so you should use the ohm gear to measure the specific resistance value to avoid wrong use.
Another situation is knowing the exact resistance value of the adjustable resistor. More typical is our adjustment to the transistor operating point.
Some circuits have more precise requirements for the collector current of the transistor, especially the early germanium tube has poor consistency, so most of the potentiometers are used to adjust the bias resistance. When the lc meets the requirements, measure the resistance of the potentiometer. Then replace it with a fixed resistor of the same resistance.
2. Judging the condition of the electrical appliance or circuit by measuring the resistance value
This requires some understanding of the resistance of the object to be measured, and by measuring the resistance to see if it meets our expectations. Such as the measurement of coils. For common power transformers below 220 volts and 100 VA, the DC resistance of the primary coil is generally between tens to thousands of ohms, and the smaller the wattage, the higher the resistance. If the measurement is only below a few Ω or more than tens of KΩ, it means that there is a short circuit or open circuit fault.
You can also estimate its power by measuring the resistance of the electric heating appliance and according to P=U²/R. For example, the DC resistance of an electric kettle is 27.5Ω, then 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, it is also possible to judge whether the circuit or wire is conducting by measuring the resistance.
However, there are also some components that do not have an exact resistance value, such as semiconductor devices and thermal and photosensitive components.
Due to the nonlinearity of semiconductors, the resistance values measured by different instruments or different gears of the same multimeter are very different. Therefore, 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 resistance of the diode, the better. The resistance value of thermal and photosensitive devices can change with temperature or light...etc.
