We noticed that when measuring the resistance at the initial moment on the display of the multimeter, dials start to flash, stopping at a certain value. Inside, digital algorithms are used that do not instantly give the desired answer. It is especially difficult to have a multimeter conducting a measurement of low resistance. Its accuracy is low, fractional parts will not be obtained. How to check resistance with a multimeter is the topic of today's review.
Measurement of resistance with a multimeter
In contrast to capacitances, resistance is able to measure each tester. This is a simple operation. The trick is that the mechanical models work with the voltage without a battery, and to estimate the parameters of the resistors you need some kind of charge to form the auxiliary voltage. Of course, it is possible to circumvent the restrictions by creating a resistive divider, using an external source - for example, a socket. Difference of digital multimeters - devices do not work without make-up.
A minus of modern models is the limited scale. You want to measure the resistance of a resistor with a multimeter, and you come across solid difficulties. The maximum limit does not exceed 2000 kΩ.This is only 2 megohms, hams know that this is not the upper limit for a decent resistor. The insulation resistance of electrical appliances should be 20 MΩ.Check its quality with an ordinary multimeter will not work. The first rule of measurement of resistance by a multimeter: "The size of the scale corresponds to the measured value."
Understanding compliance is not easy. In the old days, the nominal value was put on the resistor body. For too small models it is difficult to see the numbers. The face value does not depend on dimensions. You have to guess: baby for a couple of ohms or MOhms. The difference is a million times, I do not want to make a mistake. Most resistors today are marked with colored stripes. Do not learn the table by heart. We recommend using a simple method: find an online calculator on the Internet for solving your own problems. Similar is located at http: //www.chipdip.ru/info/rescalc/.
Everything is arranged in the form of a table, and it is shown that the resistors are marked with four or five strips. Valid colors are listed in the rows of the table formed by the authors of the site. The numbers of the bars go in columns. The choice of the desired range occurs in the form of clicks on radio boxes. A single color is possible for each strip. At the top, current changes are displayed on a schematically drawn resistor, which adds convenience. Usually the extreme band is thicker than the others, in practice it is impossible to notice.
Then they try to get the circuit of the device to orient themselves. If the approximate denomination is known, it is difficult to make a mistake. Secondarily look at the strip. For example, gold and silver color are found exclusively from the extreme thin strip. In practice, a rare person will be able to distinguish it from yellow and gray. Without experience is too difficult. It will be necessary to start both options on the calculator( from left to right and from right to left), then start measuring with a multimeter with the maximum of the received ratings.
So, to get the value in the online calculator you need to put down all the bands. Real-time Chip & Dip work does not work - a small drawback. As a result of the efforts in the text field appear:
- Resistor value, resistance in standard units. For example, ohms.
- Comma accuracy tolerance. The worst resistors show a deviation of 10%( both sides separately).As a result, the variation of resistance values is strong. Therefore, a multimeter resistance test is required.
The calculator form is not the best, but it is located on the site of the well-known Chip & D store where you can order the necessary parts. According to the value found, the scale of the multimeter is set with a margin. Valid for a 10 kΩ resistor limit is 20k. We remind you that on the front panel a group of scales measuring resistance is marked with the Greek letter omega Ω.
How to check the resistor with a multimeter
Usually the test starts with the measurement of the nominal, as shown above. The corresponding number will appear on the display. Please note that the nominal parameter can vary greatly, maintaining accuracy tolerance. The accuracy of the digital multimeter is 0.5 Ohm, the device shows only integer values. Taking into account that the internal resistance of the multimeter is additionally present, it is impossible to estimate the parameters of a resistor with a small value.
- When measuring resistance, sometimes the readings are close to zero, or vice versa - a break is fixed. So the resistor is out of order. In the first case, the nearest turns were closed, in the second case - the thread burnt out. Most resistors consist of a ceramic base and a high-resistance core wound around it. Each element is characterized by the maximum power dissipation indicated in the technical data. If the parameter is exceeded, the effects described above occur. Often the resistor body darkens. Not any blackness means breakage - in most cases the paint is less resistant to heat than it lived and darkens.
- A lot depends on the tolerance. Cheap resistors even in one set differ by 15 percent or more. It does not mean that the multimeter is lying, you just need to take this fact into account when assembling the circuit. Approach wisely. If it is written that you want to get a resistive divider with equal shoulders of 100 ohms, the worst will not happen if you take nominal values of 90 ohms. The main thing is to observe equality.
The parameters of small resistances are required to be estimated by indirect methods. Let's say assemble a resistive divider, as shown in the figure. We give a brief explanation. First, we see two resistors, and one reference resistor. This is a small nominal resistance with a minimum tolerance of 0.05%( gray bar, not silver).What will provide the maximum accuracy during the work. The supply voltage of +12 V is not taken by chance. This is the maximum value, easily obtained, for example, using the power supply from a personal computer. The higher the voltage, the more accurate the measurement. We got to the main subtlety: the voltage can be measured with amazing accuracy - up to tenths of a mV.
Resistive Divider Assembly Diagram This will help determine the potential difference across the resistor under study. Then the nominal value is calculated from the proportion:( 12 - U) / U = Ret / R. Where Ret is the resistance of the reference resistor and U is the measured value( see figure).The picture shows where to connect the multimeter probes, the earth is taken from the power source( often black wire).Let's see the benefits of using the scheme. Suppose there is a 1.5 Ohm resistor with a tolerance of 10%.Obviously, a direct measurement of resistance will give the display a value of 1 or 2. This is clearly not enough. Now we take the reference resistor of 2.7 ohms, collect the circuit and see the voltage value of 4.4 V. We calculate the proportion:
( 12 - 4.4) / 4.4 = 2.7 / R;
from where we find that R = 1,56 Ohms. We could not measure the resistance with a multimeter at such small values of the nominal. In addition, the accuracy is great - to the hundredths! The main thing - it becomes clear that the resistor corresponds to the technical documentation and is suitable for the intended use. The method described is permissible to try to measure the resistance of the wire, with a large length. For example, a kilometer of a copper conductor with a section of 6 square meters.mm is a few ohms. The resistance of the cable below, we will focus on the whole bay.
Remember, to measure the resistance of the ground loop you will need to find a reference point. This circuit, which is guaranteed to be grounded. Either the potential to remove from Uet, and the formula according to alter under the required case. By the way, there is no need to use exactly the voltage of 220 V AC.+12 V is much safer, not the fact that the accuracy will be lower, given the presence among the scales of the digital multimeter limit of 200 mV.This will allow in the presence of a good reference resistor to measure the grounding resistance of a multimeter very accurately.
Measuring with a multimeter the resistance of nonlinear elements
In the lessons on the element base, it was said that in the open state the voltage drop across the silicon diode is twice as high as germanium. And semiconductor elements are made of gallium arsenide. Before estimating the resistance of the diode in the forward direction, you need to understand that this is a non-linear element. Its characteristics depend on the applied voltage. The resistance measured by different multimeters will not be the same: each tester forms an auxiliary voltage on the probes, which is different for different devices.
To find your way on the current-voltage characteristics of the diode( graph, which shows the dependence of the output current on the voltage applied to the contacts), you need to know the characteristics of the multimeter. Often, auxiliary values are not indicated in the passport; a test will be required. Take a medium capacitor. Charge auxiliary voltage. We put the range on the measurement of resistance and, not forgetting the polarity( red probe - plus), apply to the capacitor. When the resistance on the display completes the run from zero to infinity, proceed to measuring the DC voltage( not forgetting the polarity).
The result is the value of the auxiliary voltage. Now, using it, it is possible to find the current: I = U / R, where R is read from the display in the resistance measurement mode( the same happens with the dialing mode of the diodes marked with a characteristic fat arrow with a cross bar at the end).Now we look at the volt-ampere characteristic and see if the obtained point coincides with the intersection position of U and I. If the deviation is within the normal range, the diode is uniquely valid. Otherwise, if the diode opens and closes, the part can be used in circuits that are not critical to accuracy.
Measuring the resistance of
instruments with a multimeter. If you take a 60-watt light bulb, it is easy to quickly make sure that the resistance of the spiral is only 68 Ohms. At an applied voltage of 220 V, a device would flow a current of more than 3 A, which corresponds to a power of 700 W.The reason is the nature of the AC voltage of 50 Hz. The resistance check of the electric cooker is carried out taking into account the indicated simple fact. In talking about acoustics, we mean a certain average frequency for the sound spectrum, which is, for example, 2.5 kHz. Therefore, the resistance of the spark plug and the resistance of the speaker are designed to be measured by indirect methods in conditions close to real. A divider is assembled, a test pattern is created.
And the resistance of the ignition coil can be measured with a tester. To do this, you will have to find complete technical data on the number of turns and wire section.