The article describes how to check the resistance with a multimeter. In addition, it is used to measure current strength, voltage between two points, and also ring electrical circuits. Depending on the type of device, it can be used to test diodes, transistors and many other radio components.
What are multimeters?
Previously, a pointer multimeter (analogue) was used, but now many have switched to digital, as it is more convenient.
The pointer device is still used by professionals. It works better in the area of radio waves and electromagnetic fields, does not need an independent power supply, without which digital multimeters cannot work. At the same time, the wear and tear of the batteries significantly affects the accuracy of their readings. They can fail from electrostatic discharge, which does not threaten the analog tester.
The pointer multimeter worksas a microammeter equipped with switches, shunts and voltage dividers, allowing it to be switched to the operating modes of various devices. In contrast, a digital device displays the results of comparison and calculation of the difference between the measured parameters and standards.
Basics of instrument operation
Each multimeter, whose characteristics differ from others, has its own measurement specifics, but there are mandatory rules for all types of devices.
One switch is used to switch to a specific built-in device, as well as to the required measurement range of its parameters.
Measurements are made by touching metal probes with insulated handles to conductors.
The measured value of the parameter must be within the range set by the switch. Measurements are made first at higher ranges, and then the required accuracy is adjusted with the switch.
The voltmeter is connected to two points with different potentials.
To measure the current, create a break in the electrical circuit and connect an ammeter to it.
Resistance is measured on an element disconnected from the circuit by passing an electric current through it from a battery built into the device.
The probe with the black wire is connected to the COM jack with the "-" pole, the one with the red wire is connected to the VΩmA jack with the positive pole.
Different models of multimeters are produced, differing in their work features. Each one comes with manufacturer's instructions on how to makemeasurements and switch operating modes.
Digital multimeter device
The basis of operation for most models is the same. Icons, measurement limits, and additional features may vary slightly here. All control and monitoring elements are located on the front panel: mode and range switch, LCD display, probe connectors.
The most advanced devices automatically select measurement limits.
Probes are designed to transmit a signal from the elements of electrical circuits to the device. For them, the device has three adjacent sockets. When measuring, always hold only the insulated handles.
Working principle
The electric multimeter in most budget models runs on a 1CL7106 chip.
When voltage is measured, the signal is applied from the switch to input 31 through resistor R17.
To measure the value of direct current, a multimeter is connected to the break in the circuits. The current strength is perceived by the resistors depending on the set range, after which the voltage drop from them is fed to the input 32.
The diagram shows only the main functions. Many models have additional. Which multimeter is better, each user decides depending on the specifics of the measurements.
Resistance measurement circuit
Whatever type of multimeter, the use of an ohmmeter is in almost everyone. Most often, it is used to check the resistance of resistors, transformers, inductors and the he alth of fuses. Below is a simplified circuit for measuring resistance.
Here reference resistors R1…R6 and current-setting resistors R101 and R103 are used. In the measurement mode, the reference and input voltages are compared, equal to the ratio of the measured and reference resistances.
The device is used to detect open circuits, breakdown of capacitor plates, checking the integrity of printed conductors on electronic circuit boards.
How is resistance measured?
How to check the resistance with a multimeter, you can read in the instructions, but the method is common for many models. On the tester, the resistance section is marked with the "Omega" icon. Common models such as M832, M83x, MAS83x have 5 measurement limits: 200 Ohm, 2 K, 20 K, 200 K, 2 M. In addition, the 6th position is used for continuity of circuits. The buzzer is triggered when the resistance between the probes is less than 50 ohms. When they are connected to each other, the device shows a resistance value slightly above zero. When a small resistance value is measured, this value is subtracted from the reading.
For example, if you have a resistor whose resistance is approximately 1.5-7K, you should select a range with a limit of 20K to measure with the M832 multimeter.
Unlike other devices, an ohmmeter can measure unknown resistance on any range, this will not lead to its failure. If the setting does not matchnecessary limits, the display will show one or zero. In the first case, it is necessary to increase the upper limit of the measuring range, and in the second - to reduce it.
Pay attention! Before checking the resistance with a multimeter, beginners usually touch the current-carrying leads of parts and probes with both hands. As a result, the resistance of the resistor and the body is measured, which introduces an error into the readings of the device. It is especially large when the denomination is measured in megaohms. The workpiece output and probe can only be held with one hand. This requirement should be observed when checking any radio components.
When repairing electronic equipment, it is often necessary to measure the resistance of a resistor soldered into the circuit. To get accurate readings, you need to solder one of the conclusions. The measuring circuit must only consist of an ohmmeter and a resistor. If it is soldered into the circuit, the resistances between the terminals and other radio components will be summed up. If a part has many pins, it must first be completely desoldered to take measurements.
Resistance measurement example
It is required to measure the resistance of a coil whose value is unknown. Usually the upper limit is chosen as the maximum. When the switch is set to the "2M" position and the measuring probes are connected to the terminals of the coil, only zeros will appear on the screen. This means that the electrical resistance of the turns is, but the measurement limits are chosen incorrectly.
Then you need to set the switch to the "200 K" position, which corresponds to the range of 0-200 K and reconnect the multimeter probes. A resistance value of 00.5 kΩ will appear on the screen. If there are zeros in the readings ahead of the decimal point, then it is necessary to reduce the measurement limits further. At the next switch position, the instrument will show 0.73 kOhm. This value is already more true.
If there is a need to get a more accurate result, it is necessary to reduce the range to 0-2 kOhm and repeat the measurement. The screen will show 0.751 kOhm.
If you switch to the measurement range of 0-200 ohm, the device will show "1", which means that the measured value is out of the upper limit.
Before ringing the coil with a multimeter for a break in it, you must set the switch to this mode, and then connect the probes to its terminals. The presence of an audible signal indicates that the circuit is working. If the buzzer is "silent", then there is a break in the coil.
Probes for multimeter
The styli in budget testers are not of high quality, despite the fact that some of them look spectacular. When buying, you should choose such that the wire is elastic and tightly held at the entry point.
Conductive ends are made in the form of needles so that you can pierce the insulation of the wire or find leads in microcircuits with a small step. Bronze is used as a material, which does not hold sharpening well. In addition, the needles break off at the embedding points.
In the cold, the insulation of the wires becomes hard and the device is inconvenient to use.
Another disadvantage is unreliable contact in the socketdevice. When calling schemes, it is often lost.
Probes for a multimeter often have to be brought to condition with your own hands. For this, the wires are soldered to the tips, and the connectors are selected in the sockets by others. The tip should be tinned so that when you press on the point to be checked, the resistance value does not depend on the pressing force.
It is advisable to replace the wires with a larger section in order to reduce their resistance. The wires in the kit have a resistance of 0.2-0.5 ohms, and sometimes even higher.
Checking the ohmmeter before work
During the operation of the multimeter, the current-carrying conductors of the measuring probes wear out, which negatively affects the measurement results ("jumping" readings). They should be checked before work. To do this, the switch of the device is set to the lowest range and the probes are short-circuited to each other. After that, its isolated conductors are probed. If the contact is bad inside, the display will start to go astray. You can also check the probe in continuity mode. If the buzzer sound disappears and reappears, this indicates unreliable contacts.
Instrument power supply
A 9 V Krona battery is inserted into the device. If the battery icon appears on the multimeter screen, this indicates that it has run out and needs to be replaced. Otherwise, the readings of the device will be incorrect.
Some multitesters have a HOLD button. When pressed, the instrument readings are fixed for easy reading. To return to working mode again, you need to pressbutton.
Conclusion
Each model of the multimeter comes with an instruction manual, which should be carefully studied, as each type of instrument has its own characteristics.
Before you check the resistance with a multimeter, you should determine its approximate value. If the value is a few ohms, the part can not be soldered from the board. With a dimension in megaohms, the resistor should be soldered and measured without touching the leads with your hands.
