The article talks about how to check the resistance with a multimeter. In addition, with its help measure the strength of the current, the voltage between two points, as well as ring the electric circuit. Depending on the type of device, it can be used to check diodes, transistors, and many other radio components.
What are multimeters?
Previously, a multimeter arrow (analog) was used, but now many have switched to digital, as more convenient.
The dial gauge is still used by professionals. It works better in the area of radio waves and electromagnetic fields, does not need autonomous power supply, without which digital multimeters cannot work. Moreover, the accuracy of their readings is largely affected by the wear of the batteries. They can fail due to electrostatic discharge, which does not threaten the analog tester.
The dial multimeter works like a microammeter, equipped with switches, shunts and voltage dividers, allowing you to switch it to the operating modes of various devices. In contrast, the digital device displays the results of comparison and calculation of the difference between the measured parameters and standards.
Instrument Operation Basics
For each multimeter, the characteristics of which differ from others, there is a specific measurement, but there are mandatory rules for all types of devices.
To switch to a specific built-in device, as well as to the required measurement range of its parameters, one switch is used.
Measurements are made by touching metal probes with insulated handles to the 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 a switch.
A voltmeter is connected to two points with different potentials.
To measure the current strength, a gap is created in the electric circuit and an ammeter is connected to it.
Resistance is measured on an element disconnected from the circuit by passing an electric current through it from the battery built into the device.
A probe with a black wire connects to the COM jack with a "-" pole, with red - to a VΩmA jack with a positive pole.
There are different models of multimeters that differ in the features of work. The manufacturer’s instructions are attached to each of them: how to take measurements and switch operating modes.
Digital multimeter device
The basis of functioning for most models is the same. Icons, measurement limits and additional functions may vary slightly here. All controls and controls are located on the front panel: switch modes and ranges, LCD display, connectors for probes.
The most advanced devices automatically select measurement limits.
The 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 on to insulated handles only.
Principle of operation
The electric multimeter in most budget models runs on the 1CL7106 chip.
When the voltage is measured, the signal is supplied from the switch to input 31 through the resistor R17.
To measure the value of direct current, a multimeter is connected to the open circuit. The current strength is perceived by the resistors depending on the set range, after which the voltage drop from them is fed to input 32.
The diagram shows only the main functions. Many models have additional ones. Which multimeter is better, each user decides depending on the specifics of the measurements.
Resistance measurement circuit
Whatever type of multimeter is, the use of an ohmmeter is in almost every one. Most often, with its help, the resistance of resistors, transformers, inductors and the serviceability of fuses are checked. The following is a simplified resistance measurement circuit.
Here, reference resistors R1 ... R6 and current-setting resistors R101 and R103 are used. In measurement mode, the reference and input voltage are compared, equal to the ratio of the measured and reference resistances.
The device is used for detecting 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 can be read in the instructions, but the method is common for many models. On the tester, the resistance section is indicated by the Omega icon. The common models of the type M832, M83x, MAS83x have 5 measuring limits: 200 Ohm, 2 K, 20 K, 200 K, 2 M. In addition, the 6th position is used for continuity of circuits. The buzzer sounds 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 with a resistance of approximately 1.5-7 K, select a range with a limit of 20 K. To measure with an M832 multimeter
Unlike other devices, an ohmmeter can measure an unknown resistance on any range, this will not lead to its failure. If the installation does not meet the required limits, one or zero will be fixed on the screen. In the first case, it is necessary to increase the upper limit of the measurement range, and in the second, reduce it.
Note! Before checking the resistance with a multimeter, beginners usually touch with both hands the current-carrying findings of parts and probes. As a result, the resistance of the resistor and the body is measured, which introduces an error in the readings of the device. It is especially large when the face value is measured in megaohms. The output of the part and the probe can be held with only one hand. This requirement should be observed when checking any radio components.
When repairing electronic equipment, it is often required to measure the resistance of a resistor soldered into a circuit. To get accurate readings, you need to unsolder one of the conclusions. The measuring circuit should consist only of an ohmmeter and a resistor. If it is soldered into the circuit, the resistance between the terminals and other radio components will be summed. If the part has many conclusions, for measurements it must first completely vypayat.
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 "2M" and connected to the terminals of the coil of the measuring probes, one zeros will appear on the screen. This means that there is electrical resistance of the turns, but the measurement limits are chosen incorrectly.
Then you need to set the switch to "200 K", which corresponds to the range of 0-200 K and reconnect the probes of the multimeter. A resistance value of 00.5 kOhm appears on the screen. If the readings in front of the comma have zeros, then you need to reduce the measurement limits further. At the next position of the switch, the device will show 0.73 kOhm. This value is already more true.
If there is a need to obtain a more accurate result, reduce the range to 0-2 kOhm and repeat the measurement. 0.751 kOhm will appear on the screen.
If you switch to a measurement range of 0-200 Ohms, the device will display "1", which means that the measured value is beyond the upper limit.
Before calling 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 a sound signal indicates that the circuit is operational. If the buzzer is "silent", it means that there is a break in the coil.
Multimeter probes
Probes 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 entrance.
The conductive ends are made in the form of needles so that it is possible to puncture the insulation of the wire or find the findings in microcircuits with a small pitch. The material used is bronze, which does not hold sharpening well. In addition, the needles break off at the embedment sites.
In the cold, the insulation of the wires becomes stiff and it is inconvenient to use the device.
Another drawback is the unreliable contact in the socket of the device. When ringing patterns, it is often lost.
The probes for the multimeter often have to be brought to condition with their own hands. To do this, the wires are soldered to the lugs, and the others are selected into the sockets. The tip should be tinned so that when you click on the tested point, the resistance value does not depend on the pressure.
It is advisable to replace the wires with a larger cross section in order to reduce their resistance. The wires in the kit have a resistance of 0.2-0.5 ohms, and sometimes higher.
Check ohmmeter before work
During operation of the multimeter, the current-carrying conductors of the measuring probes wear out, which negatively affects the measurement results (readings “jump”). Before work, they should be checked. To do this, the instrument switch is set to the lowest range and the probes are shorted together. After probing its insulated conductors. If there is poor contact inside, the display will begin to falter. You can also check the probe in dialing mode. If the buzzer sound disappears and reappears, this indicates unreliable contacts.
Power supply
A 9V Krona battery is inserted into the device. If the battery icon appears on the multimeter screen, it indicates that it has been discharged and a replacement is required. Otherwise, the readings will be incorrect.
Some multi-testers have a HOLD button. When it is pressed, the readings of the device are recorded for easy reading. To return to the operating mode, press the button.
Conclusion
Each multimeter model is sold with instructions that should be carefully studied, since each type of device has its own characteristics.
Before checking the resistance with a multimeter, determine its approximate value. If the value is several ohms, the part can be left out of the board. With dimensions in megaohms, the resistor should be evaporated and measured without touching the findings with your hands.