Multimeters: types, measure resistance, correctly measure

To measure any electrical quantities - such a need sometimes arises for any modern person. Not to call the master every time or contact your friends when a simple malfunction occurs in everyday life? Any motorist is constantly faced with situations when it is necessary to check the supply voltage of the battery or check the integrity of the electrical fuses under the hood of the car. The purchase of a device, the name of which is a multimeter, and work with it will allow you to come closer to solving the problem that has arisen.

How to measure resistance with a multimeter

From the name of the device it follows that it is a combined device that combines several devices for various purposes.

The purpose of the ohmmeter, the scope of its application

The device used in a multimeter to measure the electrical resistance of a circuit is called

ohmmeter. In its name, the first part comes from the unit of electrical resistance Ohm. It is used to measure the electrical resistance of a section of the circuit, resistors. Check the serviceability of the inductance coils, the integrity of the windings of power transformers, fuses. In the simplest case, you can assess the health of a light bulb.

It can be used when troubleshooting high frequency coaxial television cable circuits. The reason for the disruption of the television image may be incorrect installation of television plugs: unreliable soldering of the main core of the cable and its shielding braid, a short circuit between them. With the help of an ohmmeter, you can quickly identify such defects.

Radio amateurs often use an ohmmeter to assess the integrity of conductors on printed circuit boards, the correct functioning of semiconductor diodes and transistors. You can check the capacitor for breakdown between its plates.

Resistance measurement method

The measurement is based on Ohm's law, which is known to many from the school physics course. It determines the strength of the current (I) flowing in the electrical circuit as a quantity proportional to voltage value (U) applied to this section and inversely proportional to the resistance ® of this plot. Or:

I = U / R

The device has a DC power supply, which is a standard battery or a set of standard batteries. If you connect the measured resistance to the probes of the device, then by measuring the amount of current flowing through the resistance, you can determine its value on a scale (digital indicator), which will show its value already in units resistance.

Classification of multimeters

The multimeters used to measure resistance can be analog or digital in their design. For analog devices, the value of the measured resistance value can be determined by the graduated scale of the device, on which the arrow of the device stopped during the measurement. With digital instruments, the measured value is displayed as a numerical value on its own display.

Analog Devices

Analog multimeters have a different name - turnout. Experienced users and professional electricians continue to work with them. They appeared several decades ago, much earlier than digital devices. They are based on a pointer microammeter with a set of additional resistors and shunts of high accuracy class.

Resistance measurement with a multimeter is carried out by installing the switch located on its front panel at various points of the "Ω" sector. Depending on the value of the measured resistance, the position of the switch determines the boundaries of the range in which the result is expected. These can be marks: Ohm (Ω), kOhm (1k), tens of kOhm (x10), hundreds of kOhm (x100).

Resistance values ​​over 1 MΩ are usually not measured with an analog multimeter. This is due to the non-linearity of the instrument scale. It has the greatest accuracy on the right side (approximately the first 2/3). Then its digitization is compressed. Accordingly, it is better not to use the left side of the scale, switch to another measurement limit.

Battery voltage, participating in the measurement of the resistance value, is limited by its nominal value, therefore, when measuring resistance of a large value, the current flowing through it has very little magnitude. The pointer of the device is barely deflected in the left part of the scale, in which the measurement error has the maximum value. In any case, the measurement accuracy does not exceed 2%.

Before measuring the resistance, the ends of the probes of the device must be closed among themselves and by rotating the handle of the variable resistor connected to the front panel, set the position of the arrow corresponding to the zero mark of the measurement scale resistances. If it is not possible to set the arrow to "0", then a decision is made to replace the battery.

Digital devices

Digital multimeters are used today by most people who need to measure resistance. The result of the measurement is displayed on the indicator board with numbers that correspond to the value of the measured resistance. To do this, the switch on the front panel of the device must be set to one of the positions of the "Ω" sector. Depending on the value of the measured resistance, the choice of this position should be such that the measurement limit is higher than the resistance value to be measured.

The latest models of multimeters have 5 measurement limits, which start at 200 (up to 200 ohms) and pump in 2000k (2.000.000 ohms).

The measuring probes of the multimeter must be connected to the extreme points of the part (resistor). If the nominal resistance of the resistor is greater than the measurement limit of the selected range, then the digital indicator of the device will display "1". After that, it is necessary to change the measurement limit upwards. With the correct choice of the range, the numbers on the indicator will show the value of the resistance value of the resistor. The number "1" on any of the selected ranges indicates a malfunction of the resistor or, which happens quite often, the absence of contact between the probes of the device and the resistor.

Also, there are frequent cases of breaks in the wires of the measuring probes. To check their serviceability, it is necessary to reliably connect their ends to each other, having previously set the lower measurement limit with a switch. In this case, the numbers on the indicator should show a value close to zero. After such a check, a decision should be made: the tested resistor or measuring probes are faulty. In the latter case, they must be thoroughly repaired. Good condition of the measuring part will save a lot of time in the future.

By the way, the same trouble can happen when working with an analog multimeter.

Comparison of different types of multimeters

Digital multimeters are easier to work with than analog ones. Many novice users find that using an analog multimeter requires special training and a lot of practical experience with them. This is indeed the case.

The arrow indications must still be correctly interpreted depending on the selected scale ("Ω") and the position of the switch multiplier. The accuracy of analog multimeters is also poor. It depends on the accuracy class of the microammeter used in them. The accuracy class is indicated on the scale of the device.

On the other hand, the readings of analog multimeters are more stable. The information of the arrow of the device is averaged and does not change with instantaneous fluctuations of the measured value. This property is inherent in the magnetoelectric microammeter system. The readings of the digital multimeter in this situation will change chaotically. And the reason for such sharp fluctuations can be the banal alternating contact of the measuring probes with the part being checked.

Analog multimeters are less susceptible to various electromagnetic emissions. Circuits of digital devices contain a certain amount of semiconductor elements, and they are very susceptible to such external influences.

Both types of multimeters use a battery. In digital devices, the circuit provides for a power supply discharge sensor. At his command, the device turns off, signaling this. In the same situation, the analog multimeter continues to work with incorrect readings.

Many digital multimeters have a dial function with audible alarms. It is very convenient. If the resistance of the circuit being measured is less than 50 ohms, an attention tone will sound. The most "advanced" models are equipped with the function of storing the measured value (the "HOLD" button on the front panel of the device). It is convenient to work with such samples in hard-to-reach places. But the button must be pressed not before, but during the measurement. Otherwise, the readings will be unreliable.

User tips

The following tips will be useful both for inexperienced users of the multimeter, and for those who have been familiar with it for a long time:

• When measuring the resistance of resistors, do not touch the leads of the part with your hands. This also applies to the metal parts of the instrument probes. If this condition is not met, the readings of the device will not correspond to reality. The parallel connection of the resistor and the part of the human body that has its own resistance will be measured. The readings of the device will be underestimated.
• If it is necessary to check the resistance of the element soldered into the circuit, it is necessary to first de-energize the circuit.

The element must be removed from the circuit and only after that the necessary measurements must be made. Otherwise, the resistance is measured not of a specific element, but a parallel connection of itself and the section of the circuit in which it is installed.