How to check a resistor with a multimeter without unsoldering: part marking, testing stages, continuity of the posistor

Any electrical circuit has resistance in it. Therefore, in radio engineering, the most common element is a resistor. When repairing electrical appliances, it is important to be able to test such parts. You need to know how to check the resistor with a multimeter without soldering the element. The part most often fails if the conductive layer burns out or its connection with the clamp is broken.

Testing procedure

Resistors can be of various shapes, but standard models have a linear I – V characteristic. Device verification consists of three stages:

1. Inspection of the external condition of the device.
2. Break testing of the part.
3. Comparison of indicators with par.

The first two points are not difficult to perform, and difficulties may arise with the last step of checking the resistor with a multimeter. The problem lies in determining the nominal resistance value. With a concept, it is easy to recognize the indicator. But many modern devices are not supplied with accompanying documentation with technical characteristics. In this case, you can determine the value of the rating using the marking.

Multimeters can be digital and pointer. The latter work without additional power supply, like a microammeter. Voltage dividers are switched together with shunts to certain modes for measurement. Digital models show on the display the difference between the obtained value and the standard. This type of appliance needs a power supply, which provides measurement accuracy that decreases when the battery is discharged. These devices are used to determine the state of radio components.

Types of markings

On Soviet components, the nominal value was indicated directly on the case. In this case, no decryption was needed. But if the integrity of the part was violated, the paint was burnt, it was problematic or even impossible to read the text. It was possible to clarify the denomination according to the schematic diagram included in the package of any household appliance.

Modern components are color coded with 3-6 rings in different shades. This solution allows you to determine the nominal value, even if the element is significantly damaged. This point is especially relevant in the frequent absence of a schematic diagram from the device.

GOST 175–72 establishes clear standards for the digital and color designation of components. The stripes are located next to one of the pins and are read from left to right. Colors can be as follows:

• silver;
• gold;
• black;
• Brown;
• Red;
• Orange;
• yellow;
• green;
• blue;
• purple;
• Gray;
• White.

The tolerance defines the deviation of the series value from the nominal value at which the component can operate. If the calculation of the scheme was made correctly, then this value should be taken into account; otherwise, adjustment is carried out after assembly of the part.

Many Chinese manufacturers, trying to significantly reduce the price of products, do not set a tolerance value. As a result, the element continues to work until its safety margin exceeds the limit. If the difference between the rating and the obtained indicator exceeds the tolerance, then the element must be replaced.

Resistors with the smallest allowable value up to 10% have 5 rings. The first three denote the coefficient of resistance, measured in ohms. The fourth corresponds to the multiplier, and the fifth to the tolerance. Devices with a deviation of more than 10% are marked with 4 stripes. The markings are similar to the previous version, but there is no tolerance indicator.

With a maximum deviation of 20%, the resistors are marked with 3 rings. The first two are assigned the resistance value, and the third acts as a multiplier. Elements with 6 stripes are rare. The last ring in them marks the coefficient of change during temperature fluctuations. It determines the resistance when the resistor body heats up. It is convenient to decipher the color marking using online calculators that calculate the denomination after entering the necessary data.

Elements for a mounted installation, such as a diode, smd resistor or capacitor, are small in size, and it is simply impossible to put all the necessary information on them. Therefore, encrypted digital designations are used to mark them. Usually, three numbers are indicated on the case, two of them determine the value, and the last one acts as a factor.

External diagnostics

Before checking the posistor with a multimeter, it must be inspected and visually checked for serviceability. The case must be solid, without cracks and chips on the surface, and the leads must be securely fastened.

If the resistor is faulty, then its case will be completely burnt or ring-shaped foci. A darkened surface is not always a sign of a breakdown; it indicates heating when the permissible power is occasionally exceeded. An internal break cannot be recognized by the appearance of the element.

Check for denomination and breakage

At this stage of testing, the compliance of the obtained value with the tolerance and nominal value is checked. The indicator should not go beyond the limit set by the switch on the device. The range is set with a value slightly higher than the nominal. You can check the resistance of the resistor with a multimeter as follows:

1. Probes are connected to the sockets marked V Ω mA and COM (with the first connected to the positive red, and to the second - negative black).
2. Checking the functionality of the wires. To do this, they are closed among themselves. The tester should return a value equal to or close to zero. Small values ​​are determined by subtracting from the readings of the device. A nonzero value is often obtained when the battery is insufficiently charged.
3. The probes are brought to the terminals of the part to be checked. If the device has an infinite resistance indicator (the display shows "1"), then there is an open in the resistor.
4. The obtained data are compared with the nominal value (the tolerance must also be taken into account). The coincidence of the data indicates the health of the part. The readings may also differ slightly due to the error of the device itself, especially when measuring without soldering.

During testing, you should not touch the probes with your hands (this is a common mistake for beginners). The human body also has resistance, and when measuring the resistor values ​​in kilo-ohms, the test results may be distorted.

Variable resistor operation

The process of testing a variable element is much like working with standard models. It includes the following steps:

1. Measurement is carried out by connecting the probes to the extreme legs. The resulting figure is compared with the nominal.
2. One probe is connected to the center leg, and the other to the remaining free one.
3. The trimmer knob is rotatable. The readings of the device should be within the zone from 0 to the value obtained at the first stage.

Measurements can also be carried out without setting a limit value. Ohmmeter mode allows you to set any range values. This setting will not damage the tester. When the display shows "1" (infinity), you need to increase the threshold until the desired result appears.

Inspection of a part without soldering

On-board resistor testing is only possible for low-impedance components. If their value exceeds 80-100 ohms, then other elements may distort the value. To disconnect the part from the rest, you must release one leg. Such verification is carried out in rare cases. Before work, you need to check the presence of shunt circuits on the diagram. The bottom line is particularly affected by semiconductor elements.

The dialing method is often used for testing. The designation of the switch for this mode is a diode with a signal. The parts to be checked must have a response limit of no more than 50-70 Ohm, otherwise a weak signal will be obtained, which will be difficult to distinguish. If the resistance is below the limit, the device will beep through the speaker. To ring the resistor with a multimeter, you need to select the points of the circuit with the probes and create a voltage between them. The device requires sufficient power to operate correctly.

Working with a multimeter is quite simple if you understand the rules for setting limit values ​​and measuring resistance. You should also know how to use the tester's switches and probes. The process is greatly facilitated if there is a schematic diagram included in the package with household appliances.