Checking the varistor: finding the fault with a multimeter

Repair and diagnostics of malfunctions of electronic devices is carried out by finding failed elements with their subsequent replacement. It is often not possible to visually determine which radio component is faulty, therefore measuring instruments - testers are used to detect breakdowns. With their help, checking the varistor is usually not difficult.

Purpose and characteristics

A varistor is an electronic device with two contacts and a nonlinear symmetric current-voltage characteristic. The term "varistor" comes from the Latin words variable - "changeable" and resisto is "resistor". At its core, it is a semiconductor resistor capable of changing its resistance depending on the voltage applied to its terminals.

Resistors of this type are made by sintering at a high temperature of a semiconductor and a binder material. Powdered silicon carbide or zinc oxide is used as a semiconductor, and glass, varnish, and resin are used as a binder. The element obtained after sintering is subjected to metallization with further formation of leads. By their design, the devices are made in the form of a disk, tablet, cylinder, or film-like form.

Possessing the property of sharply reducing its resistance when a certain voltage appears at its terminals, a varistor is used in electronic circuits as a protective element. When a voltage surge of a certain magnitude occurs, the semiconductor device instantly reduces its internal resistance up to tens of Ohms, thereby practically short-circuiting the circuit, preventing the pulse from damaging the rest of the elements schemes. Therefore, an important parameter of the varistor is the voltage value at which the breakdown of the device occurs.

The principle of operation of the element implies its connection in parallel with the power supply circuit. After it is triggered and the voltage at the input is reduced, it will self-recover to its original value. Due to the low inertia, this happens instantly.

main parameters

Before checking the varistor for serviceability, it is necessary to understand not only the principle of its operation, but also to know what characteristics it has. Like any electronic element, a varistor has a number of characteristics that allow it to be used in various circuits. The main parameter is the current-voltage characteristic (CVC). It clearly shows how the current changes at a particular voltage value. Studying the I - V characteristic, you can see that the varistor, having a symmetrical bidirectional characteristic, works both in the forward and reverse zones of the sinusoid, resembling a zener diode.

In addition to the I - V characteristic, the following characteristics are noted in the study of the varistor:

• Um is the highest permissible operating voltage for a variable or constant current.
• P is the power that an element can dissipate on itself without deteriorating its parameters.
• W is the permissible energy in joules that a radioelement can absorb when exposed to a single pulse.
• Ipp - the highest value of the impulse current for which the impulse shape is determined.
• Co is the capacity, the value of which is measured at the varistor in its normal state.

But in practice, special attention is paid mainly to the parameter Um. This characteristic shows the voltage level at which the cell breaks down and current begins to flow.

Types of devices

The variety of types of varistors encountered is due to the fact that manufacturers strive primarily to increase their performance. Therefore, SMD technologies of leadless mounting are used, which allows achieving short response times with a jump in the input voltage. Typical response time of elements with leads is in the range of 15-25 nanoseconds, and SMD - 0.5 nanoseconds.

There is a class of low-voltage varistors and high-voltage ones. The first ones are produced with an operating voltage of up to two hundred volts and a current strength of up to one ampere. The latter have an operating voltage of up to twenty kilovolts. Low-power elements are used as protection against voltage surges arising in the household network, and powerful ones are used in transformer substations and in thunderstorm protection systems.

Element marking

Regardless of the manufacturer, there is a standard for marking varistors. It is customary to put an alphanumeric code on the element itself, in which the main parameters are encrypted. For example, for a disk type, this designation looks like S6K210, where:

• S - material from which the varistor is made;
• 6 - diameter of the element body, indicated in millimeters;
• K - deviation tolerance value;
• 210 is the value of the operating voltage, expressed in volts.

For the planar type, the same marking is used, only the first letters are CN, indicating the type of product.

In the diagrams, the radioelement is graphically indicated as a crossed-out rectangle. A shelf is made on the strikethrough stick, above which the letter U is placed. The element is signed on the diagrams in Latin letters RU.

Multimeter Test Methods

To check a varistor, however, like any other radio element, the easiest way is to use devices specially designed for this. Multimeters are used as such devices. The main parameter that can be measured with them is the internal resistance of the element. But before you directly start checking the varistor, you should prepare.

In addition to a multimeter, you will need:

• soldering iron;
• solder;
• flux;
• datasheet.

The measurement of the resistance of an element can be carried out without unsoldering it from the circuit, but in order to obtain reliable data, at least one of its outputs should be disconnected from the board. All preparation comes down to the fact that the semiconductor element is first visually inspected for the absence of: splits, blackening, cracks. If a bursting case is immediately visible, then the check can no longer be carried out. Such a varistor is clearly defective.

A soldering iron, flux and solder will be needed in order to unsolder one of the terminals of the element or even remove it entirely, and after checking, if necessary, solder it back. The datasheet for an item is an official document issued by the manufacturer. It contains all the main data and characteristics.

The datasheet is used to know exactly what the operating resistance of the radio component is at rest. If, when measured with a multimeter, the resistance of the varistor does not differ by more than 10%, then it is considered serviceable. If the resistance is significantly less than that indicated in the datasheet, then it will need to be replaced. It is important to note that in the normal state, the varistor resistance reaches several hundred megohms, so the tester should be able to measure in this limit.

Measurements with a pointer device

Such a device is considered analog. Its design uses an electromechanical head. It is a frame placed in a magnetic field. Depending on the current strength, the arrow in the frame deflects, stopping in a certain position. The range of deflection of the arrow is graduated with numbers, according to which the resistance is calculated.

Before you start checking the varistor, you will need to adjust the pointer multimeter. For this, it is calibrated. Its essence boils down to setting the zero position of the arrow by rotating a special knob when the probes are closed with each other.

For this, the switching button selects the operating mode corresponding to the “Ω», and the wafer switch is set to the tester's largest resistance limit. Most often it is designated as "x100", which corresponds to megaohms. Resistance is measured from a power source (battery) installed in the device. Therefore, if you cannot set the arrow to zero, then the battery will need to be replaced.

Carrying out measurements directly, one probe of the tester touches one terminal of the varistor, and with the other - to the other. As a result, there are three possible outcomes:

1. The arrow will deviate to zero or show resistance in the kilo-ohm region. It is concluded that the element is faulty (breakdown).
2. The measurement result lies within hundreds of megohms. This indication indicates that the varistor is working properly.
3. When touching the terminals of the radio element, the arrow does not react to this in any way. Possible reasons are as follows: the operating range of the device is not enough to measure the resistance value of the varistor, the device is faulty, the radio element is faulty (open circuit).

Digital tester

Using a digital multimeter, it will be a little easier to test a varistor for performance than an analog one. This is due to the fact that the digital tester in its design has an LCD display, which clearly displays the measured resistance.

This type of tester is based on an analog-to-digital converter, the principle of which is based on comparing the measured signal with the reference one. It should be noted that if the blinking battery icon is displayed on the screen when the tester is turned on, the battery will need to be replaced. The procedure for measuring the resistance of a varistor can be represented in the form of the following actions:

1. 
   The switch sets the maximum resistance measurement limit. Usually this limit is indicated by a number and a letter. If just numbers are written, then the unit is Ohm, the letter K after the number stands for kilo-ohm, and the letter M stands for mega-ohm.
2. The probes are fixed on the two terminals of the varistor, and the opposite ends of the wires with plugs are inserted into the tester's sockets, designated Ω and COM. Since the polarity of the applied signal to the varistor does not matter, it does not matter which wire is connected to one or another terminal of the element. Although it is accepted that a black cord is inserted into the COM connector.
3. The device is turned on by pressing the ON / OFF button on the tester.
4. If a unit is highlighted on the indicator, it means that a small measurement limit is selected.
5. If the screen displays numbers other than one, then this is the value of the measured resistance.

When interpreting the measurement result, you should also take into account the tolerance. Each radio element has its own tolerance index. For example, if the tolerance is 10 percent and the internal resistance of the varistor is listed as 100 MΩ, then the results should be between 90 and 110 MΩ. If it is found that the measured resistance of the element is below or above this range, then it can be considered faulty.

Using a rheostat

Checking the varistor is possible not only by measuring its internal impedance. The internal resistance value may correspond to the declared value, but the threshold voltage of the varistor will be incorrect. To check the breakdown value, a multimeter with a laboratory autotransformer or rheostat is used.

In the test circuit, a movable contact of the rheostat is connected to one of the terminals of the varistor, and a fuse is connected to the other. The multimeter probes are fixed parallel to the terminals of the semiconductor element, and it itself switches to the voltage measurement mode. A potential difference is applied to a free pair of contacts, the value of which exceeds the breakdown value of the component.

With the help of the movable contact of the rheostat, the voltage smoothly changes until the moment the varistor is triggered. This moment is determined by a voltmeter. Initially, the readings of the multimeter will rise, and then they will sharply drop to zero. This will blow the fuse. The maximum fixed non-zero value will be the threshold voltage.

It is important to note that when measuring, especially with a rheostat, the body may be electrocuted. Therefore, one should not forget about safety precautions, one should unswervingly observe it.