A varistor (literal translation from English - a resistor with a variable resistance) is a semiconductor with a non-linear volt-ampere characteristic (Vakh).
Everything electrical appliances calculated to its operating voltage (in the houses of 220 V or 380 V). If there was a power surge (instead of 220 V, 380V was applied), the devices could burn. Then the varistor comes to the rescue.
- 1 Varistor principle
- 2 Key Features and Parameters
- 3 Varistor Types
- 4 Varistor Reference and Marking
In the normal state, the varistor has a very high resistance (according to various sources from hundreds of millions of ohms to billions of ohms). He almost does not pass through a current. It is worth the voltage to exceed the permissible value, as the device loses its resistance thousands or even millions of times. After normalizing the voltage, its resistance is restored.
If the varistor is connected in parallel with the appliance, then at a voltage surge, the entire load will fall on it, and the devices will remain safe.
The principle of operation of the varistor, if explained on the fingers, comes down to the following. With a jump in the electrical network, it acts as a valve, passing electric current through itself in such a volume as to reduce the potential to the required level. After the voltage is stabilized, this “valve” closes and our circuitry continues to work in the staffing table. This is the purpose of the varistor.
Key Features and Parameters
It should be noted that this is a universal device. He is able to work immediately with all types of current: constant, pulsed and variable. This is due to the fact that he himself does not have polarity. In the manufacture of high temperature is used to solder the powder of silicon or zinc.
Parameters that need to be considered:
- conditional parameter, determined at current 1 mA, V;
- maximum permissible alternating voltage, V;
- maximum permissible constant voltage, V;
- average power dissipation, W;
- maximum impulse absorbed energy, J;
- maximum pulse current, A;
- device capacity in normal condition, pF;
- response time, ns;
To choose the right varistor sometimes it is necessary to take into account the capacity. It strongly depends on the size of the device. So, tvr10431 has 160nF, tvr 14431 370nF. But even parts of the same diameter can have different capacities, so the S14K275 has 440nF.
In appearance there are:
- in the form of tablets;
Rod can be equipped with a moving contact. They will look like the name. In addition, there are low-voltage, 3—200 V and high-voltage 20 kV. The first current ranges from 0.0001—1 A. It does not affect the designation according to the scheme. In the radio equipment, of course, apply low-voltage.
To check the operation of the varistor need to pay attention to appearance. It can be found at the circuit input (where power is supplied). Since a very large current passes through it - as compared with the protected circuit - this, as a rule, affects its case (chips, burnt places, darkening of the lacquer coating). And also on the board itself: at the place of soldering, the assembly tracks can peel off, the board darkens. In this case, it must be replaced.
However, even if there are no visible signs, the varistor may be faulty. To check its health, it will be necessary to unsolder one of its conclusions, otherwise we will check the circuit itself. For dialing, a multimeter is usually used (although you can, of course, try a megohm meter, it is only necessary to take into account the voltage it creates so as not to burn the varistor). It is easy to call him, the connection is made to the contacts and its resistance is measured. We put the tester at the maximum possible limit and see that the value is not less than a few hundred MΩ, provided that the voltage of the multimeter does not exceed the response voltage of the varistor.
However, infinite resistance, provided that the ohmmeter is quite powerful (if you can use this word), it also indicates a malfunction. When checking a semiconductor, you must remember that it is still a conductor and it must show resistance, otherwise we have a completely burned part.
Varistor Reference and Marking
If a replacement is needed, the varistor reference will come to the rescue. To begin, we will need the marking of the varistor, it is located on the body itself in the form of Latin letters and numbers. Although this element is produced in many countries, the labeling is not fundamentally different.
Different manufacturers and markings are different. 14d471k and znr v14471u. However, the parameters are the same. The first digits "14" is the diameter in mm., The second number 471 is the voltage at which the operation (opening) occurs. Separately about the labeling. The first two digits (47) is the voltage, the next is the coefficient (1). It shows how many zeros to put after the number 47, in this case 1. It turns out that the device under test will operate at 470 V, plus or minus the error that is put next to this number. In our case, this letter “k” is located after and denotes 10%, i.e. 47 V.
Other markings are s10k275. Error rate faces the voltage, the voltage itself is shown without a coefficient - 275 V. From the considered examples, we see how the marking can be determined: we measure the diameter of the device, we find these dimensions on a varistor, other figures show voltage. If it is not possible to determine the marking, for example, kl472m, you will need to look on the Internet.
Diameter. Imported tvr 10471 can be replaced by 10d471k, but be careful with 7d471k, the latter has a smaller size. The greater the value, the more power dissipated, roughly speaking. Putting the device of smaller diameter, we risk to burn it. For example, the 10d series has a working current of 25A, and the k1472m is 50A.
To choose the right element correctly, it is necessary to take into account not only the supply voltage. Produce a lot of calculations, for example, leaving the desired speed (response), or a small operating voltage. In this case, the use of so-called protective diodes. These include bzw04. When applying it, it is important to observe polarity.
Noise immunity. One of the drawbacks is the creation of interference. To combat them use capacitors, for example, ac472m Connect in parallel with the varistor.
In the diagram, the varistor is denoted as a resistor, an empty rectangle with a line striking under 45 degrees and has the letter u.