In picture tubes of most televisions, demagnetization systems are used, in which a posistor is built-in. How to check such a circuit in case of failure on its own, you need to know the owners who want to carry out repairs on their own. The element has physical properties that can be checked with a conventional ohmmeter.
Item Properties
It is worth exploring what a posistor is, how to test it in a circuit - it will become clear later. This element is capable of changing properties depending on temperature. Measure its physical value resistance. At room temperature, the ohmmeter values show units or tens of ohms.
When heated, the resistance begins to change upward. Ohmmeter values already show hundreds of kilograms, which indicates the normal state of the element - such a posistor is working. How to check if there is a suspicion of a faulty circuit? Ways to resolve this issue are given below.
Due to its properties, posistors are widely used in microelectronics for various purposes:
- Power circuit protection. With increased current consumption, the element heats up and increases the resistance to a maximum when current cutoff is observed.
- In the heating circuits. Thanks to the posistors, an automatic heating control system is implemented.
- In the schemes of temperature sensors.
The internal structure of the element
The resistor changes its resistance with heating, as does the resistor. How to check the first item? With this, everything is simple. At the resistor, the values fluctuate within insignificant limits. A posistor is able to completely block the current passing through it, like a temistor. Only the latter exhibits an inverse temperature dependence.
To know how to check the health of the posistor, you should determine its main performance characteristics. These include:
- nominal resistance at normal ambient temperature (usually it is 20-25 degrees);
- switching resistance is determined at the point of the characteristic of the dependence of resistance on temperature, when the first parameter is increased 2 times in comparison with the nominal value;
- the maximum voltage that the element can withstand without failure;
- values of current loads: rated, switching, maximum possible and overturning; for verification, these parameters are important only if the posistor will be used in high-precision circuits.
The element in the demagnetization circuit
How to check the posistor on the TV? The answer to the question follows from the principle of its work. The element malfunction is manifested by image distortion from magnetization. To eliminate this defect in the design of the screens, a grid is used, connected in series with the posistor. This design is called the outer loop, covering the entire surface of the screen from the inside.
The posistor is often soldered into the screen mask circuit, which complicates its on-site verification. Before taking measurements, you should unsolder at least one end from the grid. The best option would be to extract it completely from the circuit.
To heat the element using a conventional or mounting hair dryer. To check without external heating, you will need to collect the electrical circuit and determine the type of posistor by marking. Based on the passport data of the device set the response current of the element and the corresponding temperature.
The serviceability of the posistor can be conditionally set when heated with a hairdryer. If the resistance grows, then the element is suitable. However, with this verification option, the probability of an erroneous result remains. After all, the resistance of circuit elements changes over the years, which leads to instability of the assembly.
Why do I need a system in picture tubes?
On television screens without a demagnetization system, the image would be distorted with a slight influence of the electromagnetic field. It is emitted by all household appliances, the Earth's surface is riddled with invisible waves.
So amplifiers, large speakers, heating elements are often located next to TVs. Without a screen mask, the image would be constantly distorted. During initial operation, a small current flows through the posistor, not causing it to heat. In this case, the mask physically experiences stress from the emerging field.
This is an applied magnetic field and demagnetizes the mask at the moment the TV is turned on. Often this process is accompanied by a sound comparable to a blow to a gong. The larger the diagonal of the screen, the higher the tonality of the sound. The posistor at this moment passes a high amplitude current through itself, which leads to its heating. An increase in resistance occurs and the element closes the circuit.
Variants of faults in picture tubes
If the image is distorted during the first start-up or ripples and stripes are observed, then the posistor is most likely to blame. How to test a multimeter element in a circuit? On a cold circuit, this is easier to do, because the resistance of the posistor is minimal.
Often soldering contacts simply falls off from long work. A posistor refers to circuit elements that constantly operate in a heated state. An ohmmeter checks the connection of the screen mask with the output of the second posistor leg. If it is minimal, this indicates a reliable connection. Perhaps the item does not trigger the cutoff.
If the posistor is faulty and shorted, then the power supply fuse will blow when it is first turned on. Provided that this happens without a visible short circuit in the circuit, you can check the malfunction by completely disabling the screen mask and the posistor.
Element in a chain of coolers
If the back of the refrigerator is not heating up - the radiator, then for self-repair you need to familiarize yourself with how to check the posistor. In the refrigerator, 2 types of starters can be used: with posistors and with electromagnetic relays. The former spend some of the energy on heat loss in the resistance of the element, the latter are less reliable, but they do not heat up.
Most posistors in refrigerators should have a resistance of about 20-30 ohms. In a heated state, there may be several kilograms. If the values are significantly higher than those given, then the element must be replaced. It is important to allow the posistor to cool to room temperature before taking measurements.