Posistor: how to check for serviceability?

In the kinescope of most TVs, demagnetization systems are used, in which a posistor is built-in. How to check such a chain in case of failure yourself, you need to know the owners who want to carry out repairs themselves. The element has physical properties, which can be checked with an ordinary ohmmeter.

Element Properties

It is worth studying what the posistor is like, how to test it in a circuit - it will become clear later. This element is capable of changing properties as a function of temperature. Measure its physical size resistance. At room temperature, the ohmmeter values indicate units or tens of Ohms.

During heating, the resistance begins to change in the larger direction. The values of the ohmmeter already show hundreds of kilo, 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? The ways of solving this problem are given below.

Due to their properties, posistors are widely used in microelectronics for various purposes:

• Protection of power circuits. With increased current consumption, the element heats up and increases the resistance to a maximum when current cutoff is observed.
• In 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 heat, like a posistor. How to check the first element? With this, everything is simple. The resistor values range very little. The posistor is also capable of completely blocking the current passing through it, like the thistor. Only the latter has an inverse temperature dependence.

To know how to check the serviceability of a posistor, you must determine its main performance characteristics. They include:

• Resistance nominal at normal ambient temperature (more often it is 20-25 degrees);
• The switching resistance is determined at the point of resistance-temperature characteristic when the first parameter is increased by 2 times in comparison with the nominal value;
• The maximum voltage that the element can withstand without failure;
• Values of current loads: nominal, switching, maximum possible and rollover; For checking these parameters are important only if the posistor is used in high accuracy circuits.

Element in the demagnetizing circuit

How to check the posistor on the TV? The answer to the question follows from the principle of his work. The malfunction of the element is manifested by distortion of the image from magnetization. To eliminate this defect in the design of screens, a grid is used that is connected in series with the posistor. This design is called an external loop, covering the entire surface of the screen from the inside.

The posistor is often sealed in the screen mask circuit, which complicates its on-site verification. Before carrying out the measurements, one must at least one end from the grid. The best option is to completely remove it from the schema.

To heat the element use a conventional or mounting hair dryer. For testing without external heating, it is necessary to assemble the electrical circuit and determine the type of the posistor by marking. Based on the passport data of the device, the operating current of the element and the corresponding temperature are set.

The serviceability of the posistor can be conditionally established by heating with a hairdryer. If the resistance increases, then the element is good. However, with this version of the verification there is a chance of an error. After all, the resistance of the elements of the circuits changes over the years, which leads to instability in the work of the assembly.

Why do we need a system in kinescopes?

On TV screens without a demagnetization system, the image would be distorted with a slight influence of the electromagnetic field. It radiates all household appliances, the surface of the Earth is permeated with invisible waves.

So amplifiers, large speakers, heating elements are often located next to the TVs. Without a screen mask, the image would be constantly distorted. During initial operation, a small current flows through the posistor, which does not cause its heating. At the same time, the mask is physically subjected to stress from the emerging field.

This is an applied magnetic field and demagnetizes the mask at the moment the television is turned on. Often this process is accompanied by a sound comparable to a blow on the gong. The larger the screen diagonal, the higher the tonality of the sound. The posistor at this moment passes a high amplitude current through itself, which leads to its heating. The resistance increases and the element closes the chain.

Faults in picture tubes

If the image is distorted at the first start or ripples and stripes are observed, then the posistor is most likely to be guilty. How to check the multimeter in the circuit? On a cold circuit, this is easier, because the resistance of the posistor is minimal.

Often the soldering of contacts simply falls off from a long work. The posistor refers to circuit elements that constantly work in a heated state. With an ohmmeter, check the connection of the screen mask with the output of the second leg of the posistor. If it is minimal, this indicates a reliable connection. Probably, the element does not work on cutoff.

If the posistor is faulty and shorted, the fuse of the power supply unit blinks the first time it is turned on. Provided that this happens without an apparent short circuit in the circuit, you can check the malfunction by completely disabling the screen mask and the posistor.

Element in the chain of coolers

If the rear of the refrigerator is not heated - the radiator, then for self-repair it is necessary to get acquainted with how to check the posistor. In the refrigerator, two types of starters can be used: with posistors and with electromagnetic relays. The former spend part of the energy on heat loss in the element's resistance, the latter are less reliable, but they are not heated.

Most of the posistors in refrigerators should have a resistance of about 20-30 ohms. In the heated state it can be several kiloammers. 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 carrying out the measurements.