The voltage of the lighting network is not always stable; it often fluctuates, sometimes dropping below its nominal value, and at other times rising above it. Most commonly, the voltage drops during peak load hours, typically in the evening. Occasionally (for example, in the case of a poorly maintained lighting network and its overloading), the voltage drop can be quite noticeable, reaching 20...25% and sometimes even more. Voltage surges above the nominal value can occur at night, on holidays, and at other times. Televisions contain many electron tubes designed for a specific power supply voltage. The lamps and cathode-ray tube (kinescope) perform best when their supply voltage precisely matches the normal value. This also ensures their longest operational life. This means that if the television is connected to a 220-volt nominal voltage network, it operates best and most durably when the network voltage is maintained at 220 volts at all times.

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Too low voltage causes a decrease in brightness, often a reduction in image dimensions, especially in the horizontal direction. Images become pale, pictures start 'running' up or down, and lines begin to flicker.

The impact of voltage drop on the television transmission quality is particularly noticeable when electron tubes have been operating for an extended period. Under normal voltage, these tubes continue to function satisfactorily, but even a slight voltage drop can cause a sharp decline in their performance.

To compensate for fluctuations in the network voltage, the industry manufactures special devices such as autotransformers and voltage stabilizers. Among these, autotransformers are simpler and more cost-effective.

An autotransformer is connected between the lighting network and the television, meaning the autotransformer is connected to the lighting network, and the television's power cord plug is inserted into the autotransformer's socket. Autotransformers allow the voltage applied to the television to be regulated either by rotating a corresponding switch or by repositioning the plug.

The most common autotransformers are those with stepwise voltage regulation, such as the PAT type, whose external appearance is shown in the diagram.