Baretter and urdox. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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In the first half of the last century, two elements that are unusual in the modern view, baretter and urdox, were developed and widely used. The design of the barter is very similar to the device of a conventional electric incandescent lamp. The filament is made of thin iron wire and placed in a hydrogen atmosphere. The resistance of the filament increases rapidly when it is heated by current, so the voltage drop across it also increases. As a result, the current is stabilized in the circuit with the barter connected in series.
In vintage radios, it was quite common to use transformerless power from a DC or AC mains. The filaments of all lamps and the barretter were connected in series and connected directly to a 110 or 117 V mains. The barretter provided a stable filament current with significant voltage fluctuations in the mains.
Any incandescent lamp has the properties of a barter - the resistance of its filament at room temperature is 10 times less than at nominal incandescence. This property can still be used to create very simple stable current generators.
In receivers with transformerless power supply, another problem arose: at the moment of switching on, when the filaments had not yet warmed up, the current pulse reached large values and often “burned out” them. By the way, the problem still exists - ordinary lighting lamps, not to mention Christmas tree garland lamps, burn out most often at the moment they are turned on.
Urdox, built into the barter, helped solve the problem. Its design resembles a resistor, but the conductive layer is made of a material that dramatically reduces resistance when heated. At the moment of switching on, the resistance of the urdox is high and all the mains voltage is applied to it, saving the lamps. The released heat warms up the urdox, its resistance drops, while the resistance of the filaments increases and the voltage is redistributed - most of it falls on the lamps and the barretter, and a smaller part on the urdox. After a few seconds, the lamps glow normally, and the urdox consumes only a small amount of power.
It should be noted that elements similar to urdox are still used, for example, to limit the inrush current in transformerless power supplies. These are thermistors with a negative temperature coefficient of resistance (TCS). When heated, their resistance decreases tenfold.
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