Protection of the control AC from overload. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Speakers
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When setting up the UMZCH, to check its sound by ear, an acoustic system is connected to the amplifier instead of a load equivalent. For these purposes, small control speakers with inexpensive and relatively low-power dynamic heads are often used. To avoid failure of the heads due to overloading the speakers with a strong signal or when a constant voltage appears at the output of the amplifier, you can protect the speakers with self-resetting fuses.
The figure shows a diagram of a control speaker with sensitivity adjustment and such protection.
The speaker sensitivity is regulated by a powerful variable wire resistor R1. The FU1 polymer resettable fuse can be thought of as a thermistor with a high non-linear positive temperature coefficient of resistance. When the speaker is overloaded, the fuse heats up, its resistance increases sharply, and the power supplied to the dynamic head is limited. For the types of fuse and dynamic head shown in the diagram, the residual power supplied to the speaker with a winding resistance of 4 ohms after the fuse is blown will be about 100 mW with a signal power of 25 W supplied to the speakers.
To protect a speaker system with a speaker with a power of 3 ... 8 W, a resettable fuse for a current of 0,25 ... 0.4 A is suitable. In this case, it is desirable to use the most high-voltage types of fuses, for example. MF-R025, MF-R030. LP60-025, LP60-030, LP60-040 (all - for an operating voltage of 60 V and a rated operating current from 0.25 to 0,4 A). Solder the fuse at a distance of at least 10 mm from the case. By placing the fuse in the speaker cabinet. it must be excluded from blowing it with air currents created by the diffuser of the dynamic head, otherwise the fuse will operate at a much higher current. A variable resistor can be used with any wire-wound resistor with a power of 5 W or more.
Since a self-restoring fuse is a non-linear element, it is undesirable to protect acoustic systems intended for regular listening to high-quality phonograms with it, since it will introduce non-linear distortions.
Author: A.Butov, village of Kurba, Yaroslavl region
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