Device for protection of radio equipment against excess mains voltage

Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies

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An article under this heading was published in "Radio" No. 6, 1997. The need to use such protective devices in practice is obvious. If the electricity supplier brings to the consumer a three-phase network connected according to the so-called "star" scheme, the equalizing or "neutral" wire often "burns out". Depending on the redistribution of loads to the phases, the mains voltage at the consumer can reach 380 V. There is no need to talk about the consequences.

The repetition of the device described in the article and its operation revealed some shortcomings.

So, if a network radio is powered from it, it is practically impossible to use it due to interference in the network created by a working triac. It is possible to reduce interference somewhat by connecting capacitors with a capacity of 0,1 μF each for a rated voltage of 400 ... Placing the device in a metal shield and connecting it to a common wire further reduces interference. If the mains filter is structurally a separate unit, the connection between it and the protection device should be made with twisted wires enclosed in a shielding braid, which is connected to the common wires of the units.

The first switching on of the device with a load connected to it - a 100 W incandescent lamp - was accompanied by the effect noted in the article by V. Setalov "Automatic Light Switch" ("Radio", 1984, No. 8, p. 54), namely, the glow of the lamp was incomplete and flickering. Obviously, it was necessary to increase the amplitude of the pulses opening the triac, and this required the use of a transistor of the KT315 series with a current transfer coefficient of more than 150. I will make a reservation that I did not have the opportunity to select a triac.

An asterisk is indicated near the reference designation of the capacitor C3, but the text does not explain what frequency of the generator should be set: 1500 Hz or, as in some similar devices, 15000 Hz?

Powering a Sharp TV (model CV-5133SC) from a protection device revealed another unexpected effect. If you turn on the TV (without the "preparation" mode) at the same time as the device, the purity of the color deteriorates sharply. It was not possible to return the TV to normal operation by repeated switching off and on when powered directly from the mains. Only by demagnetizing the kinescope from the side of the screen (the TV set is off) with a choke used in sound recording to demagnetize the magnetic tape, this "effect" was completely eliminated.

An explanation is possible as follows. At the moment the TV is turned on, a highly distorted, unbalanced signal is supplied to it due to the delay in stabilizing the supply voltage of the protection device. In this case, a non-sinusoidal current with a constant component flows through the demagnetizing loop of the TV, which very strongly magnetizes the shadow mask of the kinescope. One way to avoid disruption to your TV is to keep your security device plugged in at all times. The second is to complicate the device somewhat. In the conductor break, to the output 13 of the DD2.3 element (see Fig. 1 in the article mentioned above), include an additional node (see the proposed diagram), which will allow the passage of the triac control pulses only after the stabilization process of the supply voltage of the protection device is established.

All reference designations of elements are continuations of those adopted in the scheme of the protection device.

Author: V. Kvyatkovsky, Moscow

See other articles Section Protection of equipment from emergency operation of the network, uninterruptible power supplies.

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