Network filter with full galvanic isolation from the network. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Network filter with full galvanic isolation from the network. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network

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In some cases, for example, during the repair of equipment or in special operating conditions, it is necessary to ensure complete galvanic isolation of this device or device from the mains. This is usually done with a transformer with equal primary and secondary windings.

Unfortunately, it is difficult to purchase such a transformer ready-made, and you have to do it yourself, which is very laborious. But it is possible to provide galvanic isolation between the mains and the load using any two identical network transformers, each with a power of at least three times the load power. For example, you can take two standard transformers with a power of 600 W or more, designed to receive 220 V AC voltage from the 36 V mains (such transformers are used in various workshops at enterprises). And turn them on as shown in the diagram.

(click to enlarge)

Transformer T1 is turned on as it should be, that is, the 220 V winding is connected to the mains. The second transformer T2 is included as a boost.

An alternating voltage of 36 V from the secondary winding T1 is supplied to the secondary winding of exactly the same transformer T2. Which works like a booster. As a result, on the primary winding T2, which is secondary in this connection, there will be an alternating voltage of 220 V. This voltage can power the load.

With transformer powers of 600 W, the load power should not exceed 200 W, otherwise a decrease in the output voltage under load is noticeable. You can use transformers with secondary windings for a different voltage, it should be noted that the higher the voltage of the secondary windings, the greater the efficiency will be provided.

Capacitors C1 and C3 must be rated for a voltage of at least 250 V. Capacitor C2 - for a voltage of at least 50 V. The author used old BMT-type paper capacitors commonly used in power supply circuits of asynchronous motors.

Of course, you can use a more modern element base. The capacitances of the capacitors can be two times or more different from those marked in the diagram. The task of these capacitances is to suppress interference over the network, since this circuit, in addition to galvanic isolation, also performs the functions of a network filter.

It is desirable that the transformers are the same or at least of the same type. If you use transformers for different voltages of the secondary windings, you can get an output voltage greater or less than the input. For example, using T1 with a 42 V secondary and T2 with 36 V, you can get an output voltage of about 255 V.

If you reverse the circuit, that is, apply voltage to a transformer with a secondary winding of 36 V, and remove voltage from a transformer with a low-voltage winding of 42 V, then the output voltage will be about 190 V.

It is even more convenient if you have transformers with secondary windings with taps, for example, 24 V, 36 V, 42 V. By connecting them appropriately, it will be possible to obtain various output voltages from 125 V to 380 V.

Author: Lyzhin R.

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There are a couple of questions. 1. What is the efficiency of such a scheme. 2. Are there devices that may not work correctly or not at all with galvanic isolation.

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