A simple UPS based on an electronic transformer. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A simple UPS based on an electronic transformer

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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The article describes a very simple switching power supply. Its simplicity is due to the use of an "electronic transformer" - a serial product designed to power halogen lamps.

To power modern electronic devices, pulse sources are increasingly used, in which the voltage is converted at a high frequency. This allows, as is known, to significantly reduce the size and weight of such power supplies compared to linear ones.

However, for novice radio amateurs, when making their own switching power supplies, the greatest difficulty is the establishment, during which expensive high-voltage transistors often fail. And the supply voltage in the primary circuits, galvanically connected to the network, is quite high - about 300 V, which, firstly, imposes increased requirements on the interwinding insulation of the converter transformer, and secondly, makes the establishment of such a unit unsafe.

Quite simply, this problem can be solved if you use a ready-made device - an "electronic transformer". Similar devices appeared on sale several years ago in specialized stores selling professional electrical and commercial equipment.

"Electronic transformers" are designed to power 12-volt halogen showcase lighting. They are powered by a mains voltage of 220 V, a frequency of 50 Hz, and at the output they have high-frequency alternating current pulses with an amplitude of 12 V.

The described device uses a 105 W transformer, but there are more powerful models. Depending on the country of manufacture and the trading organization, the price of this product ranges from 250 to 350 rubles.

In fact, such a transformer is a very compact switching voltage converter. The conversion frequency (depending on the model) is from 50 to 80 kHz.

For practical use in the power supply "electronic transformer" does not require any modifications. It is only necessary to manufacture a surge protector, a matching transformer, a rectifier and an output filter.

The figure shows a power supply diagram based on a similar product. This device has been working in conjunction with a stereo amplifier with an output power of 2x20 W for two years. Its output voltage under load is 100mA at a nominal supply voltage of 220V - ±25V. If the load current is increased to 2,2A, the output voltage is reduced to ±20V.

A simple UPS based on an electronic transformer

The supply voltage through the switch SA1 and the fuses FU1 and FU2 is supplied to the filter L1C1-C3, which protects the network from interference of the pulse converter. The middle point of switching on capacitors C1 and C2 is connected to the shielding casing of the power supply. It should be especially noted that the casing must be isolated from the common wire and the metal chassis of the powered device (load).

Next, the mains voltage is fed to the input of the "electronic transformer" U1, from the output terminals of which (with the inscription 12 V) a reduced voltage is supplied to the matching transformer T1. The alternating voltage from the secondary winding of the transformer rectifies the VD1-VD4 diode bridge and smoothes the L2C4C5 filter.

Inductor L1 used ready-made DF90PTs from ZUSCT TVs.

Transformer T1 is made independently. The output voltage of the power supply depends on the number of turns of its secondary winding. The transformer is made on a K30x18x7 ring magnetic circuit made of M2000NM ferrite. The primary winding contains 10 turns of PEV-2 1,2 wire. The secondary winding (for an output voltage of ± 24 V) is wound with a PEV-2 0,8 wire folded in half. It contains 22 turns. By connecting the end of the first half-winding with the beginning of the second, the middle point of the secondary winding is obtained.

Choke L2 is also homemade. It is wound on the same as the transformer ferrite ring K30x18x7 M2000NM with PEV-2 0,8 wire. Both windings contain 20 turns.

Rectifier diodes must be placed on a heat sink with an area of at least 50 cm2. Moreover, the diodes, in which the anodes are connected to the negative output of the bridge, should be isolated from the heat sink with mica spacers, and the heat sink itself should be isolated from the power supply housing.

Smoothing capacitors C4 and C5 consist of three K50-46 connected in parallel with a capacity of 2200 microfarads each.

The shielding casing can be made of thin sheet brass or tin plate. It must be provided with holes for cooling.

The parts of the power supply are placed on a board made of insulating material and connected by segments of an insulated mounting wire. The design should completely exclude the possibility of touching the conductors and parts of the device connected to the input of the "electronic transformer", which are under mains voltage.

A properly assembled power supply starts working immediately and does not need to be adjusted. During the test, 240 ohm resistors with a dissipation power of at least 3 W should be connected to the output of each arm. It is not recommended to turn on the unit without load.

If the powered device contains highly sensitive circuits (for example, microphone amplifiers), then it is useful to shunt the smoothing capacitors C4 and C5 with ceramic capacitors with a capacity of 0,33 μF for a nominal voltage of at least 63 V (several capacitors can be connected in parallel).

Author: A.Sharonov, Chita

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