Smooth start of impulse and transformer power supplies. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Smooth start of impulse and transformer power supplies. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Power Supplies

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When the power supplies of amplifiers, laboratory and other power supplies are turned on, interference occurs in the network caused by the inrush currents of transformers, charge currents of electrolytic capacitors and the start of the powered devices themselves.

Outwardly, this interference manifests itself as a “blinking” of light, clicks and sparks in power outlets, and electrically it is a drawdown of the mains voltage, which can lead to failure and unstable operation of other devices that are powered by the same network. In addition, these inrush currents cause burning of the contacts of switches, power outlets. Another negative effect of the starting current is that rectifier diodes with such a start work with current overload and can fail. For example, the inrush charge current of a 10000uF 50V capacitor can reach 10 or more amperes. If the diode bridge is not designed for such a current, such operating conditions can damage the bridge. Especially strong starting currents are noticeable at a power of more than 50-100W. For such power supplies, we offer a soft starter.

Smooth start of impulse and transformer power supplies. Scheme

When connected to the network, the power supply starts through the current-limiting resistor R4. After some time, necessary for its start, charging the capacitors and starting the load, the resistor is shunted by the relay contacts and the power supply is brought to full power. The turn-on time is determined by the capacitance of the capacitor C2. The C1D1C2D2 elements are a transformerless power supply for the relay control circuit. Zener diode D2 plays a purely protective role, and with a good control circuit, it may be absent. The BS-115C-12V relay used in the circuit can be replaced with any other relay with a contact current of at least 10A, with the selection of zener diodes, capacitor C1 and the choice of transistor VT1 for a voltage greater than the relay operation voltage. Zener diode D3 provides hysteresis between the on and off voltage of the relay. In other words, the relay will turn on abruptly, not smoothly.

Capacitor C1 determines the switching current of the relay. In case of insufficient current, the capacitance of the capacitor must be increased (0,47 ... 1 μF 400 ... 630V). For protective purposes, it is advisable to wrap the capacitor with electrical tape or put a heat shrink tube on it. Fuses are selected for twice the rated current of the PSU. For example, for a 100W power supply, the fuses should be 2*(220/100)=5A. If necessary, the circuit can be supplemented with a network symmetrical / unbalanced filter, connected after the fuses. The connection to the body, present in the diagram, can only be regarded as a common wire for connecting the tester. In no case should it be connected to the chassis of the device, output it to the common wires of network filters, etc.

Publication: radiokot.ru

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