A simple emergency power supply. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies
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The electrical circuit shown in fig. 1.3, it is convenient to use in the country and where electricity is still unstable.
Rice. 1.3. Electrical diagram of the emergency power supply
A simple device, assembled according to the recommended scheme, will automatically turn on the backup lighting (or other active load with a power of up to 10 ... 12 W) in the event of a power failure of 220 V.
The VT1 transistor of the KT825 series (you can replace the one indicated in the diagram with the KT825 transistor with letter indices D and E) provides a maximum load of up to 25 watts. It must be installed on a radiator with a cooling area of at least 100 cm2. If a less powerful load is planned (up to 5 W), then it is possible to use a control transistor of the KT818AM-KT818GM type in the circuit.
A car battery with a capacity of 55-190 A / h is used as a backup power source.
Automotive incandescent lamps are used as back-up lamps.
Principle of operation of the device
The network power supply unit (PSU) generates a reduced rectified voltage of 13 ... 14 V. The PSU includes a step-down transformer and a rectifier bridge.
The ripple of this power supply is smoothed out by a high-capacity electrolytic capacitor C1.
The voltage from the power supply through the diodes VD1, VD2 and the limiting resistor R1 freely flows to the connected battery and charges it with a low current. With a charging current of 80-110 mA, a car battery can be charged without harm for a long time, up to about ten days in a row. The voltage drop across the VD2 diode creates a reverse bias for the base-emitter junction of the transistor VT1. The transistor is in the closed state and the load (EL1, EL2) is de-energized. Switch S1 is used to force the emergency operation. This may be needed for discharging. Battery or backup lighting system checks (lamp integrity).
Establishment
The device does not need adjustment.
When the mains power is turned off, the stationary power supply is de-energized, and the base circuit of the transistor VT1 receives current through the resistor R2, the transistor opens and the load is powered by the battery. As soon as the supply of energy in the network resumes, the transistor VT1 closes, the load is turned off, and the battery is charged according to the considered scheme.
About details
Resistor R1 brand MLT-2, resistor R2 - type MLT-0,5. The battery and load lamps are connected to the device with stranded insulated network wires with a cross section of at least 1 mm and with a minimum length (to reduce energy losses in the wires). Capacitor C1 brand K50-24, K50-ZB or another for a voltage of at least 25 V.
The best option for a step-down transformer of a mains power supply is a universal power transformer CCI 127/220-50-12.
Author: Kashkarov A.P.
See other articles Section Power Supplies.
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