Dedicated power supply. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Power Supplies
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On fig. 4.6 shows a diagram of a power source that remains operational even when the mains voltage is turned off, while the device switches to battery power. The step-down transformer used in this circuit is selected based on the maximum current required by the load. Usually the largest current is consumed by the buzzer.
AC voltage from the secondary winding of the transformer is supplied to the rectifier bridge, made on diodes D1-D4. Further, the pulsating voltage is smoothed out by an electrolytic capacitor C1 and applied to a voltage regulator on transistors Q1, Q2 and a zener diode D5 In general, most of the circuits described in this book are not critical to the stability of the supply voltage. Therefore, the stabilizer can be removed without including transistors Q1, Q2, Zener diode D5 and resistor R2 in the circuit. Connect the positive plates of electrolytic capacitors C1 and C2 with a wire. With diodes D6 and D7 spare battery. B1 is off all the time when there is voltage in the network, which means on the capacitor. When it fails, this battery is switched on under load. Diodes D1-D4, D6, D7 are selected based on the value of the current consumed. Zener voltage D5 12 ... 14 V.
Assembly of the device. For reliability, it is better to place the loop signaling board in a metal case. It is better to take it in a size larger than necessary in order to be able to add something else in the future. The same applies to the voltage source.
Any insulating material is suitable for the board. It is also convenient to use foil getinaks or fiberglass. However, if you are going to put only one circuit on the board, you can do it faster without resorting to printed wiring. When assembling the power supply, attention should be paid to securely fastening the transformer to the bottom of the metal case.
The trinistor and transistor Q2 in the power supply must be installed on heat sinks for better heat dissipation. For this purpose, two aluminum plates of 13 cm each are suitable. In fact, the surge in power consumption only occurs when the buzzer is operating, and this time is relatively short.
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