The following circuit is the basic form of the commercial Uninterruptible Power Supply (UPS), the circuit gives a constant regulated 5 Volt output and an unregulated 12 Volt supply. Within the event of electrical supply line failure the battery pack takes over, with no spikes on the regulated supply.
This circuit could be modified for different regulated and unregulated voltages by applying various regulators and batteries. For a 15 Volt regulated supply, make use of two 12 Volt batteries in series including a 7815 regulator. There’s plenty of flexibility in this circuit.
TR1 has a main matched towards the local electrical supply that is 240 Volts in the UK. The secondary winding should be rated at least 12 Volts at 2 amp, but could be greater, as an example 15 Volts. FS1 may be a slow blow type and protects from short circuits on the output, or indeed a bad cell inside a rechargeable battery. LED 1 will light ONLY when the electrical power supply is present, having a power failure the LED will go out and output voltage is maintained by the battery.
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