This circuit represents a basic version of a commercial Uninterruptible Power Supply (UPS). It delivers a steady, regulated 5 Volt output alongside an unregulated 12 Volt power source. Should there be an interruption in the electrical supply, the battery seamlessly assumes control without generating any voltage spikes in the regulated supply.
This circuit can be customized for various regulated and unregulated voltage requirements by using different regulators and batteries. For instance, to achieve a 15 Volt regulated supply, you can employ two 12 Volt batteries connected in series along with a 7815 regulator. The circuit offers considerable adaptability.
TR1’s primary winding is specifically designed to match the local electrical supply, which is typically 240 Volts in the UK. The secondary winding should have a minimum rating of 12 Volts at 2 amps, but you can opt for a higher rating, such as 15 Volts. FS1, a slow-blow fuse, serves as protection against output short circuits or potential faults in a rechargeable battery cell. LED 1 illuminates only when the electricity supply is available, and in the event of a power failure, the LED extinguishes while the output voltage is upheld by the battery. The circuit depicted below simulates the operation of an active circuit with mains power connected:
You can access the nominal unregulated power supply between terminals VP1 and VP3, while a 5 Volt regulated supply is accessible between VP1 and VP2. Battery B1 is charged through the charging paths provided by resistor R1 and D1. The inclusion of D1 and D3 ensures that LED1 does not light up in the event of a power failure. This battery is intentionally set up for trickle charging, with the charging current determined by the following formula:
(VP5 – 0.6 ) / R1
where VP5 is the unregulated DC power supply voltage.
D2 must be included in the circuit, without D2 the battery would charge from the full supply voltage without current limit, which would cause damage and overheating of some rechargeable batteries. An electrical power outage is simulated below:
Note that in all cases the 5 Volt regulated supply is maintained constantly, whilst the unregulated supply will vary a few volts.
The duration for which the regulated supply can be sustained without an electrical supply is contingent on two factors: the load drawn from the UPS and the battery’s Ampere-hour (A/h) capacity. For instance, if you were utilizing a 7A/h 12 Volt battery and the load from the 5 Volt regulator was 0.5 Amp (with no load from the unregulated supply), the regulated supply would remain operational for approximately 14 hours. Utilizing batteries with larger A/h capacities would extend the standby time, whereas using batteries with smaller A/h capacities would result in shorter standby durations.