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12V Alarm Power Supply: Circuit Diagram & Build Guide
Description
This device generates an output voltage of 12-volts, with a current capacity reaching up to 1-amp. In case of a power outage from the main supply – the reserve battery activates without delay. And once the primary power source is restored – the battery initiates its automatic recharging process.
More about components
7805
The 7805 requires a larger heat sink due to its substantial energy dissipation, particularly when tasked with recharging a depleted battery. Its heat sink operates at 9v1 and must be kept disconnected from ground. This integrated circuit converts a higher DC voltage to a stable 5-volt output, essential for powering sensitive electronic components within the circuit. It’s a very common and widely available linear voltage regulator.
7812
The 7812 necessitates a smaller heat sink because its power dissipation is limited to a maximum of 2-watts. Consequently, its heat sink can be more compact. This integrated circuit, similar to the 7805, is a linear voltage regulator designed to step down a DC voltage to a stable 5-volt output. Its lower power requirements translate to a smaller and more efficient heat sink design. It's frequently employed in applications where a lower current draw is desired.
Schematic Diagram
The 7805 needs the larger heatsink because it has to dissipate a lot of energy - especially when called upon to recharge a flat battery. Its heatsink is at 9v1 - and must NOT be connected to ground. The 7812 never has to dissipate more than 2-watts - so its heatsink can be smaller.
Veroboard Layout
Many of the components, which are shown lying flat on the board, are actually mounted standing upright. The links are bare copper wire on the component side of the board. The heatsinks are folded strips of aluminium, about 2mm thick. Use a well-insulated panel mounted fuse holder for the mains supply to the transformer - and fit it with a 1-amp fuse.
Use a genuine alarm type back-up battery. They are maintenance-free. Their terminals can be held at 13v8 for many years - with no apparent ill effects. They have a life expectancy of about five years. However, they tend not to recover from a very deep discharge. If you wish - you can use a smaller or larger capacity battery.