with LM7806 for 6V and LM7809 for 9V. The circuit is using a SLA battery of any capacity from 7AH to 12AH.
Working of the circuit is simple, when the power from the transformer power supply is going to the circuit the load will be connected with the transformer power supply and when the power from the transformer supply is absent the load will be automatically shift to the battery.
The circuit is simple to build and adjust, using few low cost components and works completely automatic. The maximum output of the circuit is upto 1A. The output current is also depends on the transformer used, If you have low current requirements in your project for example 200mA or 300mA then use a 1A transformer. But if your project requires upto 300mA for example 500mA or 1A then use a 2A transformer, because it provides current for battery charging and load at the same time.
A simple automatic two transistor battery charger is also used in the circuit which require some adjustments at first time. For adjusting the battery charger circuit connect a power supply in the place of 12V battery and set the power supply to 14.4V. Then adjust the 10K variable on the point where the LED goes ON. After this setting the circuit ready for use now remove the power supply and connect the backup battery.
The voltage regulator ICs dissipate heat during operating therefore use a suitable heatsink with them.
We often feel the need of an automatic UPS (Uninterruptible power supply) or a battery back circuit for our 5V, 6V and 9V projects. So here we have designed a good quality circuit for these purposes. The required output voltage can be achieved simply by replacing the LM7805 IC
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