# Circuit Design - Automatic Switching between two DC Supplies

#### Joe747

Joined Sep 20, 2023
3
I have 2 DC supplies:
1) Supply from a regulator ( LM7809) supplying 9V
2) Supply from a Lead Acid battery ( 12V )

Requirements :
*When the supply from the Regulator stops supplying any voltage the back up supply (battery) should start supplying automatically.
* There should be an LED indicator to show when the battery is supplying

Is it possible to use a Relay switch for this, and how can it be set up ?

Any sort of guidance guidance will be appreciated

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#### WBahn

Joined Mar 31, 2012
29,519
A lot depends on the details of what is needed.

What is powering the regulator?

How much current does the load need?

Does the voltage to the load need to be kept at 9 V when the regulator stops supplying power, or is it okay to all of a sudden start supplying it with 12 V?

Does the indicator showing that the battery is supplying power need to be lit even if the battery isn't actually supplying any power (for instance, if the regulator power goes a way but the load is removed and, hence, not drawing any current at all).

What is the lowest voltage to the load that is acceptable?

There are a number of different ways to approach this problem, some very simple and some much more complicated, and the answers to questions like this are what determine which approaches can/should be ruled out and which ones should be considered first.

#### Joe747

Joined Sep 20, 2023
3
A lot depends on the details of what is needed.

What is powering the regulator?

How much current does the load need?

Does the voltage to the load need to be kept at 9 V when the regulator stops supplying power, or is it okay to all of a sudden start supplying it with 12 V?

Does the indicator showing that the battery is supplying power need to be lit even if the battery isn't actually supplying any power (for instance, if the regulator power goes a way but the load is removed and, hence, not drawing any current at all).

What is the lowest voltage to the load that is acceptable?

There are a number of different ways to approach this problem, some very simple and some much more complicated, and the answers to questions like this are what determine which approaches can/should be ruled out and which ones should be considered first.
Thank you Wbahn for the reply

*The regulator is the final component from my ac to dc supply.
* The load current can work of minimal current.
* The voltage to the load need to be kept at 9V
*The LED must only be lit when the battery is supplying
* The voltage at the load should be as close to 9v as possible

Furthermore, the battery should charge when not supply.

Thank you

#### WBahn

Joined Mar 31, 2012
29,519
Why not power the regulator from the battery and float charge the battery?

#### AnalogKid

Joined Aug 1, 2013
10,792

#### vu2nan

Joined Sep 11, 2014
340
Two Schottky diodes and an electromagnetic relay would be required.

The power supply predominates on account of its higher voltage (14V).

The battery takes over when the power supply is off.

The battery is charged through resistor R1. It's resistance value decides the charging current.

Overheating of the regulator is avoided, with the resistor R2 limiting its input voltage.

The LED lights up, when mains power is off, indicating that the battery has taken over.

Nandu.

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#### ericgibbs

Joined Jan 29, 2010
18,255
Hi Vu2,
The LED will be connected directly across 12V, no limiter resistor.
E

#### vu2nan

Joined Sep 11, 2014
340
Hi Eric,

That's supposed to be a 12V LED with in-built resistor!

Will include the resistor.

Thanks.

Nandu.