# SCR as a Switch in back-up power

#### samrat

Joined Apr 2, 2008
30
Need to understand how the SCR can act as a switch to trigger a back up battery of 12 volts ON when the power goes OFF...explanation with a circuit diagram would be really handy.

#### mik3

Joined Feb 4, 2008
4,843
Need to understand how the SCR can act as a switch to trigger a back up battery of 12 volts ON when the power goes OFF...explanation with a circuit diagram would be really handy.
what is the DC voltage output of the power supply?

#### samrat

Joined Apr 2, 2008
30
Hi MIK3.. the output DC voltage is 12 volts from a rectifier cct that was stepdown from from a 240 v AC supply. The 12 volts should also be used to charge the 12 volt battery. The SCR should trigger the power switch to turn to the 12 volt battery once the main power is cut off. This 12 volt battery is for a standard lighting system at home that can operate on 12 volts.

#### samrat

Joined Apr 2, 2008
30
I know that a SCR is triggered by a positive pulse and stays ON but for this assignment, it has to be triggered when the power supply is OFF. Thats where I am stuck.

#### mik3

Joined Feb 4, 2008
4,843
I know that a SCR is triggered by a positive pulse and stays ON but for this assignment, it has to be triggered when the power supply is OFF. Thats where I am stuck.
You can make things simpler if you forget the SCR and use a relay. The power supply will drive the relay when the power is on. When the power goes off the relay will release and connect the battery to the circuit you want. Is it possible or you have to do it with an SCR?

#### samrat

Joined Apr 2, 2008
30
I have thought about the relay but a scr is what is needed to be used.

#### mik3

Joined Feb 4, 2008
4,843
I have thought about the relay but a scr is what is needed to be used.
Ok see the attachment then. But note that when the power comes back again on the power supply the SCR wont shut off. if you want you can replace the SCR by an NPN transistor as when the power comes back to release the relay. Or if you still want the SCR use a transistor in parallel with the SCR to reset it when power comes back.

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#### samrat

Joined Apr 2, 2008
30
Thanx Mik3..really appreciate it....

#### samrat

Joined Apr 2, 2008
30
Mik3, the top right side of the diagram you sent was not very clear. Couldn't read the writing. Can you please specify??? thanx

#### samrat

Joined Apr 2, 2008
30
Hey Mrmeval, can you please resend the web address that you sent? Couldn't find a link to the one that you sent earlier.

#### mik3

Joined Feb 4, 2008
4,843
Mik3, the top right side of the diagram you sent was not very clear. Couldn't read the writing. Can you please specify??? thanx
The word on the left is 'power supply' , the word on the top is 'to the circuit' and the word on the right is 'battery'