How to make battery backup for circuit?

killjoy said:

now thats clear to me. ^_^
who cause the diode to reversed biassed?

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ericgibbs said:

hi,
From the 12V supply when its measured after the diode, it will be 11.3V.

So on the battery diode you have 9V on the Anode and 11.3V on the Cathode.

Thats a reverse voltage on the diode of 11.3 - 9V = 2.3V

As you may know a diode has to have a forward bias voltage of about 0.7V before it conducts.

OK.?

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killjoy said:

ah sir last question. will the battery supply immediately after the 12v is off?thx

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ericgibbs said:

hi,
Yes it will.

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killjoy said:

bump bump*

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