Help designing a 5V UPS circuit.

[HellasTechn]

Hello Friends.

Here is my idea: i have a circuit that runs on 5Vdc and a 12V supply with an 7805Vreg feeding it.
My idea is to make a "ups" with a 9V Nimh (200ma rechargeable) battery so that when the 12V are gone the circuit will be running from the 9V battery again through the 7805Vreg.
My circuit at full load will drain about 100ma so i am thinking abut the 9V Nimh batt because they are cheap and good for long term low current use. Besides i would only need it active for about 30min or so.
I could build the simplest desing with a relay latching on the 12V supply and fliping to the 9V when 12V are out but then again the problem is that i need the 9V to slef charge and standby when 12V are present...
Something like a charge controller in other words.

Gudelines and ideas are welcomed. Thank you.

 

[kellys_eye]

Feed the charging circuitry direct from the 12V supply line.

OR the output of the 12V line with the output of the 9V Nimh using diodes. When the 12V supply is on, the diode at the output of the Nimh source is reverse biased and doesn't allow current to flow into the 7805. When the 12V supply goes off this reverse-biasing is removed and the Nimh output flows to the 7805 instead.

OK, so there are small losses in the OR diodes but if it's simplicity you're looking for....... and use low volt-drop diodes as gating devices.

Modular Nimh charging circuit (boards) are all over the usual online markets (eBay, Banggood etc).

 

[HellasTechn]

I see... You have a point there. I am not sure about the diode method though but its not a bad idea.

any other suggestion ? maybe use of other battery ?

There are many, cheap 18650 or 14500 batteries that i could use in series but again how could i charge them ?
Are there chargers that can charge each individual cell ?

 

[kellys_eye]

Your battery choice is a flexible decision. Size, availability, amphere-hours (capacity), chemistry etc. If the UPS function isn't expected to be operated very often - mains supplies generally being reliable these days - then even alkaline (non-rechargable) batteries could be used.