Need a circuit to power a 9 volt DC device which includes a 9v backup battery

Use two schottky diode in series and connect the DC supply and the battery both at the same input of the circuit. It will work perfectly. You do not need any additional circuitry.

Thanks Sourav

My knowledge of electronics now is a bit shaky and I have never used schottky diodes. I have built model train controllers with 'inertia' braking etc, motor vehicle capacitor discharge ignition system (when motor vehicles used HT coils and distributors for the spark plugs)  and other simple circuits in the past but that was 30 years ago.

Also would this isolate the backup battery when power is being supplied from the power pack ie the battery would NOT receive any power from the plug pack?

I would preferable want to have a circuit which supplies the load from the power pack and a regular (non rechargable) battery which powers the load if the power from the power pack is lost and has no connection to the power pack.

Thanks again.

Following on from my previous reply -

I drew a diagram but could not work out how to include it in the reply so will try and describe it.

connect POS from 9v DC power pack to POS of DIODE (hope I have that correct) and then the NEG of DIODE to the load - I will refer to that diode as D1

connect NEG from 9v DC power pack directly to load

connect NEG rail? to NEG of 9v backup battery

connect the POS of 9v battery to POS of DIODE (D2)

connect the NEG of D2 to a point on the POS rail between the load and D1 - would that prevent voltage from the 9v DC power pack connecting to the backup battery?

Oh I wished I could have included an image of what I mean above and so hope you understand.

Do not use the R2... That will be not required. You can try this and it will auto switch the circuit.

Thanks Sourav

I was wondering if only a diode connecting the backup battery that some (small) voltage would still be present at the battery terminals and also wondered if that small voltage would cause problems (don't try and charge a non recharable battery).

BTW is the only method to include an image in a reply is to have that image located/stored somewhere on the internet and to then add the url for that image in the reply?

Hello again Sourav, hope you read this.

I have eventually got around to building a circuit as per your diagram and have purchased all the required parts (and some extra such as a 'breadboard',  PB board mounted fuse to protect the 9V adaptor and a green LED + resistor to indicate that there is power from the 9v adaptor).

The MOSFET I purchased is a SUP53P06-20 P-CH 60V 53A. I am wondering if the resistor values you gave quoted would be affected by this particular mosfet and also the following -

actual voltage from 9v dc adaptor is 8.8V

load is around 10mW constant and 'spiking' to around 300mW for a few minutes once every 2 days.

I have never worked with mosfets before.

Thanks

John

Kindly hook up the components in the breadboard and check whether it is working or not.