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Switching from internal battery supply to AC supply

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casey73

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First time here. I'm building a mini "console" similar in shape of an old recording studio console for my grandson at my daughter's request. He is a toddler and the console involves lights, switches, sound effects, etc. She would like it to be battery powered, and I suggested allowing for an internal DC supply (9 VDC) that can be powered from a 120AC wall outlet as well.

I want to make it user friendly so I came up with a simple idea I've never done before but should work in theory. I want to get other opinions in case I'm overlooking something as I've never worked with electro mechanical relays. So here is my idea.

There will be a master power switch and an internal battery power supply consisting of 6 each D cell batteries. I will also have a 9VDC AC line powered supply. My idea was to use a relay that will switch the power supply feed via a relay that is activated as soon as the AC power cord is plugged into the wall. The 120AC will activate the low voltage power supply, and in turn activate the relay in the "on" position to switch the source of power from on board batteries to the rectified AC supply.

Will the relay, if I stay in specs of the relay, be able to stay in the on position for an extended time without overheating and/or failing?
 
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Easier and cheaper to just use two diodes. If you are worried about the forward drop of a Si diode, use a Schottky instead.
 
Why not simply use a wall wart supply with a switching socket as used in many pieces of equipment.
 
As per MikeML's diode suggestion, the schematic is below (the power to your circuitry comes from the terminal marked "to amp V+"). For this to work, the voltage of the line-powered supply must be more than the voltage of the battery (e.g. 10.5 volts), or else the battery will be supplying the power to the circuit even when the power supply is connected.
diode-or-bridge.png

You may want to use the wall wart power supply to keep the 120VAC away from the toy as well, which can be safer.

If you use power-efficient components (e.g. LEDs instead of light bulbs, an automatic standby timer, etc.) the batteries can be made to last a long time and the external power supply is not warranted. Once you work out the current consumption of your circuit, you can easily calculate the battery life.
 
You can get dc power sockets with a built in switch for the purpose as mentioned, the switch opens the battery + and connects the circuit to the wall wart.
 
Power supply is an electronic power supply that uses a switching regulator in order to control the conversion of electrical power in a highly efficient manner. If you want use an external power supply its allows portability of equipment powered either by mains or battery without the added bulk of internal power components. Thanks for sharing with us.


Best Regards !!!
Riello-ups
 
Be careful using a diode OR as Schottky diodes can have a high reverse leakage at warm temps and you could end up (over)charging the battery from the mains. Maxim do ICs that can handle this.
When I have had to do this, I have float charged a Li-Ion cell using one of many available charger ICs and run the ciruit direct from the battery supply. Linear Techs LT4054 does the job for small cells.
 
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