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Old 17th Jan 2022, 8:53 pm   #1
Uncle Bulgaria
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Default Battery/Mains Switcher & LED supply splitter box. Elegant solutions sought.

My brother has a stall he lights with some LED strings and several spotlights. Sometimes the fairs he attends have mains hookups, sometimes he has to use a lead-acid 'leisure' battery. Sometimes the mains supply is intermittent.

I thought he'd appreciate a box that would allow connection of a 'wall wart' supply alongside his battery, switching the battery off if mains is present. A meter on the front to monitor battery voltage is always fun, and a low-voltage cutoff would prevent accidental excessive discharge of the battery.

DC outputs would be a barrel type, as any way to make it easier and faster to pack down is of benefit. Fiddling with screw terminals wouldn't make him happy. Simple discretes I may have in my stores are preferable to specialised ICs.

I have found some examples online of circuit elements that do some of what I want, and I have bread-boarded the elements together. This does what I expect, but I'm certain that the great minds here will immediately see more elegant ways of achieving my ends.

I've come across a voltage-divided 555 low-voltage cutoff and a MOSFET and zener diode circuit to expand the meter display, as only voltages between about 11-15V are worth showing.

I've assumed a NO/NC relay activated when the 'wall wart' is powered, to disconnect the battery from the NC contact. Perhaps this is too power-hungry and there's a transistor or MOSFET solution that would be preferable. These devices are still very new to me so I'm still at the stage of working out how to make them switch on an LED.
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Old 17th Jan 2022, 10:20 pm   #2
joebog1
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Default Re: Battery/Mains Switcher & LED supply splitter box. Elegant solutions sought.

There is one thing about a nice relay!!. They last almost forever if kept out of the weather. I use a single double pole changeover relay for my solar power installs. Except I dont have mains, I have a generator or the solar batteries. Thats my two sources.

A decent relay uses about 100 mA ( assuming 12 volts ) and works like this:
If you have mains power the relay pulls in, and switches the mains to the load.
If there is no mains power the relay relaxes ( saving 100 mA ) and connects the load to the supply.
If there is an inverter and the system is totally 240 volt mains powered driven by the batteries I switch in the inverter.
I dont in fact switch any DC at all. ( VERY expensive big fat specialist relays )
If you have wallwarts driving the LED lights, find out if the LED's are 12 volts or otherwise.
( "most" are 12-36 volts )

Hope that has given you an idea.

Joe
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Old 19th Jan 2022, 12:00 am   #3
Oldcodger
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Default Re: Battery/Mains Switcher & LED supply splitter box. Elegant solutions sought.

It might be worth looking at changing all the lighting to LED. e.g. I got hold of a car main beam 55w 12v LED lamp. Testing it on an old mobility scooter 12v battery I found that with the lamp drew 46mA but the light output was still good and there's no waiting for the lamp to cool after running for hours. With LED @12V you could run the lighting off of a transformer ,but rectified on a float system if mains available .
As Joe says ,relays are the way to go . But, for switching between mains/no mains, you'd still need a mains relay( as primary interface) to isolate the system from mains supply.
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Old 19th Jan 2022, 12:59 pm   #4
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Default Re: Battery/Mains Switcher & LED supply splitter box. Elegant solutions sought.

The simplest solution is to use two steering diodes and a mains powered DC supply that has an output voltage greater than the battery voltage.
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Old 19th Jan 2022, 10:52 pm   #5
Uncle Bulgaria
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Default Re: Battery/Mains Switcher & LED supply splitter box. Elegant solutions sought.

Thank you for all the suggestions. I'll post a circuit of where I am when I manage to get it looking understandable!

The spotlights are LED - I'm sorry, that was unclear. The fairylights are the little AA battery-in-a-dangling-box-with-LED-globules-on-copper-wire type. He's a jeweller so the fairs are reasonably civilised indoors events.

I plan to use a 12V wall-wart I already have, saving space and having mains in the box. As such, this precludes OR diodes.

Last edited by Radio Wrangler; 8th May 2022 at 7:04 pm. Reason: Final para removed as requested
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Old 20th Jan 2022, 9:05 am   #6
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Default Re: Battery/Mains Switcher & LED supply splitter box. Elegant solutions sought.

What about float charging the battery, wall wart float chargers abound.
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Old 20th Jan 2022, 10:16 pm   #7
broadgage
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Default Re: Battery/Mains Switcher & LED supply splitter box. Elegant solutions sought.

Several approaches.

1) If the load is modest AND if the load can accept 13.8/14 volts connect the lamps to the battery and leave the battery on charge via a constant voltage float charger.

2) If the above not applicable, then a changeover relay that with the mains present switches the lamps to a mains powered 12 volt SMPSU. When the mains is absent the lamps are switched to a suitable 12 volt battery. If desired include a small battery charger.
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Old 8th May 2022, 10:41 pm   #8
Uncle Bulgaria
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Default Re: Battery/Mains Switcher & LED supply splitter box. Elegant solutions sought.

The changeover relay sounds neat and simple. Would it be connected as shown? I've got this circuit for a low voltage disconnect (LVD) which I've breadboarded with parts I have, along with a meter expansion circuit.
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