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Topic: Automatic switching between two power sources (Read 1 time) previous topic - next topic


Thanks so much for putting some brain-cycles to this.  Like most things tried for the first time there are always aspects those with more experience will think of.  In this case the switchover time was something I completely hadn't thought of.
How are you going to charge the battery?
In this case the battery and taking it on tour is actually a bit of an afterthought - I have a donor 12V wall wart already grafted in, so at the minute it's tied to the house and was going to be a semi-permanent.  This is a bench worklight, using 4.8m of LED strip-lighting, cut to 24cm strips with brightness controlled by switching different patterns of them through TPIC6B595 shift registers via an iR remote controlled by an ATtiny85. 

There are a few places where it could be used if I added battery power though.  Remotely, the battery would be the primary source of power and it's charged by the vehicle, or a solar panel, or on a boat by the outboard.  At home where mains is available there really isn't a need for a battery.  In contrast, remotely some occasions both are present, for example for the hours when a generator is running at a campsite, or mains power is available to the site.

I think a version 2.0 designed from the start with mains charging the battery is probably the go.

This community is awesome as ever, thanks !
"There is no problem so bad you can't make it worse"
- retired astronaut Chris Hadfield


Apr 22, 2012, 06:55 pm Last Edit: Apr 22, 2012, 06:58 pm by dc42 Reason: 1
Easiest way is to use diode OR'ing and to arrange that the wall wart as a higher output voltage than the battery, say 15v instead of 12v. That will ensure that the wall wart supplies the power as long as it has a mains supply.

In fact, if the wall wart is unregulated, then you don't need a diode on the output of the wall wart, just one on the battery output. If in addition the load is well below the rated current of the wall wart, then you will probably find that the wall wart produces a bit more than its nominal output. This, together with the diode in series with the car battery, means that a 12v wall wart will probably work just fine in this arrangement.
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