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Old 30th August 2010, 08:46 PM   #1
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Default Any tips on how to include two power supplies safely and efficiently?

On a unit I'm planning I would like to have a battery available as well as an option for connecting a supply from the wall (probably using a rectified AC signal), any tips other for getting them to supply the same circuit without having to fiddle with switches etc? I was hoping there'd be a way of making sure the wall-supply didn't overpower the battery or maybe even recharge it if it's the right type
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Old 30th August 2010, 09:18 PM   #2
Xoc1 is offline Xoc1  United Kingdom
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A simple way is to connect each supply via a diode. The supply with the highest voltage supplies the current.
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Old 30th August 2010, 11:37 PM   #3
dangus is offline dangus  Canada
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With a sealed lead-acid battery, you just have to choose a charging supply that current limits at something like C/5 or less (C being the AH rating of the battery), and a voltage limit of about 2.4 volts/cell. And make sure the charger doesn't load the battery when unplugged.

With nicads and the like, the proper way would be a smart charger like the venerable Temic (Telefunken) U2400B or a microcontroller that will stop charging when the battery is full by watching voltage and/or temperature. Otherwise, a trickle charging arrangement is invariably a compromise which tends to overcharge the battery. That may be OK for commercial products that only have to outlive the warranty period, but less attractive if you're the one buying replacement batteries.

There may be useful smart battery charger chips to be salvaged from scrapped laptops and cellphones. Or get something from Maxim like the MAX712 or MAX713:
NiCd/NiMH Battery Fast-Charge Controllers
Google Maxim nicad or nimh charger and you'll find projects with board layouts.
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Old 31st August 2010, 01:37 PM   #4
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Good ideas, cheers! I'm tempted to use the diode trick but wouldn't voltage be knocked off? I need a 9v supply and I'm trying to keep the supplies to 9v for ease of availability.
Could I use a J-FET as a gate? It would be better to have the wall supply as the first priority, I was also considering a logic gate.
That battery chip-charger is a very good idea too, I'm saving that for a future project!
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Old 1st September 2010, 03:17 PM   #5
DF96 is offline DF96  England
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Use Schottky diodes for low voltage drop.
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Old 2nd September 2010, 06:26 PM   #6
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Yeah I think schottkys are the way to go for this especially with two in the same casing available for the task. Is forward current important or should I calculate how much power I need? My next project only has a few small-signal transistors in it and an LED
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