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I'm looking for ideas for a 5V power supply that will run from either 220V or 110V and have a battery backup. I want to use a standard 7AHr 12V sealed led acid battery.

I was looking at capacitive transformerless power supply but the unit I use pulls about 1.2Amps every now and then. Capacitive transformerless power supply can only supply low current outputs. It looks like I will have to use a transformer.

Is there a battery monitoring and charging chip, that say works with 12V. I can always use a DC/DC convertor to convert the 12V to 5V DC. I found it uses much less power that a 7805.

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    \$\begingroup\$ You are aware that a "capacitive transformerless power supply" bites when not handled with extreme care? As in bringing real and intense physical pain? Anyways, there are plenty USB-like wall wart chargers rated for 1 or 2A, why not use one of those? \$\endgroup\$ – jippie Jan 3 '14 at 16:30
  • \$\begingroup\$ Using a 6V 7Ah SLA might be better. Do you have to use a 12V SLA? \$\endgroup\$ – RedGrittyBrick Jan 3 '14 at 16:59
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You have two separate problems, charging a 12 V battery from 110-220 VAC, and creating 5 V from the 12 V.

The first is easily solved with a off the shelf power supply with universal input and that puts out the right float charge voltage for your battery. Most power supplies nowadays have universal input, so that part is easy. Most lead-acid batteries are fine with being indefinitely held at the float charge voltage, which is a bit over 13 V for a "12 V" battery. You can probably tweak up a 12 V power supply to make 13 V or so, or you may be able to get a power supply intended to charge and maintain a 12 V lead-acid battery. Those are usually sold as battery chargers instead of power supplies.

As for converting 12 V to 5 V, that's a well solved problem so no need to go into detail here. For such a voltage drop, it should be a switcher. You should be able to find off the shelf DC-DC converter modules that can do this, or make your own with any number of buck converter chips. There is also a drop in buck switcher replacement for the old 7805 linear regulator. Again, there are many well trodden paths here.

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  • \$\begingroup\$ Hi Olin, thank you for that. I agree about the power supply and the DC/DC converter. 1. I would have liked to use a transformerless circuit but it does not look like I can get the power I require. 2. How do I monitor the level of the battery and power it off before it discharges too much? \$\endgroup\$ – andrebruton Jan 4 '14 at 9:17
  • \$\begingroup\$ I'm looking for a circuit for a universal input power supply. I do not want to use off the shelf unit. \$\endgroup\$ – andrebruton Jan 4 '14 at 9:18
  • \$\begingroup\$ @andre: A capacitive charge pump power supply is inappropriate here. Your power levels are too high, but more importantly, the output is not isolated from the power line. For a lead-acid battery, it is good enough to monitor voltage to decide when to not let it discharge further. As for a circuit for a universal input power supply, that's over your head given the kind of questions you are asking here. That is not a beginner project. Not only is it dangerous, but there is too much to explain. Go buy a power supply. \$\endgroup\$ – Olin Lathrop Jan 4 '14 at 13:47

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