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I've got a 220 volts fishbowl pump that is conected and working 24x7.

However, when a power outage happens (they happen often where I live) I need to switch to a 12 volts version of the same pump, so the pump never stops working.

I can, buy 2 different version of the pump, one ready for 220 and the other one for 12 volts, or, if possible, use the same pump.

Do you guys know of any circuit that maybe already exists for this?, or should I make one "home made"?.

MarkU
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Artemix
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3 Answers3

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The simplest approach is to get a SPDT relay whose coil matches your primary power source (220 VAC), and whose contacts are rated for your secondary power source (12VDC at whatever amps). Use the normally-closed contact to switch the secondary power to the secondary pump. Whenever the primary power is available, the relay will pull in and prevent the secondary pump from running. When the power fails, the relay will drop out and fire up the seconday pump.

Dave Tweed
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The simplest approach is to plug the 220V pump into a small UPS. They start at about $40.

There are three things required beyond the pump you have now- a backup power source such as a 12V battery, some means of keeping the battery charged, and a power transfer circuit. The UPS supplies all three plus an inverter so you don't need to purchase another (12V) pump.

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(120V type linked above). Obviously you'll want one with enough capacity to last through the longest power outage.

The cheapest approach might be to use a 12V SLA battery or a salvaged car battery with venting, a 12V pump, a 220V coil relay, and a float charger made from an old AC adapter. Depending on salvage skills this could be almost free, not counting the pump.

Spehro Pefhany
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Simplest solution would be to buy a car battery charger that brings 220V down to 12V attach it to a 12V lead acid battery and wire that in parallel to a 12V pump. No switch, no inverter, and as soon as the power goes out, your pump continues running. The only caveat here may be, depending upon the design of the charger, you may need to add a diode to make sure the charger doesn't drain your battery when the power goes out.

220V -> 12V car battery charger -> diode -> Battery + pump

Lastly, the car battery charger should be rated to handle enough current to allow the pump to run optimally.

horta
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  • According to Wikipedia, the float-charge voltage on a 6-cell ("12V") lead-acid battery should be 13.05 to 13.6V @25°C depending on type (and, obviously, limited in current), with a tempco of -3.9mV/K if you want long battery life. – Spehro Pefhany Apr 05 '14 at 04:08
  • Correct. I don't get your point tho. I highly doubt a pump which would be made up of an electric motor would have much sensitivity to 13% variation in input voltage. Also, I've seen most 12V power bricks give between 12.5 and 14 volts anyways. Is there something awfully sensitive to voltage that I'm not taking into consideration? – horta Apr 05 '14 at 12:47
  • No, what you've described will work for sure but there's no sure way to charge the battery properly. – Spehro Pefhany Apr 05 '14 at 14:23
  • Ah I understand what you're saying now. I changed the answer to use a car battery charger instead. – horta Apr 05 '14 at 14:35
  • I like that approach, would I need a 12v pump then?, could it be a 220v one?. – Artemix Apr 29 '14 at 19:14
  • @Artemix It would have to run off of 12V. A 220V pump won't work for this solution. – horta Apr 29 '14 at 19:45