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Hi i am trying to make a UPS for a project that requires 12Volts. I'm currently using a replay diode and capacitor. My problem is that when the switchover occurs, the voltage drops from 12Volts to 7Volts, and back up again.

Does anyone have a good circuit design, that will minimise the drop off?

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  • \$\begingroup\$ May we see the schematic? It might help. \$\endgroup\$ – Lou Nov 2 '09 at 14:06
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    \$\begingroup\$ You should probably specify the load. The best solution for 100 mA it's probably different than the one for 10A. \$\endgroup\$ – icabrindus Nov 18 '09 at 14:47
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    \$\begingroup\$ Did you mean "relay diode" \$\endgroup\$ – Brian Carlton Apr 12 '10 at 16:15
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Use Schottky diodes (which have a lower forward voltage) and you should only lose 0.5V or so. The voltage clearly shouldn't fall all the way to 7V anyhow - perhaps your diode is underrated, or you don't have enough battery current capacity?

When choosing Schottky diodes, choose one with a reverse breakdown voltage close to the maximum which you're likely to encounter (so not more than 20V) so that you get a low forward voltage too.

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How about running the project off of the battery and charging the battery? We used to do this for critical radio setups in the old days. If you have a trickle charge potential greater than that of your load, and set a cutoff for over charging you have it.

As an aside, there is supposed to be a "hum" or small ripple in the charging current to improve the battery life, but you could also have the battery at 14 or 16 volts and then place a regulator to get the 12 volts you are needing.

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    \$\begingroup\$ Mind, the charging sequence for the battery is highly dependant on it's chemistry. LiPos require a special process. \$\endgroup\$ – wackyvorlon Dec 8 '09 at 5:41
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So-called "12 V" batteries hardly ever put out precisely 12.0 V. Typically 12 V lead-acid batteries (used in practically all UPS, whether it's AGM or gell-cell) require something around 14 V to charge, and put out around 10 V open-circuit when nearly drained -- and even lower voltage when trying to power a heavy load.

If the rest of your system cannot tolerate such wide fluctuations, then some ways to work around the fact are:

  • Add a big capacitor. This will temporarily supply a power surge -- is that all you need for your system to work? Alas, a capacitor it does nothing for longer-term voltage droop.
  • switch to a different kind of battery with some other battery chemistry with less voltage droop. This may be expensive.
  • Use bigger batteries, or more batteries in parallel, or both. This reduces the effective resistance (the voltage drop for a given surge current).
  • Put some sort of voltage regulator between the batteries and the rest of your system. Perhaps a cheap linear voltage regulator will be adequate; this also gives you the option of using 2 batteries in series. Perhaps a switching voltage regulator will be better for your application; this also gives you the option of using a 6 V battery.
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