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?

  • \$\begingroup\$ May we see the schematic? It might help. \$\endgroup\$
    – Lou
    Nov 2 '09 at 14:06
  • 2
    \$\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
  • 2
    \$\begingroup\$ Did you mean "relay diode" \$\endgroup\$ Apr 12 '10 at 16:15

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.


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.

  • 1
    \$\begingroup\$ Mind, the charging sequence for the battery is highly dependant on it's chemistry. LiPos require a special process. \$\endgroup\$ Dec 8 '09 at 5:41

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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