DIY 12 Volt UPS

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.