My understanding of transformers is that they take in and give out AC signals.

However, the UPS system below has its DC batteries connected to a transformer. Is

it possible to know the rational behind this design?

(source: wasp.kz)

  • \$\begingroup\$ Not without a circuit - it'd be guesswork and how valuable an answer to you would that be. Circuits beat speculation. \$\endgroup\$
    – Andy aka
    May 31, 2015 at 15:38

1 Answer 1


Probably the batteries are connected to the windings through some electronic switch mechanism (power MOSFETs, for example). They are switched with alternating frequency to the primary windings, so that the primary sees an alternate frequency.

From the photo you can see beefy wires to some heatsinks with power devices (MOSFETs?) attached. This could be a clue in that direction.

Some UPS topologies include a DC/AC converter (inverter) that may look like this:

enter image description here

Note that the center tap of the primary is connected to a positive battery terminal.

This picture should explain better the principle:

enter image description here

  • \$\begingroup\$ I do see the MOSFETS but then two out of three red wires coming out of the batteries are feeding directly into the transformer. Could there be a switch mechanism within the transformer as well? \$\endgroup\$
    – givknow
    May 31, 2015 at 15:17
  • \$\begingroup\$ @givknow No, the transformer has no built in switch. Possibly there are multiple windings and the schematic is more complex than the simple one I linked to. \$\endgroup\$ May 31, 2015 at 15:25
  • \$\begingroup\$ @givknow From what I can see, it seems there are two separate circuits (maybe for reliability). Each one follows more or less the principle I've already shown, but each one has its own separate transformer winding. The batteries seem connected in series by two, i.e. two in series for each one of the two "circuits". And there are two group of power devices, four device for each group. So the switching mechanism could be an H-bridge, which needs 4 active switches. So you could have two H-bridges in two independent circuits. \$\endgroup\$ May 31, 2015 at 15:30

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