I am diagnosing a 22 year old UPS (APC 1400RM) problem, the big transformer (430-7140A) is getting very hot yet the UPS works and self test reports fine. I suspect something is going to fail soon and destroy some parts, so I want to diagnose and replace a failing part to prevent disaster.

I suspect the windings of the transformer may be starting to short (not enough to cause outright failure, but I can smell the varnish melting and it is VERY hot to touch). I have an inductive ammeter which I thought might help me diagnose this.

I can't find the transformer in the circuit diagram so I'm trying to understand what it does. On one side there are 2 thick wires which attach to (heat sink) plates connected to transistors. on the other side are 4 thinner wires which go to the circuit board. Would wire size tell me which side is primary/secondary? Next, is it possible to use my inductive ammeter to determine if it's starting to short? Again, it's still functional so I don't expect a 0 ohm short...

I can't find any specs on this transformer as I suspected it was made exclusively for APC. Is there another test I can do? (capacitance or resistance which might indicate a problem?)

(I checked the caps while still in the circuit and none are shorted, and none are swollen).

  • 1
    \$\begingroup\$ An alternate explanation for a hot transformer is excessive load. The transformer may be just fine - Is anything else in the product also getting hot??? Try disconnecting the battery. If this is a UPS and it has a lead acid battery, it's years past its prime. \$\endgroup\$
    – Kyle B
    Jan 4, 2023 at 6:09
  • \$\begingroup\$ The batteries are new, but even with the batteries disconnected the transformer is very hot. \$\endgroup\$
    – TSG
    Jan 4, 2023 at 14:28

1 Answer 1


Hot transformers usually happen when some diodes or semiconductor switches short out. The transformer in those UPSes is fairly robust and shouldn't fail unprovoked. If anything, the transformer has not failed but is working hard against another fault. Simple test: get another of those UPSes, and swap out the transformer. Most should be easy to disconnect by removing connectors, including connecting screws for high current paths.

If the transformer itself has failed, then you need a replacement transformer - scavenged from another UPS, perhaps a junked/non-working one. So to either exclude the transformer in an easy way, or condemn it, you'll need another transformer anyway.

If you can't get another transformer quickly, start by measuring all the active components that form the switched current paths around the transformer - any diodes and transistors/mosfets.

  • \$\begingroup\$ I don't have access to another similar UPS for references. I could order another transformer ($150 on ebay) but I wan't to avoid to much investment in 20+ year old UPS. I tested 8 big transistors (on heatsinks) while in place and resistance across leads is the same on similar transistors. Diodes report ok (while connected). Is it safe to run the UPS with secondary of the transformer disconnected? If so, then if transformer is hot then it would mean short in ups? \$\endgroup\$
    – TSG
    Jan 4, 2023 at 15:54

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