UPS Transformer Identification--General Rules

[Dumpsterholic]

Hi everybody:

I have a couple of transformers that I've pulled from junked uninterruptible power supplies. After watching several inconclusive videos on YouTube and searching this forum (and a couple others) by subject, the sheer number of dumbass questions and less-than-helpful replies has prompted me to ask if anybody knows of some useful threads that cover the topic from a general perspective.

I could post pictures of the units I now have, but lacking the accompanying circuit boards I doubt the pics would be of much use. Owing to the proprietary nature of these parts, datasheets tend to be rather scarce. Just identifying the primary and secondary windings isn't easy; knowing that the same xfmr can be used in different ways depending on the status of the UPS at any given time, I understand that they are complicated gadgets. I have what I believe to be a fairly safe test jig, and I know how to use a DMM to test for continuity and resistance. Yesterday I picked up a variac, and in due course I hope to build or lay my hands on an LC meter; eventually I will buy a DSO, as these units are getting cheaper every year.

I'm leaning toward the conclusion that scrapping my xfmrs for beer money is probably the wisest choice. Anybody care to chime in?

[Bratster]

Please post photos of them.

I have taken apart a ton of UPS' and there are generally only a couple different types of Transformers that are used. (In terms of winding configuration).

I also have always ended up throwing away any of the Transformers I have collected out of them as I never did was able to find it use.

Unless you want to build your own linear power supply. Which would be a good project.

Sent from my Nexus 6 using Tapatalk

[rstofer]

Assuming the are step down transformers, the winding(s) with the highest resistance are the primary.  There may be two such windings so the transformer can be wired for 120V or 240V.  If there are two windings, you need to determine which wire is which in terms of polarity.

Google for 'transformer kick test':
https://www.quora.com/What-is-DC-Kick-test-of-transformer-and-doesnt-transformers-winding-get-burned-during-the-kick-test

Once you have identified the primary windings and their polarity, wire them for either parallel (120V) or series (240V).  I'm just guessing on the voltage.

Now, you can use a 120V/6V transformer to feed the primary with a low voltage and measure the secondary windings.  You can also identify polarities there as well.  There may be a pair of windings that make up a center-tapped winding and you will need the polarity.

There should be plenty of videos on identifying transformer windings.

The primary windings usually have black wires but on a custom transformer, who knows?  Still, if the leads are color coded, it's a place to start.

[SeanB]

Typical those transformers will have 3 separate windings. one with 3 heavy wires is the drive from the FET switches, typically there is a short length of white and yellow ( or red) wire coming out and a centre tap which is black. the black wire goes to the battery negative, with another black wire going to the main board. The other wires go to spade connectors that push on the board.  The next will be a thin yellow pair of wires, going to a 2 pin socket. This is a 16VAC supply, used to charge the battery when plugged into the wall. The other winding is the mains side, typically with 4 wires, so the transformer can be used to boost and buck the incoming mains when it is high or low without turning on the UPS functionality. These go to a 4 pin connector that is on the main board

The mains winding will be easy to find, look for the leads with the highest resistance on those 4 wires, that will be the mains input side. there will be 2 taps as well on there, of around 20V each tap.

[David Hess]

If these are big heavy laminated transformers, then they have a good winding ratio for making a big 12 or 24 volt battery charger or power supply.

[Dumpsterholic]

OK, folks:

Thanks so much for the help! I have started this thread with the idea of providing the basics of UPS transformer identification in such a way that anybody can use this information to identify the particular unit they have. Since UPS XFMRs come in different "flavors", that may be unrealistic, but so be it. I have attached some photos and my crude drawing of just one of the XFMRs to help the visual learners out there. Note that in my crappy drawing, I have attempted to reproduce both the sizes of the various wires and the colors of their insulation.

Here are the results of the resistance and continuity measurements I made. Please refer to the drawing to match the letters I've chosen to the corresponding wires going into the XFMR. All readings are in Ohms: A-B: 3.7; A-C 3.7; B-C 1.5; D-E 1.8; D-F n.c.; F-G 1.5; F-H 1.5; F-I 1.5;G-H 1.5; G-I 1.5. So far, this is what I have surmised: A, B, and C are the primaries; D, E, and F are one set of secondaries, while G, H and I are another set of secondaries. What puzzles me is that F and G should be electrically connected. If they are two different secondary windings, then they shouldn't connect at all, right? Since I don't know if this unit has been damaged (i.e. shorted out), I don't wish to go on to the next step of applying voltage to the presumed primaries ABC.

A word about my test jig: Elsewhere online I have heard people recommend that a current limiter and/or a fuse be used. If I understand correctly, the current limiter could be a light bulb connected in series with the "live" or "hot" wire that goes to one side of the primary, so that even if a heavy load is connected to one of the secondaries, very little current will flow through the XFMR. Sound right? Some people have suggested that I use a small step-down XFMR (output of 10-12V) to apply a safe voltage to the primary of the XFMR under test, while others have suggested using a variac to gradually step up this input voltage. Since I have a variac, I will use the latter option. The remainder of my test jig is basically a long power cord going into a fused and switched power strip, plus a clear plastic "blast box" which sits atop the XFMR under test. With this setup, I can hook up my test leads to the XFMR with no power applied, then turn the thing on from a safe distance. I haven't posted pics, but this should be pretty easy for anyone to understand and implement.

Update: First, David Hess, SeanB and rstofer, thank you all for your feedback. Somehow the photos failed to upload, but no matter. I went ahead and decided that A and B were the 110V primaries and applied mains power. Then I tested for voltages on the supposed secondaries, and here is what I got: D-E 15.7V; D-F 3.5V; E-F 2.8V; G-H 9.4V; G-I 9.4V; H-I 18.8V. Happy to report no sparks, flames or smoke! Since G, H, and I are the heaviest wires, and since they appear to make up a center-tapped 18.8V output, I think that I will build myself a +/- 12VDC power supply and have done with it. In other blogs I have read about tests that help one to determine the power ratings of unknown XFMRs, but since this one feels pretty beefy and is about twice the size and weight of a 24V@4A power XFMR I purchased for a power supply kit rated at 0-30VDC@3A max, I'm not going to concern myself with the mystery XFMR's power factor. A couple of big capacitors, four big diodes, an LM7812 and an LM7912 plus some other sundry parts, and I'll end up with a nice bipolar supply capable of powering any small audio amplifier I need. Thanks again to everybody.

[David Hess]

Here are the results of the resistance and continuity measurements I made. Please refer to the drawing to match the letters I've chosen to the corresponding wires going into the XFMR. All readings are in Ohms: A-B: 3.7; A-C 3.7; B-C 1.5; D-E 1.8; D-F n.c.; F-G 1.5; F-H 1.5; F-I 1.5;G-H 1.5; G-I 1.5.

I usually find that a measurement of the cross section of the wire used for each winding is as important if not more important than the winding resistance.  This can give a good estimate of the current capability of each winding.

The transformer turns ratio may be measured by applying a low voltage sine wave to one of the windings, I usually pick the one with the thinnest wire which is likely the highest voltage, and measuring the AC voltage across each winding.  A function generator or low AC voltage line transformer is a good source.