• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Determining transformer specs?

Not open for further replies.


New Member
I have a few transformers that my work gave me out of some broken UPS modules (power rating unknown). They are pretty beefy, heavy, and have 10GA output wires. I want to use them in a couple 12-14v power supply projects, but don't want to overwork them, yet I'd like to get the most from them.

They have a bunch of numbers on them, but no company info. A Google search on the numbers turned up nothing. So, I determined which wires were which and did some measuring (all voltage measurements are AC):

Transformer 1: A single set of large gauge outputs (no center tap)
Unloaded = 16.21v
Loaded with 10.8Ω = 16.13v (1.49A)
Loaded with 2.5Ω = 15.9v (6.36A)
Loaded with 0.81Ω = 15.4v (19.01A)

I calculated coil output resistance by subtracting the loaded voltage from no load voltage, and then dividing by the current. Naturally, it varies according to the load, but is 0.045Ω is the average.

Transformer 2: This one has a center tapped output, but I loaded and measured across the whole output.
Unloaded = 30.78v
Loaded with 10.8Ω = 30.34v (2.8A)
Loaded with 2.5Ω = 29.05v (11.62A)

Using the same method above, I calculated coil resistance to be 0.15Ω average.

Transformer 3: This transformer (Stancor) DOES have the rating listed on it: 36vAC @ 6A. To verify, I measured the output:
Unloaded = 39.43v
Loaded with 10.8Ω = 37.84v (3.50A)
Loaded with 2.5Ω = 33.82v (13.53A) - over the rating, but wanted a consistent test.

I calculated the average coil output resistance to be 0.454Ω. At the 6A rated output, I calculated the voltage would drop to 36.85v, which is ~93% of the unloaded value. Is this 93% factor consistent for transformers in general, or is there another way to go about this?

Now, is there some rule of thumb in determining approximately how much they can be loaded? For example; could I use the info I have and calculate the output current at an output voltage that drops a certain % from the unloaded value?

Sorry for the long post. And thanks for any assistance you can provide.
Last edited:


The power rating depends on the core size.

I recommend looking at the datasheets of different transformers. Weight your transformers and compare their mass to the transformer datasheets: this will enable you to estimate the power rating. Once you've got that you can estimate the nominal secondary voltage which is what you get when the transformer is fully loaded.


New Member
Thanks. I should have known it wouldn't be as simple as coming up with a v-drop %. I don't have a scale, but a make-shift balance beam and some exercise weights suggests the following:

trans 1 weighs more than 10lbs (11-12lbs)
trans 2 weighs right around 10lbs
trans 3 (the only one where I know the ratings) weighs around 8lbs.

At least the info you provided gets me pointed in the right direction...


The % V-drop can also be used but as fas as I'm aware it can't be used as the basis for a calculation.

Generally larger transformers have a better regulation so the percentage in voltage drop should be smaller.

Unfortunately, the regulation is a measure of voltage drop from off load to fully loaded which is pretty useless for you since you don't know what the capacity is.

Another method is primary winding resistance, assuming all of your transformers have the same primary voltage, the larger the transformer the lower the DC resistance of the primary should be. Unfortunately the primary resistance isn't normally specified on transformer datasheets so it's not much use to you.


New Member
Yeah, I measured DC resistance of all coils, but my meter (Fluke 97) doesn't have the resolution to accurately measure that low (was getting readings around 0.8-0.9Ω), and then I have to subtract the meter lead resistance. The best I could come up with is "those two leads are not the same electrical point", so I figured it was safe to hook 120vAC up to them.

Seeing as how these were in larger UPS units, which were rated for at least 500-600w (which is about all I know about them), I can probably safety assume they are rated for around that value. But, UPS units probably aren't designed to run at 100% duty cycle, so I have to take that into account. Either way, I can probably assume I should be able to get at least 20A @ 14.5vDC. These supplies will be used for R/C chargers (lipo batts).


You can't calculate the primary resistance using Norton's theorem.

The change in secondary voltage will depend on both the primary and secondary coil DC resistance and how well they are coupled.

500W to 600W sounds about right given their weight.
Not open for further replies.

Latest threads

EE World Online Articles