Estimating a transformer`s max output current - Page 1
Forum section: electronics / beginners

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: December 16, 2015, 10:12:29 pm
content: Is it possible to estimate or calculate a transformer's maximum output current?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 16, 2015, 10:31:27 pm
content: If you are talking about conventional, iron cored, line frequency transformers, yes.Measure the DCR. (Accurately. If a multimeter reads "0.3" ohm or so, it's probably not telling you the real value, and you need a milliohmeter or some other method to be sure.)(Nominal current) = (nominal winding voltage) / (DCR * 20)The "20" makes assumptions about the transformer's design. It's a common value, but if you can show us the transformer in question, we can give a better estimate.Tim

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: December 17, 2015, 01:17:02 am
content: Thank you!Edited: I moved the pics from my Google repository to a local attachment.

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: December 17, 2015, 01:53:56 am
content: Here are the measurements on the secondary (consider first pic for reference):- between center tap and rightmost tap: 1R4- between center tap and leftmost tap: 1R0- between leftmost and rightmost taps (outer taps): 0R7On the primary, marking them a, b, c, d from top to bottom, considering the 2nd pic for reference:- a-b: 79.5 Ohms- a-c, a-d: infinte- b-c, b-d: infinite- c-d: 83.6 OhmsAC Voltages:- between center tap and leftmost tap: 13.06V- between center tap and rightmost tap: 26.13V- between outer taps: 13.06VSo I conclude that the leftmost tap is actually the "tradicional" center tap, right?

author: Brumby
author_rank: Unknown
timestamp: December 17, 2015, 03:51:42 am
content: Sorry - I'm not seeing the images.

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: December 17, 2015, 10:58:03 am
content: Quote from: Brumby on December 17, 2015, 03:51:42 amSorry about that. I edited the post with the images, attaching them to the post.

author: MagicSmoker
author_rank: Super Contributor
timestamp: December 17, 2015, 12:17:00 pm
content: Quote from: AlxDroidDev on December 16, 2015, 10:12:29 pmYou can roughly estimate the power of a small mains frequency (ie - 60Hz) transformer at 30W/kg, and if there is a single (or single, center-tapped) secondary then the output current is simply the Watts / Volts = Amps.Things are a bit more complicated if there are multiple secondaries as you then have to divide the power up among them according to wire gauge which will be difficult to do (for a beginner) unless the secondaries have very different output voltages and/or you have a reliable means of measuring low resistance values. Note that even good quality DMMs are unable to reliably measure resistance below 1 ohm, and so you will have a tough time distinguishing between two or more relatively low voltage secondaries by resistance alone unless they are wildly different in current rating (~10x or more difference).

author: Brumby
author_rank: Unknown
timestamp: December 17, 2015, 01:17:04 pm
content: Quote from: AlxDroidDev on December 17, 2015, 01:53:56 amBased on the voltages - yes. There is always the possibility there may be current capacity differences between the two halves, but (in my limited experience) I am not inclined to think so.From the measurements you have supplied, I'm guessing your transformer will look something like this (image borrowed from MPJA Online) with similar configurations...Left hand side are the primaries with your connections a, b, c, d from top to bottom (Or possibly a, b, d, c - depends on the phasing)The secondary you've already figured out.As for current capacity - that's not something I could offer much help with, I'm afraid.

author: bktemp
author_rank: Super Contributor
timestamp: December 17, 2015, 01:37:04 pm
content: Based on your measurements, assuming 220V input voltage and an estimated height of 55mm the power rating is around 30VA. The voltage under load is 2x 11V @ 1.33A.Maybe the primary has taps for 220V and 240V and you did use the 240V tap, so the nominal output voltage could be 2x 12V @ 1.33A.

author: AlxDroidDev
author_rank: Frequent Contributor
timestamp: December 17, 2015, 01:51:22 pm
content: Quote from: bktemp on December 17, 2015, 01:37:04 pmIt has 4 taps on the primary, that can be wired for 110V or 220V. I wired like shown in the left image of the picture posted by Brumby and tested it on 110V (127V, actually). That's how I got the 13.06V + 13.06V on the secondary.Quote from: MagicSmoker on December 17, 2015, 12:17:00 pmI'll weight it this evening and I'll let you know.Quote from: T3sl4co1l on December 16, 2015, 10:31:27 pmTim, in that formula, what voltage should I use? The 26V I get using the 2 outer taps or the 13V from the center tap to an outer tap?I use the higher voltage (26V), I'll get at most 26 / (0.7 * 20) = 1.85A, which is far less than I wa hoping for. If I use the center tap + outer tap, I'd get: 13/(1.4 * 20) = 0.46A, which is really less than most 12V wallwarts I have.

author: bktemp
author_rank: Super Contributor
timestamp: December 17, 2015, 03:07:05 pm
content: Quote from: AlxDroidDev on December 17, 2015, 01:51:22 pmIf the transformer is rated for 110V and you measured it using 127V the rated output voltage is most likely 2x 9 or 2x10V and the power is 25VA.QuoteYou have to use the full voltage (26V) and the full resistance across the same winding were the voltage was measured (1.4ohms). So you get 0.93A -> 24VA. That is in the same ballpark than my calculations.If you wire both secondary windings in parallel you get 13V and 0.35ohms. This gives 1.85A at 13V. That is the same 24VA.