Identifying amps or watts rating of mains transformer - Page 1
Forum section: electronics / projects

author: trevortjes
author_rank: Contributor
timestamp: December 12, 2020, 03:52:25 pm
content: Hello everyone,I have this transformer lying around of which I would like to know how much current I can draw.Specs I measured thusfar:Primary voltage: 230VACNo load voltage secondary 1 & 2: 9.64VACDC resistance secondary 1 & 2: ~10 ohmsWeight: ~100 gramsDimensions:In attachmentsWonder if there are calculations or other ways to find out how much current in total I can pull from this thing.

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 12, 2020, 05:22:08 pm
content: Here there are some old 110V x 2 : 6.3V x2 @ 60 Hz, rated 20 VALams 48 by 58 by 19 mm stack which is larger than yours, and 60 Hz.So yours might be 10 VA approximately.To test, you could put on your sec a 10 Ohm resistor of suitable rating,and measure the winding surface temperature each 10 minutes for 1 hour or so.The winding surface rise should not be allowed to exceed about 60 C .

author: Zero999
author_rank: Super Contributor
timestamp: December 12, 2020, 06:33:49 pm
content: I normally look at data sheets of transformers of a similar mass. There's no way it's as high as 10VA, more like 2VA.https://docs.rs-online.com/49f6/0900766b80f46e48.pdfIt's probably rated to around 6V when fully loaded, in which case an open circuit voltage of 9.6V is reasonable.

author: trevortjes
author_rank: Contributor
timestamp: December 13, 2020, 10:20:17 am
content: Im currently loading one secondary with a load of 16 ohms. The voltage went down to 5.2VAC.Ambient temp at the beginning ~22 degrees.Temp after first 10 minutes: ~32 degrees @ 5VACTemp after 20 minutes: ~36 degrees @ 5VAC30m: ~3840m: ~4050m: ~4060m: ~40Seems like it flattens out at about 40 degrees.Does this mean that I can safely use this transformer for 5^2/16 = 1.6W?

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 13, 2020, 04:21:48 pm
content: Good test.secondary current is 5.2 V/16 Ohm = 325 mA - wantedPrimary VA = 9.52 V No-Load * 0.325 = 3.1 VAT_wdg_surface = 40 - 22 = 18 CP.U. Z = (9.52 - 5.2)/9.52 = 0.43Class B winding wire (was) rated for 80 C rise over any ambient.Your hotspot temperature inside the winding, is unknown but probably around 2 times the 18 C surface rise.If the temp rise goes approximately by the square of the current, 325 * 1.4 = 460 mA. should give 36 C surface rise.But the p.u. impedance of this transformer seems a lot higher than the 20 VA one here.So Zero is about right, calling it 2 VA

author: trevortjes
author_rank: Contributor
timestamp: December 14, 2020, 05:34:28 pm
content: So eventho I only did the test for one of the two secondaries, I can use the measurements as an approximation for 2 secondaries of which I ofcourse have to divide the total current / 2?

author: bob91343
author_rank: Super Contributor
timestamp: December 14, 2020, 06:02:37 pm
content: The weight of the transformer is an important parameter. Compare to others of similar construction to determine the total VA it can handle. Since both windings appear to have the same resistance, each can handle half the total.As a cross check, use the winding resistance to determine the load current to reduce the output voltage by about 10%, which should agree with the numbers from the weight comparison. That should, if both windings are taken into consideration, compare to about 10% primary resistance loss.Perhaps 5% would be a better figure to use.

author: srb1954
author_rank: Super Contributor
timestamp: December 14, 2020, 09:08:38 pm
content: Quote from: trevortjes on December 13, 2020, 10:20:17 amWere you measuring the transformer surface temperature or the actual winding temperature? The winding temperature is much more important in determining the transformer rating than the surface temperature.You can measure the average temperature of a winding by comparing the change of winding resistance from cold to operating condition (after suitable stabilising time). Copper windings will increase in resistance by 0.38% per degree C so you can calculate the increase in the winding temperature from the increase in the resistance.

author: m k
author_rank: Super Contributor
timestamp: December 15, 2020, 04:08:11 pm
content: Quote from: trevortjes on December 12, 2020, 03:52:25 pmI've recently wondered the same thing and learned a rule of thumb thatfor EI-core 1kVA is 15,0kg and 0,1kVA is 1,55kg.In many cases under 100VA structure is different so thatfor EI-core 50VA is 1,0kg and 5VA is 0,15kg.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: December 16, 2020, 03:34:37 am
content: Weight is a reasonable indication for the power a transformer can handle.Or take some real measurements.If you gradually increase the secondary current, then at some point it will start overheating quickly. Just back off a bit before the thing becomes too hot. Some of them have a thermal fuse inside.If you have a variable AC voltage souce, you can also dial in the point where it starts getting into saturation, which also gets you close.

author: trevortjes
author_rank: Contributor
timestamp: December 16, 2020, 02:40:29 pm
content: Surface temperature with a not too precise thermocouple. Guess I'll need to create some sort of voltage divider network to measure this resistance as my meters dont go that low with precision.

author: srb1954
author_rank: Super Contributor
timestamp: December 16, 2020, 03:54:28 pm
content: Quote from: trevortjes on December 16, 2020, 02:40:29 pmMeasure the primary resistance (remember to completely disconnect from the mains first). In mains transformer designs the windings are generally proportioned so that the winding power losses, and hence the temperature rises, are approximately equally divided between primary and secondary.With a small transformer the primary resistance is usually in the range few hundred to a few thousand ohms. This can be measured to a sufficient degree of accuracy with even a basic multimeter.

author: geggi1
author_rank: Frequent Contributor
timestamp: December 18, 2020, 11:15:58 pm
content: The size of the core is related to the VA.Do a internet search for datasheets on cores.Find datasheet for a core with similar size as your transformer and your close to the rating.

author: The Electrician
author_rank: Frequent Contributor
timestamp: December 20, 2020, 07:28:14 am
content: Quote from: trevortjes on December 12, 2020, 03:52:25 pmHave a look at this thread:https://forum.allaboutcircuits.com/threads/assessing-an-unknown-transformer.38273/In particular look at the graph at the end of post #36.