How to test a transformers maximum output current? - Page 1
Forum section: electronics / projects

author: toli
author_rank: Frequent Contributor
timestamp: July 31, 2010, 05:21:52 pm
content: Hello Guys,I have two transformers I took out of an older Sansui amp. I've measured the output voltages with no problem using my DMM. Now I would like to see what the maximum current of each rail is. In the lab at the university we've used a variable transformer to feed the transformer we were testing. What else can I do to test the maximum current of the transformer?Unfortunately I wasn't able to find info of the transformer online.

author: djsb
author_rank: Super Contributor
timestamp: July 31, 2010, 05:37:13 pm
content: One way would be to connect a high wattage (50W or more) 10 Ohm wirewound resistor (or several in series) load across the secondary and then measure the voltage across the resistor.http://cgi.ebay.co.uk/RESISTOR-10-OHMS-5-50-WATT-ALCLAD-1PC_W0QQitemZ350330181524QQcategoryZ39997QQcmdZViewItemQQ_trksidZp3286.m7QQ_trkparmsZalgo%3DLVI%26itu%3DUCI%26otn%3D3%26po%3DLVI%26ps%3D63%26clkid%3D6730205628487125926At the same time you could fix a temperature probe to the transformer and check that the temperature stays stable. If the transformer is being overloaded it will start to get hot very quickly.David.

author: toli
author_rank: Frequent Contributor
timestamp: July 31, 2010, 05:42:40 pm
content: I dont have a 50W resistor, but I guess I can find a way to burn so much heat. Is there no other way to do so? I'm not to sure as to what temperatures are the SOA of such a transformer, and how well it will dissipate that heat (can't it just heat a part of the transformer and not all of it?). This transformer has 4 secondaries.

author: Simon
author_rank: Unknown
timestamp: July 31, 2010, 05:49:51 pm
content: I asked one of my college proffs this question and he told me to connect the primary to 10% it's rated voltage and then read the amperage on the secondary with a meter (a short circuit) I don't know how accurate this advice was though, the idea of temp sensing is a good one, at the end of the day it is about how hot it gets, as long as it stays under a safe temp that hopefully someone here can tell us I would think you can pull all the amps you want. It would be good to make a testing load for this purpose that can control the current draw. The other thing will be internal resistance of the transformer, if overloaded the output voltage will drop, but at that point your looking at seriously overloaded

author: Zero999
author_rank: Super Contributor
timestamp: July 31, 2010, 06:23:33 pm
content: QuoteAre you sure he meant to short circuit the secondary?Sounds like a bad idea.Perhaps he meant increase the load until the voltage drops by 10%That's not a very accurate method, the regulation factor depends on the size of the transformer and its construction, larger transformers generally have a better regulation factor than smaller transformers.I think the most accurate method is to weigh it and compare its mass to that of transformers with a similar power rating and minimum frequency - e.g. two 100VA transformer should have a similar mass. Another option is to measure the primary winding resistance and compare it with other transformers of the same construction, primary voltage and frequency, higher power transformers will have a lower resistance. One thing to also note is the construction, toroidal transformers will be lighter than E core transformers and will have a lower primary resistance and if it's potted, it'll be heavier so only compare like with like. Look in component supplier catalogues for datasheets of transformers which will give specifications such as mass and winding resistance.

author: saturation
author_rank: Super Contributor
timestamp: July 31, 2010, 06:27:42 pm
content: See:http://www.repairfaq.org/REPAIR/F_audiofaq8.html#AUDIOFAQ_008

author: Simon
author_rank: Unknown
timestamp: July 31, 2010, 07:11:41 pm
content: Quote from: Hero999 on July 31, 2010, 06:23:33 pmwell if your connecting a meter across it your making a short circuit infact come to think of it those were his words: measure the short circuit current at 10% the primary voltage, I have no idea how or why this should work and never tried it as it meant having a 24 V transformer capable of at least the same current the value of which is unknown.I used to just take my incognito transformers to my friendly TV repair man who would get me the parts I needed (when i lived in italy and had no access to RS or the likes) and he would give me a ball park figure based on his experience.I'm not sure comparing weights will work, you'd have to look carefully at the transformer construction type and some older transformers will probably make less W/Kg.I do like the idea of temp testing though but what is a good temp ? or rather delta temperature, it should be the amount by which the temperature increases relative to the ambient as oposed to the absolute temperature

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2010, 12:03:58 am
content: Quote from: Simon on July 31, 2010, 07:11:41 pmOh, I see what you mean now.No, I don't know why that would work either. I suppose it has something to do with the resistance of both the primary and secondary windings. Larger transformers will have a lower resistance but it's not directly proportional, probably inverse exponential.QuoteThe amount of iron is proportional to the amp/turns which is directly proportional to the power and inversely frequency and as far as I'm aware a 50 year old transformer will use similar magnetic materials as a modern transformer. Weight can go in both directions, older transformers might also have less insulation between the primary and secondary coils as regulations weren't so strickt back then.Did you read any datasheets, as I suggested?Here's a datasheet I found on Rapid with a list of various toroidal transformers made by the same manufacturer:http://www.rapidonline.com/netalogue/specs/88-3781.pdfLook, the mass a 225VA torridal transformer is 2.2kg.Now let's look at the mass of a 225VA transformer from another manufacturer:http://docs-europe.origin.electrocomponents.com/webdocs/0da0/0900766b80da0cfc.pdfIt's 2.29kg - a difference of under 5%All right, maybe that was a fluke, let's look at one from a different manufacturer:http://www.farnell.com/datasheets/22145.pdfWhat a surprise? it's bang on 2.2kg!How about a 500VA transformer?The one on the datasheet listing many transformers (see the first link) is 4.3kgA 500VA tansformer sold by Farnel is also 4.3kghttp://www.farnell.com/datasheets/11718.pdfLet's look on RS.http://docs-europe.origin.electrocomponents.com/webdocs/0027/0900766b80027b4a.pdfIt looks like I've found an exception, 3.5kg but 18.6% out isn't bad for a general rule of thumb and take note that the 225VA transformer made by that manufacturer is also lighter, same for the rest of their range, see table linked below:http://docs-europe.origin.electrocomponents.com/webdocs/0111/0900766b80111b4e.pdfQuoteI think, that's a far less accurate method than simply looking at the mass. Going from the datasheets, larger transformers get hotter, see the first link I posted, the 15VA transformer has a temperature rise of 26°C and the 1000VA transformer has a temperature rise of 75°C. I would also expect that the maximum operating temperature will vary widely, some transformers will be able to safely operate hotter than others.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 01, 2010, 01:06:49 am
content: Quote from: toli on July 31, 2010, 05:21:52 pmWrong question ...Finding the Amperes is an easy task , as long you know the voltage .Just measure the thickness of the wire ( transformer output ) .The rest are easy ..

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 01, 2010, 03:37:35 am
content: Quote from: Kiriakos-GR on August 01, 2010, 01:06:49 amnot as easy as this. find a 10A rated wire, short it. if it burnt out, its not safe to check with DMM. i used to short circuit the DMM to it.... for smaller transformer though.you can setup parallel resistors config (equal value) and check A for each resistor. so if you have 10 resistors in parallel you are measuring 1/10th of total A.

author: toli
author_rank: Frequent Contributor
timestamp: August 01, 2010, 04:32:18 am
content: Hi Guys,The transformer is a very large one and weights more than 5kg. It has many secondaries so checking the mass wont help as it won't tell me what is the maximum current of each winding.As for wire gauge, all of the wires on the transformer are exactly the same gauge, and I don't think that all secondaries have the same current rating, so I guess its not that accurate, no?

author: shodan
author_rank: Contributor
timestamp: August 01, 2010, 05:07:44 am
content: isn't maximum current of a transformer only dictated by how much current can run through it before the windings burn ?I don't think there's a non-destructive way to measure that directlyprobably indirectly, like measure the winding's wire gauge or temperature vs amperage curve (and then determine the maximum allowable temperature)I think you are not looking for the maximum current but the number of turn ratio between windings to determine nominal voltage outputs on the transformer's secondariesI think one way to do that is to apply a known AC voltage to the primary and measure the open circuit voltage of each of the secondariesif you want to measure the power output (right term ?) of that winding you then apply progressively decreasing power resistors (discrete steps of resistances) to the output winding while measuring both voltage and current (I guess it's best to also use an oscilloscope here to watch the waveform degrade) when the output voltage starts to sag I guess that is when you have found the maximum power carrying capacity of that winding (I assume it is proportionnal to primary-to-secondary turn ratio, the inductance of each winding and the magnetic coupling between the windings)if the "output impedance" (probably not the right term) on the secondary winding is too low the voltage will sag because energy is leaving the core faster that it is entering it via the primaryyou can raise the voltage to compensate for this, but at some point the core will become saturated and secondary winding voltage will sag and the waveform will flatten because of magnetic flux hysteresis (?)I'm not really sure this is the right way to map out an unknown transformer but from what I've read so far about them this is the best explaination I can give you, I hope someone more knowledgeable will correct the erroneous bits

author: toli
author_rank: Frequent Contributor
timestamp: August 01, 2010, 05:25:29 am
content: I've measured the voltage of each secondary already. I know how to do thatI want to make sure it'll be able to handle the currents I need safely (up to 3A). That's why I'm looking for a safe way to test that. From what I know (we had one lesson about transformers in the intro lab), there's the maximum current because of temperature and because of saturation. The saturation current I can easily test with a scope and a power resistor at the secondary. The problem is to test the temperature stability. I can't put the probe inside the transformer but only on the output, so there will be a difference between what I'm measuring and what's going inside the transformer. Plus, I don't really know what temp is considered the right temp.

author: shodan
author_rank: Contributor
timestamp: August 01, 2010, 06:57:32 am
content: without knowing the type of insulation on the windings it's hard to know at what temperature it will failI guess you can assume 50 C is a safe temperatureplace a thermocouple or two on the transformer and run progressively more current through it until temperature stabilise at 50Cif that's not enough current for your application then add active cooling ?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 01, 2010, 09:11:42 am
content: No need for external temp measurement - you can measure the temperature of the actual winding directly by measuring its change in resistance - it increases by +0.393 percent per degree C.You may also be able to infer a rough idea of the relative rating of each winding from the combination of its resistance and the output voltage - the voltage will be proportional to the number of turns, so dividing the resistance by this will give an approximation of resistance per turn, and hence wire size.You can approximate the overall VA rating from the size and weight, and knowing the resistance per turn of each winding you can get the relative proportions of the VA rating available from each winding.

author: Simon
author_rank: Unknown
timestamp: August 01, 2010, 09:32:46 am
content: it looks like i found the easiest method long ago: take it to someone who's been in the game for years and ask for a rough figure

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 01, 2010, 10:51:48 am
content: Quote from: toli on August 01, 2010, 04:32:18 amI think that you are wrong again ....There is only one secondary , the max power in volts and amps , will be found , on the wires that you get the highest voltage .Those are the true end points, of the secondary coil.The wire gauge , its all that you need to know so to determine the output.If you are unable to understand and calculate, the wire gauge in watts ,then you need to check your school books .Or you can ask the help, of an true electrician , he knows that stuff ..

author: toli
author_rank: Frequent Contributor
timestamp: August 01, 2010, 11:37:38 am
content: I guess you have a reputation for the way you speak.Anyway, there are many secondaries. 4 in fact. 3 of which are center tapped. All 4 are not connected together internally.I've attached a photo of the transformer.The green wires are one coil, the purple-gray-purple are another, the blue-white-blue are another, and the orange-yellow-black-yellow-orange are another. The white-orange at the top go to the 240V. The other wire are used for other main voltages.

author: Simon
author_rank: Unknown
timestamp: August 01, 2010, 11:50:12 am
content: silly question but have you tried looking up the transformer codes, you may glean some info.Yes Kiriakos has a certain eer - Style, don't mind him, he is a fiery Greek and you don't really want to stoke his fire

author: toli
author_rank: Frequent Contributor
timestamp: August 01, 2010, 11:52:36 am
content: I did try running all of the codes on google. I did the same for the second smaller transformer from that amp. I got nothing unfortunately.I'll try and test it using the 10% voltage drop and the temp. I'll also check the wire gauge. Hopefully all of these will give me a close reading. I'll start with the smaller transformer as it has just one output

author: djsb
author_rank: Super Contributor
timestamp: August 01, 2010, 12:01:44 pm
content: These post may be helpfulhttp://www.edaboard.com/ftopic359829.htmlhttp://home.earthlink.net/~jimlux/hv/xfmrmeas.htmDavid.

author: Simon
author_rank: Unknown
timestamp: August 01, 2010, 12:47:44 pm
content: do you know the amplifier specs ? that way you will know what the transformer is capable of

author: toli
author_rank: Frequent Contributor
timestamp: August 01, 2010, 01:11:45 pm
content: Its a Sansui RZ9500AV. I don't have any specs for it. It does have 5 channels. Here's all I could find about it:http://1.bp.blogspot.com/_GQkPxwyJoco/SyVI906OpeI/AAAAAAAAATY/gOs5Y3qVXeA/s1600-h/sansui+02.jpgAnd this one with a little more info:http://articulo.mercadolibre.com.mx/MLM-35485247-receptor-estereo-sansui-300-watts-pro-logic-rz-9500av-_JM

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 01, 2010, 03:02:46 pm
content: All those transformers , are build for special purpose , specially made to serve the specific device needs .Do not look for them at the open market ..As about the transformers How-to , there is lots of theory behind them.And lots of books to study ..My own way of " speaking " , sounds harsh , but serves only one purpose ,to force you, so to investigate and learn .Its all about learning..

author: tecman
author_rank: Frequent Contributor
timestamp: August 01, 2010, 03:03:15 pm
content: Hero has is right. The wattage of a transformer is directly related to the amount iron in it. Using comparable transformers as a comparison will give you the wattage +/- 10%. If it is a single winding, then watts/voltage out will be the amps. If it is a multi winding unit, looking at the voltages should give some clue. Usually one or two windings are the main outputs. Other coils will be some auxiliary voltages. Again knowing the voltage and the wattage you should have a good idea of the currents it can handle.Paul

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 01, 2010, 03:10:25 pm
content: Quote from: tecman on August 01, 2010, 03:03:15 pmNop , there is no easy answers here ..Any primary , build to operate on multiple AC input voltages , uses higher wire gauge ,and the transformer becomes much larger , than the single input ones .

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2010, 03:35:40 pm
content: Quote from: Kiriakos-GR on August 01, 2010, 03:10:25 pmPerhaps you should look at the datasheets I posted awhile ago.If it's built to operate on multiple AC voltages, then it will be slightly larger but it shouldn't make much difference, for example if there are 220V, 230V and 240V taps on the primary, you're looking at +8.33% more mass on the primary coil but the mass of the core and secondary will remain the same and iron is denser than copper so it's negligible.Quote from: mikeselectricstuff on August 01, 2010, 09:11:42 amGood answer but I suspect that measuring the temperature rise won't be very helpful.For a start, each winding is thermally coupled to its neighbour, therefore the temperature rise of all the windings at their maximum ratings will be higher than the temperature rise for one winding when it's at it's maximum rating. Then there are all the problems with transformers having different temperature ratings and temperature rises described earlier.Quote from: toli on August 01, 2010, 04:32:18 amNo but if you can accurately estimate the power rating of the whole transformer, you've already got one part of the puzzle.QuoteI think he was talking about the gauge of the wire actually wound round the core, not the external fly leads.QuoteThat's probably the best way of estimating the rating of transformer and different windings but note that it's not linear.I'd suggest measuring the resistance of the primary winding first. You know it's 240V and the power rating will be equal to that of the whole transformer. Incidentally, the resistance of the primary is also a good indicator of the power rating but you need to compare transformers with the same primary voltage or scale accordingly.Measure the voltages and resistances of the secondaries, the I2R of two windings with the same power rating (copper losses) should be similar.For example, if you have a 500VA isolation transformer with a primary and secondary voltage of 240V and the primary resistance is 2R, the secondary resistance will also be 2R, if the secondary voltage is 120V, the secondary resistance will be approximately 0.5R and if they're two 120V secondary windings, each with a power rating of 250W, they will have a resistance of about 1R.Note as I said above, it's not linear but square, in other words:IfVpri = 240VRpri = 2RVsec = 120VTherefore:Rsec = Rpri*(Vsec/Vpri)2= 2*(120/240)2= 0.5RIf there are two windings with an equal power rating, Rsec would be approximately double what one would expect.EDIT:For completeness, here's the calculation:The expected resistance of the secondary is 0.5R, actual resistance is 1R, so P = 500* 0.5/1 = 250VA. The method should be the same for any transformer: calculate the expected resistance for each winding assuming it's rated to the full power, work out the ratio and scale it down.I suppose it's possible that there will be a difference between the resistance, since it's turns ratio and windings in the middle of the bobbin will have a slightly lower resistance but nothings perfect.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 01, 2010, 03:57:29 pm
content: Quotesome one opened the books ... or so ..Even so , the electricians way, are faster ..Calculations over wire gauge , its our bread and cheese ... or Fish and Chips for the English ones..

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2010, 04:11:44 pm
content: What if you can't see the wire gauge?Going from the picture, the windings are completely enclosed and the fly leads are connected to them internally and are probably oversized.I don't see how an electrician can estimate the power consumption of a device just by looking at the wire gauge? It's often oversized, for example I have a soldering iron with a cable with a cross-sectional area of 0.75mm2fitted (between 18 and 19 AWG) which would indicate a current consumption of 6A, which at 230V would be 1380W but the name plate says 50W so the current is only 217mA.The wire gauge only tells you the ampacity of the wire used and would be useful, if you could actually see the transformer winding but it will still depend on the quality of the insulation and former.

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 01, 2010, 04:30:41 pm
content: the mass to wattage "rule of thumb" might be helpful (if it does indeed exist, i never found or search) at first assumption of the max A the transformer is capable of.i dont think wire surface area is relevant as this is relating to electromagnetic coupling strength between primary and secondary coil, we need to know the transformer efficiency and that one is only in "the real" transformer datasheet.if someone gave me the one like in the picture, here is what i will do to investigate:1) find out how many primary and secondary coil exist (usually one for each) by continuity or resistive check. if there exist discontinuity between 2 ports, probably more than 1 winding exist, but so far i never found such transformer, but who knows, it may existthis also may help analyzing if the trans is ok or not.2) find the order from shortest to longest wire tap/end by continuity or resistive check. the highest resistive between two wires is the longest winding. if unsure, you can do the voltage test (feed the primary, check the secondary voltage vice versa) but make sure you dont feed with higher than acceptable voltage (esp in secondary coil) V=IR (yup, here I = f(wire surface area)), and.... experience3) the shortest secondary tap (or lowest voltage) is the one that is capable of producing the highest A, check that for max A capability using "my" parallel resistors technique. the other tap should be lower A than the shortest.some feeder (from school books, IIRC):(P=VI), P2 = effn * P1, P2 = power on secondary coil, P1 = power on primary coil, effn = efficiency at the nth tap.but nothing beats the experience

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 01, 2010, 04:38:02 pm
content: or if you afraid to damage your DMM, then find as many wire size as you can get, short them, the biggest one that blown out is the one that you need to find of its wire gauge, from there you'll know the amperage

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2010, 05:14:19 pm
content: Quote from: shafri on August 01, 2010, 04:30:41 pmIt comes from transformer design calculations.The core size is proportional to the power and inversely proportional to the frequency, a transformer designed for 60Hz will be1/6of the size of a transformer designed for 50Hz, all other things being equal. This is why SMPS transformers are tiny compared to mains transformers, although different core materials are used so you can't really apply this rule to them unless you know the material. Generally mains frequency transformer all use silicon steel.http://www.geofex.com/article_folders/xformer_des/xformer.htmQuoteNot normally true, most of the time a winding is rated to the same current throughout, although some windings might be thinner/thicker from a tap to the end, depending on the transformer. For example in an insect killer, the UV tube may be connected to a tap with an open circuit voltage of 500V and a short circuit current of 200mA, then there will be another 5kV tap with a short circuit current of only 10mA for the high voltage grid.QuoteIt won't be linear. P = I2R.QuoteWhy would you want to do that?The fusing current of wire can be found in tables which can be found on the Internet.http://www.hmwire.com/New%20PDFs/Fusing_Currents_Aluminum_Magent_Wire_R2.011609.pdfThe gauge of wire can be measured using a micrometer.You don't want to destroy the transformer.

author: toli
author_rank: Frequent Contributor
timestamp: August 01, 2010, 05:24:14 pm
content: I see many suggest to first check the secondaries voltage. I already did that before opening this thread. With the transformer connected to 230V I checked the voltages. Before doing so I've tested for continuity, that's how I know it has 4 secondaries (even though there are 13 output wires on the secondary). This transformer was probably made for use with this amp specifically as each of the secondaries went to a different board of the amp.Anyway, I've drawn the voltages I've measured on all secondaries, the picture is attached.Please note that the primary isn't drawn, but its input is 230V 50Hz.I would also like to take a moment and thank you all for all your help.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 01, 2010, 05:37:49 pm
content: Quote from: Hero999 on August 01, 2010, 04:11:44 pmEverything are related ... even the soft wire .. the soft wire looks bigger in the eye , but its not.

author: djsb
author_rank: Super Contributor
timestamp: August 01, 2010, 05:41:28 pm
content: Do you still have the rest of the amplifier? Are there any internal fuses on the circuit board (on the secondary side of the transformer)? If so this may give an indication of a safe current.Do you have access to a variac? If you do you could short 1 secondary at a time whilst slowly increasing the voltage input (maybe upto 10% as suggested earlier) whilst measuring the input current (as suggested herehttp://www.edaboard.com/ftopic359829.html).David.

author: toli
author_rank: Frequent Contributor
timestamp: August 01, 2010, 05:56:25 pm
content: Hi David,I don't have a Variac. I've used one in the university laboratory for testing transformers, but I can't take it home, and the university is a few hours using the train so I don't like the idea of taking almost 6kg of iron with meBTW, I've check using my micrometer, all wires are 0.029" in diameter. So 21AWG.However, I can see from the side that the wires coming out of the transformer are not these that are used inside it (see picture attached). I've checked 3 of the wires (there's a small portion where I can measure the internal wiring with my micro) and they are actually higher gauge, 0.051" diameter, which is 16AWG.BTW, I dont have the amp anymore. But I wouldn't trust the fuses in there as it looked like someone already tried to fix it, and the fuse holders were damaged. One of the fuses actually had a wire running in parallel so the fuse was useless. That's why I'm not sure the fuses were the same ones as when the amp was built.

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2010, 06:19:27 pm
content: It's no surprise that some of the internal wires are thicker - they need to be because they're coiled up so there's less heat loss.Have you actually weighed it? Is it really 6kg? If so I could have a quick flick through the catalogue to find match. Have you measured the DC resistance of the primary? That might also help me to estimate the total power rating?Be wary about using fuse ratings to determine maximum ratings: fuses are often spec'ed to double the nominal current rating.

author: toli
author_rank: Frequent Contributor
timestamp: August 01, 2010, 06:31:53 pm
content: Yes, I did weight it at about 5.5kg. As for DC resistance, my current DMM isn't really accurate at these low values but it reads 3.2R on the primary. I should get my 87V in a few weeks, but until I get that I probably wont be able make any reading with better accuracy.

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2010, 07:06:33 pm
content: It will be between 300VA and 450VAI reckon 350VA minimum and absolute maximum 450VA, most likely 400VA.This data sheet has a 300VA transformer with a mass of 5kghttp://www.rapidonline.com/netalogue/specs/88-5074.pdf4.9kg, 400VAhttp://www.farnell.com/datasheets/93631.pdf5.5kg, 400VAhttp://docs-europe.origin.electrocomponents.com/webdocs/002e/0900766b8002ea32.pdf5.3kg, 400VAhttp://www.farnell.com/datasheets/12658.pdfAll the above transformers have multiple primary taps to higher voltages than 240V so I'd expect yours to be slightly lighter so 400VA is a pretty safe estimate, none specified the primary resistance but that will help you estimate the secondary resistances which I suggest you measure.

author: toli
author_rank: Frequent Contributor
timestamp: August 01, 2010, 07:12:39 pm
content: Thank you, will do that.I'm a bit overwhelmed with all the different ways to test this one thing

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 01, 2010, 07:52:03 pm
content: Quote from: toli on August 01, 2010, 05:56:25 pmThats an 0.75 in metric , AWG18 = 12V 0-5A ... so you are safe at the 3A continues operation.Lets hope that you did not measure the isolation too ..

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2010, 08:06:45 pm
content: Quote from: Kiriakos-GR on August 01, 2010, 07:52:03 pmNo, that's just the maximum current rating of the leads, not the transformer.Look at the voltages of the windings he posted above, if you assume the maximum current for each winding is 3A, the total power comes to about 840VA which is not possible with a mains frequency transformer weighing only 5.5kg.

author: Simon
author_rank: Unknown
timestamp: August 01, 2010, 08:18:05 pm
content: Quote from: Hero999 on August 01, 2010, 03:35:40 pmnope actually copper is heavier to iron by about 10-15 % if my memory serves, but still the difference can be calculated and compensated for

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2010, 08:40:20 pm
content: Quote from: Simon on August 01, 2010, 08:18:05 pmYes you're right, although only a small part of the transformer is copper, the windings are small compared to the core, so I wouldn't think it would be significant.

author: Simon
author_rank: Unknown
timestamp: August 01, 2010, 08:44:17 pm
content: well depends, theres probably no harm in considering copper to weigh what iron does but I think you would be surprised at the weight or copper versus iron in the transformer, as most calculations are already estimates you best keep acurate what you can or you get a stack up of errors that can significantly skew the answer

author: Unknown
author_rank: Unknown
timestamp: August 01, 2010, 09:28:47 pm
content: Quote from: Hero999 on August 01, 2010, 05:14:19 pmDon't you mean 5/6th?As far as I know, the power that a transformer can handle is limited by two things: the heating due to ohmic losses (I2R heating and IR voltage drop) and the flux density in the core (saturation). They are basically independent, so you can wind really flimsy wire on a large core, so the wire will melt the insulation without saturating the core. And it's also possible to wind fairly heavy wire on a small core, causing saturation even though the wires can handle more. In an ideal design, these would be perfectly matched, so either method would yield the same answer. But I wouldn't rely on this.I would probably use a formula based on core size to calculate max power (eg. this one), and do a sanity check based on wire resistance. Not sure how to calculate individual winding rating, assign each winding an amount of current proportionate to the resistance and solve for total amount of power? I don't think wire gauge is a useful parameter, since it's the total resistance (based on wire gauge and number of turns) that determines loss and voltage drop, if you assume that all parts of the core have the same temperature (which might not be true, but is probably not too far off).Checking if the temp rise remains below 20-30C or so could also work, but it does depend on the insulation class. I would expect almost any mains transformer to handle something like 60C. According toRod Elliott, the lowest insulation class can withstand 105C, although it also states that running at high temperature shortens the life span. I think the temperature rise quite steeply if the core is saturated, so that should be easy to spot. It doesn't hurt to be conservative, and it obviously depends on cooling.

author: Zero999
author_rank: Super Contributor
timestamp: August 01, 2010, 09:39:56 pm
content: Simmon,Well yes, the lightest one doesn't have a 400V tap so that's probably why.I don't know how much the primary would weigh let alone how much the extra turns for the 400V tap would weigh. I doubt it's an extra 74% extra on the mass of the primary as they're likely to use a thinner wire gauge for the 240V to 400V tap because it will draw less current and copper is expensive.alm,Yes 5/6th.

author: Kiriakos-GR
author_rank: Super Contributor
timestamp: August 01, 2010, 11:08:24 pm
content: Quote from: Hero999 on August 01, 2010, 08:06:45 pmSorry I do not follow the logic of kg's .I had speak for only one secondary , and I am staying to this theory .And 12V by 5A = 60VAoutputDo not be confused with the total VAinputAbout the voltage output of the specific TF , I did not see any voltages of the windings ..The math's above looked as an example .

author: toli
author_rank: Frequent Contributor
timestamp: August 02, 2010, 05:52:31 am
content: Yes, obviously I've removed the insulation, I'm not that stupid

author: shodan
author_rank: Contributor
timestamp: August 02, 2010, 06:38:14 am
content: considering how hard it is for the "elders" of this forum to agree on this subject, with all this confusionI think this truly is a difficult to explain subject and maybe newbies shouldn't be hit so hard with the clue stick for not understanding !as far as my understanding of transformer goes, everything I know about them comes from this pagehttp://www.ibiblio.org/kuphaldt/electricCircuits/AC/AC_9.htmlI suggest everyone who wrote to this thread reads this (even if you know how they work, a refresher isn't gonna hurt you)then we can come up with a better explanation for toli and future transformer questions

author: Zero999
author_rank: Super Contributor
timestamp: August 02, 2010, 08:25:03 am
content: Quote from: Kiriakos-GR on August 01, 2010, 11:08:24 pmDid you read the datasheets and transformer tutorial linked to on my previous posts?Why don't you follow?QuoteHe posted the voltages, none of the secondaries are 12V.https://www.eevblog.com/forum/index.php?topic=970.msg12502#msg12502