Transofmer winding load - Page 1
Forum section: electronics / projects

author: reyntjensm
author_rank: Regular Contributor
timestamp: December 05, 2020, 08:44:06 pm
content: I have a transformer with 1 primary winding and two secondary windings. I want to use both windings to make 2 independent power supplies. In the datasheets it's specified as 0,35A in series and 0,7A in parallel. My question is, can i load both windings with a different load? Say winding 1 is 0.5A and winding 2 is loaded with 0.2A?

author: bob91343
author_rank: Super Contributor
timestamp: December 05, 2020, 08:55:04 pm
content: Not a good idea. The wire size is the important thing. If you put extra stress on one winding, it doesn't help much to reduce the stress on the other one. Perhaps a 20% difference but I wouldn't go much farther than that.

author: langwadt
author_rank: Super Contributor
timestamp: December 05, 2020, 08:57:15 pm
content: no, the max current in each winding is set by the wire thickness

author: reyntjensm
author_rank: Regular Contributor
timestamp: December 05, 2020, 09:18:59 pm
content: Quote from: bob91343 on December 05, 2020, 08:55:04 pmSo 0.35A would be the max load for each fase? And it's not a good thing to have an unequal load on both windings?

author: bdunham7
author_rank: Super Contributor
timestamp: December 05, 2020, 09:27:50 pm
content: Quote from: reyntjensm on December 05, 2020, 09:18:59 pmThe uneven load is not a problem and what you proposed would not overload the primary, but itmightoverload the secondary with the higher current. Absent other information you would assume that the 0.35A rating is the max per secondary winding.

author: reyntjensm
author_rank: Regular Contributor
timestamp: December 05, 2020, 09:33:20 pm
content: Quote from: bdunham7 on December 05, 2020, 09:27:50 pmAttached you can find the datahseet for the transfomer. I'm using the 3FD-536. I'm not planning to overload the the primary winding. Both secondary windings will have a big difference in current drawn.

author: bdunham7
author_rank: Super Contributor
timestamp: December 05, 2020, 09:41:38 pm
content: Again, the difference isn't the problem. It looks very clear that 0.5A is more than they have rated the secondary winding for as they explicitly specify a 0.5A fuse. Whether it will eventually fail at that current level is questionable--it will be right on the edge.

author: bob91343
author_rank: Super Contributor
timestamp: December 05, 2020, 11:16:37 pm
content: I don't think the specs are that precise. Even if you stay within ratings, there is a possibility of failure. That possibility is directly related to the current through the winding.You can get creative and, realizing that temperature is the villain here, it's indeed reasonable to slightly overload one winding if you more than make up for it on the other winding.The wasted energy in the winding is proportional to the square of the current if we ignore the temperature coefficient of the copper. So the hot spot in the winding will still be hotter than you want.Assess the cost of failure. Will the transformer catch fire and burn down the house? Will operational loss due to a burned out winding cause consequential damage? Is the cost of the new fuse and transformer and your labor enough that you want to have a large safety factor?Nothing is ever simple.

author: Vovk_Z
author_rank: Super Contributor
timestamp: December 06, 2020, 12:12:15 am
content: 0.5 A instead of 0.35 A means twice as much heat power wasted in a winding ( (0.5/0.35)^2 = 2 ), so draw 0.5 A instead of rated 0.35 A is most likely too much.If overload is not as large or short-time then it may possibly not a problem.I can guess that up to 20-40% of overload may possibly be safe in some circumstances.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 06, 2020, 09:17:42 am
content: Mind that, if you're using a FWB + cap-input filter, the power factor is poor and you will get closer to half the DC output current, for the a given AC RMS rating. Understandable, as rectification gives you a 40% boost in voltage (over RMS) after all.As for combined ratings, they're probably bifilar so we can note that the critical factor is the heat dissipated for both wires together -- they aren't bunched up anywhere alone, so loading one winding heavier than usual won't cause a huge increase in winding interior temperature.We'll get the same heating effect, from loading both windings at 0.35A each, if we load just one at twice the power and the other at none. Twice power is sqrt(2) times the current, or about 0.5A.However, this leaves no room for the 0.2A. That would dissipate (0.2/0.35)^2 ~= 1/3 times nominal power, allowing the other winding to dissipate the leftover 2/3rds, but 3/3rds is requested, and so is unavailable.If you can relax your requirement slightly, or use literally just the next size larger transformer, you'll have it.And if your requirements are DC current, not AC RMS, then you absolutely need the next one or two sizes up, or some PFC gimmick.PFC on a low voltage transformer... what an act of desperation...Entirely possible of course -- but much better off buying SMPS modules instead. (Also, why not do that in the first place?)Tim

author: Alti
author_rank: Frequent Contributor
timestamp: December 06, 2020, 11:50:09 am
content: You can use both secondaries separate but keep in mind the insulation of a transformer is most likely specified for primary-secondary. Not secondaryA-secondaryB. Also, the currents given are rms at 50Hz/60Hz.I also think that if putting 0.5A fuses (lagging I suppose) on secondaries is recommended as adequate protection then the transformer is capable of surviving 0.5A+0.5A abuse (I do not know the ratio of the transformer, looks like 1:10 so a 100mA primary should do the trick). If secondaries are wound on same bobbin then you can draw 0.35A*sqrt2=0.5A safely from one secondary when the other one is idling.Just keep i1rms^2 + i2rms^2 < 0.25[A^2] in long term.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 06, 2020, 02:36:55 pm
content: BTW, note that the regulation of this sort of transformer is rather poor, probably not impedance-limited at this power level (12VA) but the smaller ones of identical design often are.That is, the change in output voltage, for a given load, is relatively large. Or the short-circuit current is only so many times the nominal current. (Equivalent statements.)Such transformers may not be able to clear fuses, at least not very quickly, or consistently.Fuses are, first and foremost, for fire protection, anyway: best use here is on the primary, so if the transformer does cook off, it's because the primary has turned into a smoking, molten lump, failed shorted, and is drawing fault current from the mains supply. (Or for the impedance-limited case: it simply gets really hot, but not quite hot enough to melt or smoke.)It's a lot harder to protect the transformer, in a meaningful way. It might never deliver enough current to clear the fuse, even as it's smoking off. (Heh, indeed, current falls as temperature and therefore resistance rises. That takes some minutes of heating though. If the fuse hasn't gone by then, it probably never will.)A thermal fuse (preferably integrated into the transformer itself) would be adequate primary or secondary side protection. Ironically, integrated fuses aren't exactly easy to service, so the point of protecting the transformer is moot anyway.Anyway, the regulation / fault current issue can be solved neatly with a better design, typically shell-type (same core, using single-compartment bobbin and P/S in wide layers) or toroidal. Or, again, using an overrated transformer, and a somewhat underrated fuse -- so that it can definitely supply fuse-clearing fault current when needed.Tim

author: exe
author_rank: Unknown
timestamp: December 06, 2020, 04:25:31 pm
content: In this case transformer performance is limited by temperature. Usually it's possible to do some crude calculation of different load scenarios, esp. if datasheet mentions separately core losses and winding losses. From there, I think, one can infer temperature rise under different scenarios. Basically, for medium-size transformers most of the loss is from winding resistance, which is a bit tricky to calculate as it is rises with temperature.Or, much easier solution, just buy a properly-sized transformer. I suggest make it 50% bigger than the expected VA load due to reasons mentioned above.The ultimate test is to measure temperature of transformer under expected full load and expected cooling conditions after 1h (or more) of work.