Multiple (toroidal) transformers in one enclosure - Should I expect problems? - Page 1
Forum section: electronics / beginners

author: tifkat
author_rank: Regular Contributor
timestamp: August 23, 2020, 08:32:43 am
content: Hello again,In my search for the perfect single toroidal transformer for my bench DC Power Supply project I've hit a wall on two fronts: nothing exists with exact specs, and getting a custom transformer wound is looking to cost me upwards of $500 dollerydoos, plus across country shipping.However I have managed to find no less than 3 'stock' transformers for around $200 total (including international or national shipping) which together would meet my needs.Generally speaking, what kind of issues might I face having three individual transformers all mounted in a single enclosure along with my circuit boards?I plan to parallel connect all primaries, but the secondary windings will all be isolated from each other (between transformers at least)I'm considering the prototyping stage here mostly. For the final version I may shell out for the single $500 transformer.tifkat

author: MosherIV
author_rank: Super Contributor
timestamp: August 23, 2020, 09:53:16 am
content: Weight ! 3 transformers is going to make your PSU incredibley heavey, the case to handle the weight itself will make the case heavy itself.Heat. Transformers, even without load, output some heat. Now you have 3 of them and 1 of them being under load will generate more heat.Magnetic coupling (mainly for electrical noise) now you have 3 transformers, each radiating its own magnetic field. This usually cause 50/60Hz ripple in low voltage cables that are near the transformer.Just my thought.

author: Gyro
author_rank: Super Contributor
timestamp: August 23, 2020, 10:00:37 am
content: Inrush current. Toroidal transformers can draw a very significant peak current they happen to switch on at a mains zero crossing and the core briefly saturates. Three in parallel could pull a lot of current - Hard to judge off hand whether it would be any greater than a single transformer of 3 times the rating though.I have seen an application many years ago where two transformers were stacked on a single bolt though.

author: Benta
author_rank: Super Contributor
timestamp: August 23, 2020, 11:32:18 am
content: I wouldn't worry about magnetic coupling, toroids are very good in this respect, there's practically no stray field.As Gyro says, inrush current is a real issue, both with a sigle 500 VA or with three smaller.You should consider a soft start circuit of some kind. The simplest is switching on the transformer at crest mains voltage.A second limiter before the filter cap bank might also be needed.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 23, 2020, 11:57:16 pm
content: I've done some measurements on a 230Vac to 12Vac Toroidal transformer, with a 1Ohm wirewound shunt to measure the inrush current with my scope, and I've measured 300V peak voltage over that shunt.So inrush currents are no pushover.Then I started with Triac control and I could nearly eliminate the rush in current, but had to adjust the phase difference fairly accurate for good results.Well, at least it worked good about half the times, which I realized after quite a lot of measurements.It turned out that the remanent magnetism in the core caused enough of a phase difference to be significant.So at last I turned the transformer "on" on a positive cycle, and "off" on a negative cycle.It's probably a lot easier to add a series resistor, and then bridge that resistor with a relay after a few seconds.A 20Ohm resistor will limit any inrush current to some 15A or so, which is not such a big deal.This is the standard way for turning on high powered audio amplifiers. You can even find kits for this on Ali for Ali prices.Ceramic power resistors can safely be grossly overloed for short times, so a 20W resistor will be plenty.The gold colored aluminium power transistors are very sensitive for overloads, do not use them.The root cause of the inrush current is the saturation of the transformer core.Some of the cheaper transformers are very close to saturation, while others are less critical.Also note that if you have a bridge rectum fire and big elco's behind the transformer, these are a quite serious short circuit during turn on of the circuit.Triac control can not handle this, while a series resistor with relay also takes care of this.There is extensive experience and documenation on multiple transformers close together.This was very common in the obsolete audio amplifiers with those post transistor transparent glass lightbulbs.Mr Carlson has given a good tutorial for this on youtube. The idea is to put the 2 transformers as far apart as feasible, and also orient them in such a way that the stray magnetic fields have a 90 degree offset to each other.For just power, it does not matter much. Stray field for toroidal transformers is quite small to begin with. things like hum is not relevant for just power transformers themselves.Heat may be a problem, especially if you put everything too close together. Some airflow is beneficial, so if you plan to add a fan, than do it in such a way that the transfomers also get a bit of moving air.

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 24, 2020, 12:59:18 am
content: Yes, toroidal transformers have the largest inrush current between all types so without some soft-start circuit you'll have to use fuse two or three times larger nominal rated then it could be. But there is not strong need in sophisticated soft start and even usual powerful NTC may greatly impruve and help. NTC cold (rated) resistance has to be about (or a bit larger) than primary DC resistance.

author: julian1
author_rank: Frequent Contributor
timestamp: August 24, 2020, 01:14:12 am
content: Just in case an obvious possibility has been rejected unwittingly. Is it not possible to take an off-the-shelf toriod transformer with a specific VA rating, and wind on additional secondaries as needed?http://www.diy-central.com/?p=137

author: rsjsouza
author_rank: Super Contributor
timestamp: August 24, 2020, 11:59:11 am
content: Good tips all around. IME I have seen the same soft start techniques applied not only to toroidal but also to very large conventional transformers and other large inductive loads.Quote from: Doctorandus_P on August 23, 2020, 11:57:16 pmDitto. Their thermal mass is quite large. If properly sized, even a 1W or 2W should be enough to tame inrush.Quote from: Doctorandus_P on August 23, 2020, 11:57:16 pmIn other words, if your ass is on fire, be prepared for very high zapping.