Question about toroidal transformers

[alfredrofe]

Originally posted by 00940
A simple question, but I couldn't find any quick answers by searching.

I've got a 30VA 2X18VAC toroid. I'm drawing about 15W out of it.

Is it possible to just wind an enamelled wire around it in order to create a new CT 25-0-25 secondary, to draw around 50mA ? I've around 15m of 0.6mm enameled on hand would this be suitable ? Due to space constraints, it's not really possible to add another xformer.

Thanks for any answer

This is possible, it is best to do some preliminary calculations

25 vac x 10 turns per volt on average for a torroid x 50mm per turn each winding will be 12,5 metres

250 turns @ .6mm (.6 x2 for bifiliar wind) width side by side of the inner donut circle, measure inner diameter and compute no of layers to space over the donut for max flux exposure

25 vac known
To test for turns per volt wind ten turns onto the torroid measure the voltage and divide by ten = turns per volt
measure the lenght of each turn I use a fibre glass dressmakers tape measure, you can borrow from Mrs Rumpole if you dare.

Is this 50ma per winding or total

check .6mm covert to sqmm 1.8sqmm usually 3amps/sqmm for safety = 1.5 amps for .6mm, if my calcs are correct,

Bifiliar winding helps to make an equal 25vac per winding

make sure you wind each turn as close as possible to 90 degrees to both inner and outer ring of donut

spread the winding over the entire donut for max flux exposure

Just a few things yo keep in mind for maximum success, I guess there are a few others they elude me at the moment

Best of luck, planning and patience be with you
Alfred

[AndrewT]

Hi,
your 30VA transformer must be de-rated to about 70% when feeding a capacitor input filter.
This means the maximum continuous power draw must be 21W.
You already have 15W out of it. You are running at 71% of maximum continuous capability.
This is already higher than the half rated capacity that is usually recommended to keep temperatures down rather than approach the maximum.

Taking an extra 2.5W brings your total up to 17.5W or 83% of maximum continuous capability.

[4me2ctv]

What is the voltage between the secondary windings? Do they need to be 5000V isolated from each other?

Thanks for helping. During operation, the peak voltage between windings would be 1300V. The reason for the 5000V isolation is because of peak spikes which can occur in a high voltage power supply at power on when using chokes for filtering. For my power supply, I need 4 secondaries each isolated for 5000V from each other and from the primary.

Thanks,
Adrien.

[MondyT]

Hi

For low voltage secondaries on most 50Hz power toroids of around 30VA, you will need around 13 turns per volt (10 or so turns per volt for 60Hz toroids). Transformers of this size will normally be run at around 4 to 5 A/mm^2 of copper. Seeing you have un-knowns, it would be wise to go for no higher than 4 A/mm^2. This should then give you approx the same regulation on your new secondary as the original ones.

For dual windings, Bi-filar or two wires side by side, is the way to go and as said before, wind over the full 360 degrees of the core to avoid excessive leakage which will aggravate any stray field and decrease the regulation of the winding.

Now a word of warning!!

I strongly recommend that you *do not* consider “rolling your own” toroidal windings on any voltages higher than SELV (safety extra low voltage, 50V rms or 120V DC with zero ripple) circuits. It is tempting to add windings, but toroidal windings are much more critical than EI transformer windings when it comes to insulation safety. You can use a split chamber, fully shrouded bobbin with an EI transformer and this comes with all creepage and clearances built in as long as the winding is done with reasonable care and you don’t go bonkers laminating the core. There is no such thing with mains toroids and all insulation clearances etc need to be created by construction.

Even if the primary is factory made and insulated, there are many reasons why adding windings is not a good idea and here are just some…

1. A factory produced primary winding, with its major insulation correctly applied and waiting for secondary winding, is a very delicate bit of kit and must be treated with kid gloves! There is a great chance that the insulation will be compromised by dirt, wire clippings, dust, grease, moisture etc. before any secondary winding is even attempted, and that is without the handling involved in the hand winding of just a few turns. This compromise will practically render the major insulation useless over time.

2. To enable the required creepage and clearance distances required for safety, the application of reinforced insulation around the transformer lead outs is critical and needs to be done correctly and with the correct quantities of the correct insulating materials. You must remember that the integrity of the major insulation between primary and secondaries must remain good with all the thermal cycling and dielectric stress that takes place during the entire life of the transformer and this includes conditions during faults and transformer breakdowns. Even if you test for dielectric strength when the transformer is complete, there is no guarantee it will remain sound once the transformer has reached its working temperature unless all the correct materials and methods are used. Most of these methods and materials have taken a great deal of time to perfect!

3. *All* materials used in safe transformer manufacture are chosen and used for a reason. The correct dielectric strength of all insulating materials used is not the only factor that is essential for safety. Even with SELV circuits and windings, the flammability and burning characteristics of every material used are vital and must not be taken for granted. Each different transformer temperature class has different material criteria. Unless you know exactly what you are doing, even adding a small winding could compromise your safety and the safety of those around you if the materials you used are not approved for transformer use within a given temperature class.

4. Low voltage windings that sit at elevated voltages to ground or each other, may consist of relatively few turns of heavy wire but the insulation between them needs to be of the same standard as the major insulation as failure here could result in short circuit secondary turns. Toroidal transformers by nature have much lower leakage reactance than EI constructed transformers and therefore the short circuit currents can be pretty high and things can get very hot, very fast, representing a serious fire risk.

I could go on!

Not to put too much of a downer on it, I would recommend you stick to voltages no higher than 50V rms (remember that will be 25v per winding if a dual winding is made) and to *never* remove any insulation that is on the transformer in the first place. As mentioned previously, if you can accommodate your winding by just adding flexible PVC insulated wire for the turns *over* the final transformer insulation, this is the best solution and will be pretty safe. If you do this, you do not tamper with the transformers insulation system and you also are only adding materials that have the correct flammability rating etc.

Sorry for the ramble but life is just too precious to get it wrong!

I hope this helps

Cheers
Ray

[MondyT]

Hi

For low voltage secondaries on most 50Hz power toroids of around 30VA, you will need around 13 turns per volt (10 or so turns per volt for 60Hz toroids). Transformers of this size will normally be run at around 4 to 5 A/mm^2 of copper. Seeing you have un-knowns, it would be wise to go for no higher than 4 A/mm^2. This should then give you approx the same regulation on your new secondary as the original ones.

For dual windings, Bi-filar or two wires side by side, is the way to go and as said before, wind over the full 360 degrees of the core to avoid excessive leakage which will aggravate any stray field and decrease the regulation of the winding.

Now a word of warning!!

I strongly recommend that you *do not* consider “rolling your own” toroidal windings on any voltages higher than SELV (safety extra low voltage, 50V rms or 120V DC with zero ripple) circuits. It is tempting to add windings, but toroidal windings are much more critical than EI transfor

[4me2ctv]

Thanks Ray. This is exactly the advise I was looking for.

Adrien.