#### Ravindran

hey guys, i have a toroidal transformer from a home theater which started blowing fuse all of a sudden.
i found a sticker on the top of the toroidal here is a image of it
.

it is 67W transformer
which has a input of 220v primary
and 3 secondaries which are:
15.5v-0v-15.5v which as thickness as

24v-0v
3.6.v-0v
the remaining above 2 secondary and primary has same thickness

question1: what is the amp for all the secondary?
i found out it can be calculated by W=VA
which gives
4.3225A
2.791A
18.6A for each which iam not sure that its correct!.
question2: is the thickness of the copper wire associated with the ampere?
question3: can i change the thickness of the copper wire by like .10mm or even like drastically ?
question4: is there any alternative way for solving the problem other than doing the trivial rewinding job ?
question5: if rewinding is the only way how many turns should i do and how do i calculate the number of turns ?

#### Ian Finch

Paid Member
The copper wire size could be related to current requirements but more likely, its just a convenient single size that meets the maximum current requirement of all secondary windings with acceptable resistive losses. Using a single wire size reduces the number of manufacturing operations and allows more output voltage options when multiple windings are required. Put simply, the wire diameter is not proportional to voltage - it's just a sufficient size for the maximum current demand of any and all secondary windings with acceptable resistive losses.

• To calculate the overall VA rating of the transformer, you would have to sum the total of VA requirements of all secondary windings.
• If you want to change the wire diameter, increase rather than reduce wire size, to ensure the losses and hence heating and inefficiency do not rise.
• Assuming the primary winding is designed for connection to mains power you can test secondary voltages anyway. No need to guess, as long as you are safely protected from contact with mains voltages - otherwise death can be instant!

With care, there's no need to guess what voltages are produced by the transformer. Simply use a good quality multimeter set to AC volts to test them unloaded but never use a cheap meter to check mains voltages directly - Meters can and sometimes do short and arc internally with high AC potentials and some do explode. Yes, I've seen this happen to a pro. electrician who was unaware of meter safety categories too.

The size and mass of the transformer should give you a basis for comparison of the total power (VA) rating of the transformer. You can estimate your transformer's original spec. by comparing yours with toroidal products detailed by size and mass, in your local supplier's catalogue. I use the tables published online by Farnell/Element 14/ RS and other international component sellers, in their catalogues.

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#### Ravindran

If you want to change the wire diameter, increase rather than reduce wire size, to ensure the losses and hence heating and inefficiency do not rise
i kindly thank you for your responce and causion measures.
i unwinded every secondaries and found no short in it ,
the 15.5-0-15.5 secondary had around 166 turns which gives 5.35 turns per volt is this correct ? if this is correct i have to do like 1100+ tunrs for primary right?
And do you know how to calclate the number of turns with dimention of the windind area and respective volt

#### JMFahey

Why did you unwind it?
You turned a commercial/usable product (a power transformer) into scrap metal, to be sold by the pound to the metal recycler.

Ian Finch suggestions are fine for what he describes, but here we have the reverse problem:

* we already have the VA rating (it´s printed on the label)

* we do not have the VA requirements of all secondary windings

*
The copper wire size could be related to current requirements but more likely, its just a convenient single size that meets the maximum current requirement of all secondary windings with acceptable resistive losses.
Not too sure about that, why?

It being a PT from a defunct Home Theater I would expect one winding being the "main" one, for the power amp(s), so the 15.5-0-15.5 VAC one, which would mean some +/-22V rails, compatible with a couple chipamps, which would be the thickest wire one, plus a Logic/remote control 3.3VAC one (for a regulated 3.3V supply) + a very low current 24 VAC one (for fluorescent display? .... very popular in old Home Theaters and Mini Components
I would not use thick wire for such a low power winding.

2 secondary and primary has same thickness
I simply can´t trust that info, sorry.

Unless you mean low power secondaries (not 15-0-15) have thin wire, as thin as used in primary.

question1: what is the amp for all the secondary?
i found out it can be calculated by W=VA
which gives
4.3225A
2.791A
18.6A for each which iam not sure that its correct!.
None,all are equally wrong assumptions.
You have different secondaries, with unknown consumption, all different.
All you know is total: 67 VA, distribution is anybody´s guess.

question2: is the thickness of the copper wire associated with the ampere?

As Ian said, usually yes. Sometimes very low current windings are wound with thinnest practical size, sometimes an already used one, simply because "it´s already there".

question3: can i change the thickness of the copper wire by like .10mm or even like drastically ?
How would you do that?
You can draw wire thinner through the proper die; not sure how would you increase diameter.

question4: is there any alternative way for solving the problem other than doing the trivial rewinding job ?
We are beyond that point, aren´t we?

question5: if rewinding is the only way how many turns should i do and how do i calculate the number of turns ?

You must learn to design a transformer according to your needs.
I am certain there are a couple good tutorials out there, not the subject for a Forum answer.

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#### rayma

question1: what is the amp for all the secondary?
i found out it can be calculated by W=VA
which gives
4.3225A
2.791A
18.6A for each which iam not sure that its correct!.

The given power rating is the input power for the primary winding.
There is no information given on the individual power for each secondary,
although normally current ratings for each should be specified.

There is just not enough information to redesign the transformer, so duplicate exactly the original design.
Use the same or larger diameter magnet wire. Do not use smaller wire.

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#### as_audio

Why did you unwind it?
You turned a commercial/usable product (a power transformer) into scrap metal, to be sold by the pound to the metal recycler.
Answer is in post 1 : toroidal transformer from a home theater which started blowing fuse

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#### Ian Finch

Paid Member
Well it wasn't the transformer was it? If you suspect a transformer short, surely you'd take the easy path first by removing power then disconnecting one lead of each winding to isolate it so each winding could be be tested for a short separately or even easier, start by checking all windings together as they are, using a multimeter or even a small lightbulb and battery etc. before dismantling the transformer. If it's only rated at around 60VA though, it does mean that was quite a tiny HT amplifier.

Some HT amplifiers I can see in the Indian market indeed go down to just 5 or so TDA2030s doing all the power amplification. Now that's classy style entertainment on a budget

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#### Ravindran

Why did you unwind it?
You turned a commercial/usable product (a power transformer) into scrap metal, to be sold by the pound to the metal recycler.
i kindly thank you for your responce . i begin to unwind not just because it blown fuse and also i checked the pcb for any short(which i havent found any) then i unbolted the transformer and checked ,that is where i found a brown spot under the toroidal which also smelled like toasted.And most of all the first image shows the mylar used for insulation between the pri and sec is completely vandalize.

It being a PT from a defunct Home Theater I would expect one winding being the "main" one, for the power amp(s), so the 15.5-0-15.5 VAC one, which would mean some +/-22V rails, compatible with a couple chipamps, which would be the thickest wire one, plus a Logic/remote control 3.3VAC one (for a regulated 3.3V supply) + a very low current 24 VAC one (for fluorescent display? .... very popular in old Home Theaters and Mini Components
I would not use thick wire for such a low power winding.
yes,you are absolutely right about that and as the post 1 shows the 15.5-0-15.5 is the thickest of all.

I simply can´t trust that info, sorry.

Unless you mean low power secondaries (not 15-0-15) have thin wire, as thin as used in primary.
dont be sorry it is may be because of my misinterpretation,
let me rephase
the 15.5-0-15.5 secondary is the thickest approx 1mm (not sure measured using a cm scale)
the remainig secondaries which are 24 and 3.6 and the primary has the same thickness
here is a image of the following secondaries

image of primary

None,all are equally wrong assumptions.
You have different secondaries, with unknown consumption, all different.
All you know is total: 67 VA, distribution is anybody´s guess.

How would you do that?
You can draw wire thinner through the proper die; not sure how would you increase diameter.
hypothetically speaking if i couldnt find the exact thickness of the copper wire at the stores near by me ,is it okey to change the thickness (as for as i understood by
its just a convenient single size that meets the maximum current requirement of all secondary windings with acceptable resistive losses
Sometimes very low current windings are wound with thinnest practical size, sometimes an already used one, simply because "it´s already there".
Use the same or larger diameter magnet wire. Do not use smaller wire.

it seems like i can increase the thickness and should not decrease )

We are beyond that point, aren´t we?
we definitly are.i am sitting here staring at this solid donut
You must learn to design a transformer according to your needs.
I am certain there are a couple good tutorials out there, not the subject for a Forum answer.
okey,but can you consider answering the post #3 if possible

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#### Ravindran

The given power rating is the input power for the primary winding.
There is no information given on the individual power for each secondary,
although normally current ratings for each should be specified.

There is just not enough information to redesign the transformer, so duplicate exactly the original design.
Use the same or larger diameter magnet wire. Do not use smaller wire.
i kindly thank you for your responce
iam having trouble doing so.i just need to know the no. of tunrs per volt
can you consider answer to post #3 if possible

#### Ravindran

Well it wasn't the transformer was it?
i kindly thank you for your responce. and it is the transformer
If it's only rated at around 60VA though, it does mean that was quite a tiny HT amplifier.

Some HT amplifiers I can see in the Indian market indeed go down to just 5 or so TDA2030s doing all the power amplification. Now that's classy style entertainment on a budget
yes ,you are right about everything. it is a very old HT but quite huge with its subhoofer
you can see the production date marked 2010 on the post #1
i have been using it for past 3 years night and day (atleast 6hrs per day)

And by far the internet denies the existence of this HT.

#### Ian Finch

Paid Member
This is just something to be cautious about with toroidal transformers generally:
Each secondary winding is usually wound bifilar (i.e. as a twin pair of wires, possibly formed by folding a single wire in half). This is done mainly so that windings can be made with the shortest possible lengths of wire and because only half the number of turns will be necessary. Winding lots of turns of thick wire is not simple so winding shorter pairs can more easily be done in the one operation.

Check the amplifier's schematic if you have one, to see how your rectifiers and power supply capacitors should be connected. It's probably too late to remember how it was when you removed the transformer but this is important to get right, so you'd better at least try to get a copy of the correct schematic.

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#### JMFahey

i kindly thank you for your responce and causion measures.
i unwinded every secondaries and found no short in it ,
1) still not sure transformer is shorted, and you are confirming secondaries seem to be fine.
It mght have been overheated, yet still functional.

IF primary is untouched, you must test it:

* Connect primary to Mains, throgh a lamp bulb current limiter, I suggest a 50 W to 100W lamp.
A real red hot tungsten filament one, not LED/Fluo/CFL/etc.
Still available as "high efficiency" ones, showing an olive sized quartz lamp inside of a standard size glass bulb.

When switched ON, lamp must blink and then lower intensity, to red or red orange colour.
IF full bright, THEN primary is shorted.
Then junk transformer, rewinding is more expensive than getting a new one.

Suggest salvaging one from a dead amplifier, home theater, etc. ; in general PT is the LAST element to burn, not the first one as most assume.

the 15.5-0-15.5 secondary had around 166 turns which gives 5.35 turns per volt is this correct ? if this is correct i have to do like 1100+ tunrs for primary right?
And do you know how to calclate the number of turns with dimention of the windind area and respective volt
5.35 turns per volt sounds reasonable.

A quick "practical" calculation for 50Hz mains and standard iron, standing 10.000 Gauss (1 Tesla) induction is:
N0=45/S read as:"N sub zero" (turns per volt)=45 (dimensionless constant) divided by core area in squared centimeters

You have no space to "improve" that transformer, best you can do is rewind it as it was, you might/should use same wire if confirmed good (not burnt/scratched enamel).

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#### Ian Finch

Paid Member
Since the transformer is a toroidal type, It might also be good idea to wind the wire on a weaver's shuttle first, to make passing the wire through the donut a lot easier. A shuttle is a simple, one piece, traditional weaving implement which you can search on Google if you wish and then copy it, using just a scrap piece of wood . Otherwise, as recommended by JMF, buy a ready made 80VA toroidal transformer with standard 15VAC windings and add the lighter 24 and 3.6V windings later, when you have determined the turns required by testing a trial winding on that particular transformer.( For illustration only: https://www.amazon.com/weaving-shuttle/s?k=weaving+shuttle)

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#### Ravindran

Connect primary to Mains, throgh a lamp bulb current limiter, I suggest a 50 W to 100W lamp.
A real red hot tungsten filament one, not LED/Fluo/CFL/etc.
Still available as "high efficiency" ones, showing an olive sized quartz lamp inside of a standard size glass bulb.

When switched ON, lamp must blink and then lower intensity, to red or red orange colour.
IF full bright, THEN primary is shorted.
Then junk transformer, rewinding is more expensive than getting a new one.
this was the first method that came up when i google how to test toroidals and yes the bulb lit brightly(used Incandescent 60w)

rewinding is more expensive than getting a new one.
a transformer manufacturing company agreed to do the rewinding at rs.1750 INR (5507.00 ARS)considering they can salvage the secondaries,is this price genuine?

Suggest salvaging one from a dead amplifier, home theater, etc. ;
this seems like a nice i should try if i could find one with undamaged primary winding i may be able to utilize it.thanks

A quick "practical" calculation for 50Hz mains and standard iron, standing 10.000 Gauss (1 Tesla) induction is:
N0=45/S read as:"N sub zero" (turns per volt)=45 (dimensionless constant) divided by core area in squared centimeter
wait dimensionless constant 45 i thought its 42 (source -
) ,any how i have to do like 1000+ turns for primary

#### Ravindran

Each secondary winding is usually wound bifilar (i.e. as a twin pair of wires, possibly formed by folding a single wire in half). This is done mainly so that windings can be made with the shortest possible lengths of wire and because only half the number of turns will be necessary. Winding lots of turns of thick wire is not simple so winding shorter pairs can more easily be done in the one operation.
yes but only the 15.5-0-15.5 had a twin pair instead of center tapping they just winded a pair and connected ends to form neutral(0).other sec and pri are winded in unifilar way
can i do the primary in bifilar way ,is it alright to do?

#### Ravindran

Since the transformer is a toroidal type, It might also be good idea to wind the wire on a weaver's shuttle first, to make passing the wire through the donut a lot easier. A shuttle is a simple, one piece, traditional weaving implement which you can search on Google if you wish and then copy it, using just a scrap piece of wood . Otherwise, as recommended by JMF, buy a ready made 80VA toroidal transformer with standard 15VAC windings and add the lighter 24 and 3.6V windings later, when you have determined the turns required by testing a trial winding on that particular transformer.( For illustration only: https://www.amazon.com/weaving-shuttle/s?k=weaving+shuttle)
yeah i made a model of it using cardboad ,which barely able to withstand the 1mm on it.
buying a new 15vac also seems a good idea ,if i am unable to find any old usable toroids i may have to relay on this method thanks

#### Mr Evil

it is 67W transformer
Are you sure that's the power rating? Transformers are usually rated in VA, not W. It's hard to guage the size of it from the photos, but it looks too big for only 67VA.

#### Ian Finch

Paid Member
You could expect that VA rating to be adapted to what the the Indian market understands, or rather understood many years ago when that amplifier was built. The VA rating may simply not have been understood and accepted as it has been for many years now, in Europe and N. America. Also many years ago, I bought a 300VA toroid imported from Northern Ireland and the paperwork specified it as 300Watts which was the accepted format of small transformer specifications then.

Prior to the widespread use of toroidal transformers, E-I laminated transformers were often rated and specified in watts too, as I recall, though that was probably in error when it was really calculated as VA. The amplifier is also quite old and it takes time to change from the old ways, especially if there was no clear mandate on specs. formats from the governments involved.

You're right that the pics seem to show the transformer is a little oversize for ~70VA but then, we don't know the reasons behind that, such as a revision or adaptation to whatever cores and iron quality were available at the time.

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#### JMFahey

this was the first method that came up when i google how to test toroidals and yes the bulb lit brightly(used Incandescent 60w)
Did you test it with all secondaries disconnected?
If not, bright bulb in principle means shorted load (dead amplifier or supply components)
a transformer manufacturing company agreed to do the rewinding at rs.1750 INR (5507.00 ARS)considering they can salvage the secondaries,is this price genuine?
India, in a way similar to China, often shows prices significantly lower than most everywhere else, specially US/Europe/Japan, think there must be some reason for a Ton of companies subcontracting work there.
As an example, Famous Marshall guitar amplifiers were made in UK > Korea > India > Vietnam.
For some reason they seem to have skipped China, at least for the mainstream production.
So you tell me whether that money, equivalent to U\$22, is acceptable retribution for 1 day Technical work

Here in Argentina that job will cost at least twice as much, that IF they are interested in unwinding and rewinding that transformer, which is a tedious job.
EI bobbins are MUCH easier and faster..
wait dimensionless constant 45 i thought its 42 (source -[youtube])
SORRY!!!!!
I will buy a seven tail whip and lash myself 100 times for daring to disagree with the source of all knowledge.
What was I thinking?

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#### sawyers

Did the OP check continuity of the primary? The sticker shows a thermal fuse which appears to be 117C. If that has blown as a result of a circuit fault, the primary would be open circuit.

Did you check winding continuity with a meter before off winding it? Did you do any diagnosis at all? Did you find the thermal fuse when you off wound the entire transformer?

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