Resurrecting an old chum

[Wakefield Turntables]

Oooo you rotten so and so I was really enjoying your w write up

[mikmas]

Oooo you rotten so and so I was really enjoying your w write up

oops …sorry Andrew - didn’t realise I’d created an Eastenders worthy cliffhanger

Anyhoo - to continue …

For those of you unfamiliar with toroidal transformers, what you have is a donut shaped thing with loads of wire wrapped around the tasty bit, some big washers, and a long bolt running through the hole to fasten it down to the casing. Really simple mount then?… only problem is that it is a potential minefield with actual explosive potential (kind of)

Basically this:

“Mounting: Toroidal transformers usually come with a long bolt, nut and a disk for mounting in the case (sometimes with extra rubber disks). Be aware of a metal case that the top of the bolt does not touch the top of your case. Since this bolt is already touching the bottom, if it touches the top too then via the rest of the case will effectively make a shorted turn around the transformer core itself, and damage will result.”

Source: https://electro-dan.co.uk/Electronics/wiringtrans.aspx

So my problem was that I had a situation where if anything inadvertently ended up being put on top of the amp it could flex the top enough to touch the bolt and cause the end of the f**king world (or summat worse …. a blow fuse for instance*)

As the amp was destined to end up in the shed, the chances of this happening could be rated at about 110% certainly, every time

*see this thread for one of many examples

So a genuine problem looking for a genuine solution.


hmmm, I thought, and more besides ….. for quite a while.

[Barry]

Two possible solutions:

[1] use a nylon bolt to secure the toroid, or

[2] glue a piece of insulating material on the lid over where the toroid sits.

I did the latter on a Cambridge P50 slimline amp (Cambridge Audio were one of the first manufacturers to use toroidal transformers.)

[Wakefield Turntables]

Nylon bolt is the way to go imho.

[mikmas]

After sleeping on this for a night or two, I narrowed the solution down to one of the following options:

1. Suspend the laws of physics indefinitely
2. Bin the lot and move on.
3. Ignore the problem - it may never happen
4. Buy a bigger box and start again
5. Isolate the bolt from the chassis securely


Obviously option 5 was the most appealing and/or perfectly feasible choice ...

I did consider the two options suggested by Barry above but rejected both ... the nylon bolt because I hate them and sticking plastic to the top because of the vents, which I wanted to keep above the toroid, and the messiness.
The actual implemented solution was simple, secure and just as effective:

The bolt is an M5 so I drilled out the mounting hole to 7 mm and inserted a rubber grommet; on top of this a square of plastic with a hole to clear the grommet and a penny washer to restrain the bolt head thus:



After mounting the toroid I checked for continuity with chassis earth = zero, zilch, nada.
Nevertheless, I added a couple of layers of insulating tape to the toroid top plate 'just in case' ...

Job done - another good night's rest earned

[Light Dependant Resistor]

oops …sorry Andrew - didn’t realise I’d created an Eastenders worthy cliffhanger

Anyhoo - to continue …

For those of you unfamiliar with toroidal transformers, what you have is a donut shaped thing with loads of wire wrapped around the tasty bit, some big washers, and a long bolt running through the hole to fasten it down to the casing. Really simple mount then?… only problem is that it is a potential minefield with actual explosive potential (kind of)

Basically this:

“Mounting: Toroidal transformers usually come with a long bolt, nut and a disk for mounting in the case (sometimes with extra rubber disks). Be aware of a metal case that the top of the bolt does not touch the top of your case. Since this bolt is already touching the bottom, if it touches the top too then via the rest of the case will effectively make a shorted turn around the transformer core itself, and damage will result.”

Source: https://electro-dan.co.uk/Electronics/wiringtrans.aspx

So my problem was that I had a situation where if anything inadvertently ended up being put on top of the amp it could flex the top enough to touch the bolt and cause the end of the f**king world (or summat worse …. a blow fuse for instance*)

As the amp was destined to end up in the shed, the chances of this happening could be rated at about 110% certainly, every time

*see this thread for one of many examples

So a genuine problem looking for a genuine solution.


hmmm, I thought, and more besides ….. for quite a while.

Toroidal transformers are quite safe to use, when the correct mounting hardware is used. A rubber washer a excellent insulator is mounted either side of the insulated windings
of the transformer Indeed a bolt connects the chassis through the centre, but it does not touch the transformer itself. the two insulators being air by considerable gap and rubber.

I have never known of any such failure of a toroidal transformer.

[Barry]

An alternative method of mounting, which would eliminate the risk of a possible short circuited turn, would be to use three or four heavy duty nylon tie-wraps passed through the centre of the toroid and corresponding holes drilled in the base of the amplifier case.

[mikmas]

Toroidal transformers are quite safe to use, when the correct mounting hardware is used. A rubber washer a excellent insulator is mounted either side of the insulated windings
of the transformer Indeed a bolt connects the chassis through the centre, but it does not touch the transformer itself. the two insulators being air by considerable gap and rubber.

I have never known of any such failure of a toroidal transformer.

The rubber washers are there to protect the coils from contact/pressure with hard mounting materials - they will not prevent a shorted turn:

From a toroidal manufacturer (there are plenty of such warnings online)

PRECAUTIONARY NOTES
SHORTED TURN CONDITION
A completed path by any conductor passing through the center of toroidal transformer, around the outside is a shorted turn (eg: the top of the mounting bolt shorted to the chassis). As with any short circuit, this condition will give rise to high circulating currents, and high heat. The transformer may be damaged beyond repair.

https://www.plitron.com/news/technical-notes/

[Light Dependant Resistor]

The rubber washers are there to protect the coils from contact/pressure with hard mounting materials - they will not prevent a shorted turn:

From a toroidal manufacturer (there are plenty of such warnings online)

PRECAUTIONARY NOTES
SHORTED TURN CONDITION
A completed path by any conductor passing through the center of toroidal transformer, around the outside is a shorted turn (eg: the top of the mounting bolt shorted to the chassis). As with any short circuit, this condition will give rise to high circulating currents, and high heat. The transformer may be damaged beyond repair.

https://www.plitron.com/news/technical-notes/

What is being attributed as a shorted turn poorly worded , is a condition of particularly small toroid transformers to exhibit a change to their flux density. Where with no loading
they indeed can see the middle bolt attaching as a means to heat. This is quite distinct from what is suggested by the shorted turns wording which should be
reworded correctly as flux density issues.

The shorted turn suggests to any reader a failure of the internal winding of the transformer shorting- and a reader jumps to the advice given
as seeing the attaching bolt as causing this, which is quite wrong, due to insulation.

The guidance for any mounting is to keep any field capable of conduction at least 5mm away. see creepage clearance https://sound-au.com/articles/mains-safety.htm#s5
and discussion of insufficient loading on particularly small toroids here: "The flux density in a transformer is greatest at no load, and it decreases as load is increased"
https://sound-au.com/xfmr.htm#s4

[mikmas]

The guidance for any mounting is to keep any field capable of conduction at least 5mm away. see creepage clearance https://sound-au.com/articles/mains-safety.htm#s5
and discussion of insufficient loading on particularly small toroids here: "The flux density in a transformer is greatest at no load, and it decreases as load is increased"
https://sound-au.com/xfmr.htm#s4

Fascinating stuff mate ... found this bit particularly gripping - had me right on the edge of my seat .....

"Weber (Wb) - magnetic flux. 'Flux' is the rate (per unit of time) in which something crosses a surface perpendicular to the flow. In the case of a magnetic field, then the magnetic flux across a perpendicular surface is the product of the magnetic flux density, in Teslas, and the surface area, in square metres. If a varying magnetic field passes perpendicularly through a circular loop of conducting material (one turn), the variation in the field induces a electric potential in the loop. If the flux is changing at a uniform rate of one Weber per second, the induced potential is one volt. This means that numerically the flux in webers is equal to the potential, in volts, that would be created by collapsing the field uniformly to zero in one second. One Weber is the flux induced in this way by a current varying at the uniform rate of one ampere per second. The unit honours the German physicist Wilhelm Eduard Weber (1804-1891), one of the early researchers of magnetism."

....totally absorbing