Wire size !

MaxHeadRoom

Joined Jul 18, 2013
28,619
So as I understand it, you have q primary core with the HV secondary wound on top, you also have a second bobbin alongside the first with the other two secondary's??
 

Thread Starter

spike1947

Joined Feb 4, 2016
496
Firstly: Well have found the initail fault of the transformer, after starting to unwind the Primary winding, after the first layer there is a micro thermal fuse laid underneath the wrapping of the second layer, has a open circuit on it .
Secondly: Have not drawn the schematic correctly, there is another winding continuing from P3 (Neautral) to the (Thermal fuse) then the fuse conects back to P3 ( N) so weird for me , I don't really understand that, I thought it would of been between P3 !.

Not sure I have drawn this correctly though !.

Cheers
Spike
 

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ElectricSpidey

Joined Dec 2, 2017
2,758
looking at your diagram, I'm kinda doubting a fuse would be in parallel with a winding.

EDIT:
JFTR the post/image this post was referring to has been deleted.
 
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Thread Starter

spike1947

Joined Feb 4, 2016
496
Sorry Guys another mistake , P3 does not have a loop, amended drawing attached, anyway I just need another thermal fuse to sort that bit out, but I need to know if a 0.2mm dia wire is equivelant mm2 to 9 x 0.06mm ( bundled) wires .
 

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spike1947

Joined Feb 4, 2016
496
Yes - near enough - 9x0.06mm is the same as 1x0.18mm

By the way, in your diagram, which windings were on which section of the bobbin?
What did you get for the mm2 .?

First Bobin: 240v Primary winding P1,P2 and P3, and 400v Secondary winding S3.
Secondary Bobin: 6v Secondary winding and 12v Secondary winding .

cheers
Spike
 

MisterBill2

Joined Jan 23, 2018
18,176
I suggest rewinding the transformer without a thermal fuse, unless you enjoy unwinding and rewinding transformers. If you can calculate the power from each winding and then calculate the total power you will be ble to determine the full load current of the transformer, and put a fuse with that rating in series with the primary. That will provide the protection that the thermal fuse was intended to provide and be much more easily replaced. Because it was not intended to be replaced, it is clear that the thermal fuse was not intended to protect the transformer, but rather to prevent a fire if a wrong fuse were used after the correct fuse failed. So using an external fuse of the correct rating will be at least as safe. There is no safety benefit at all from putting in a cheap thermal fuse in the windings.

You have discovered that the fuse failed but the winding was not defective, but it failed, and the transformer was not replacable for any price, while nothing else had failed. So the internal thermal fuse is a cheap substitute for doing the design correctly.
 

Thread Starter

spike1947

Joined Feb 4, 2016
496
I suggest rewinding the transformer without a thermal fuse, unless you enjoy unwinding and rewinding transformers. If you can calculate the power from each winding and then calculate the total power you will be ble to determine the full load current of the transformer, and put a fuse with that rating in series with the primary. That will provide the protection that the thermal fuse was intended to provide and be much more easily replaced. Because it was not intended to be replaced, it is clear that the thermal fuse was not intended to protect the transformer, but rather to prevent a fire if a wrong fuse were used after the correct fuse failed. So using an external fuse of the correct rating will be at least as safe. There is no safety benefit at all from putting in a cheap thermal fuse in the windings.

You have discovered that the fuse failed but the winding was not defective, but it failed, and the transformer was not replacable for any price, while nothing else had failed. So the internal thermal fuse is a cheap substitute for doing the design correctly.
Yes I was thinking of not replacing the internal one, it is a thermal fuse though ( 115Deg )!, so might fit one taped to the top and wrapped.
Thanks guys for all you inputs on this, you never stop learning and I love a bit of a challenge .

Spike
 

MisterBill2

Joined Jan 23, 2018
18,176
Yes I was thinking of not replacing the internal one, it is a thermal fuse though ( 115Deg )!, so might fit one taped to the top and wrapped.
Thanks guys for all you inputs on this, you never stop learning and I love a bit of a challenge .

Spike
I am very impressed with the ability to unwind a transformer in a way that will allow it to be rewound with the same wire. That requires a great deal of motivation and a fair amount of skill..
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
Yes I was thinking of not replacing the internal one, it is a thermal fuse though ( 115Deg )!, so might fit one taped to the top and wrapped.
Thanks guys for all you inputs on this, you never stop learning and I love a bit of a challenge .
I detest the embedded thermal fuse in a transformer, it usually saves any over-current/overheat but the transformer is lost, I prefer the replaceable inline fuse of a suitable value.
 

Thread Starter

spike1947

Joined Feb 4, 2016
496
Just one more thing, I can't understand that if the second winding over the top of the Primary winding as a output of 420v, but it has only 175 turns of ( 9x0.06mm) wire, and only the first layer of the Primary has 425, so can just guess that the Primary has nearly 900 turns, I thought a step up would be that the secondary would have prob twice that of the primary !.

Spike
 
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