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Fused switching transformer ?

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Nigel Goodwin

Super Moderator
Most Helpful Member
hi ppl.:woot:

Does anyone knows something about BCK switching transformers like this one, with built in fuses?

In the pdf PSU circuit diagram, theres no fuse, but another coil in series with TB901 primary neither.

What do u think?
While it's not impossible, it's unlikely to have an internal fuse - I don't recall ever seeing one in a SM transformer?.


Well-Known Member
Most Helpful Member
Never seen a fuse in a SMPS transformer. Really would not make sense because the switching transistor/mosfet driving it would disintegrate long before the windings would heat up in a fault situation.

Linear transformer.....yes thermal fuse for protection pretty common.


New Member
This SMPS belongs to a led tv that I have repaired one year ago n had its power mosfet and
oscillator IC not working back then.

Now it was struck by a lightning, and everything works ok in the power supply, but the switching transformer.

Between the pin marked as Drain, n the one in the middle, theres a coil actually .

Already has fed the PS with 220vac without any load on the output side, and it works, but i m scared
to turn it on with full load on the output.
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New Member
TV turns on with its SMPS straight to 220vac, RCA logo shows up on the screen , +24vcc goes down to around 20vcc and then the tv shuts itself off.

SM transformer gets a lil hot and smells like enamel so I guess some turns r shorted inside or may be the supposed fuse were turns indeed.

Hard to understand why the mosfet did not blow :nailbiting:

May be I can try to open it and looks how its inside....
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New Member
Customer rejected my U$ 110 full invoice quotation, so there will be no SM transformer opening to look inside.

U$ 83 for a new power supply, and U$27 for the job.

This is ARG, how much would it cost to install a new PS in your country?


New Member
Client changed his mind, so I have opened the switching transformer and found
an open coil where I guessed that a fuse was located.:wideyed:

Now i can see that the primary coil is made of two halves.

The first half of the primary winding that was open. Then one secondary. Then the other half of
the primary.

Is that technique used to reduce the leakage inductance to control voltage spikes?

May i destroy the mosfet if i rebuild the primary without splitting it?
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New Member
Ok keepitsimple...Thanks for your help.

I have already rebuilt the open coils inside the transformer in the common fashion
(no interleaved primary) and the tv is working ok it seems...

No noise or overheating in the power supply.

I will keep it running for a couple of days to see how it goes.

Thanks a lot to all the helpers.:smug:


Active Member
I think what you've described is a primary in two "sections", rather than a bifilar winding, as such.
Winding "sections" are usually wound as complete layers on the former, interleaved with sections of secondary - often a layer (or several) of primary on the inside, a layer of tape, a layer of secondary, more tape, another layer of primary, more tape, and then a final secondary layer on the outside. The different sections are generally brought out to the pins and connected in series.
In contast a bifilar (or multifilar) winding has several wires electrically and physically in parallel, so that there is no voltage between them. They are just insulated by the enammel coating.
You can even have multiple multi-filar section, if you want!

You're right that the multi-section winding is done to improve the coupling between primary and secondary. As I understand, both these techniques relate to avoiding the problems of skin effect, so are really two sides of the same coin.

There's more than you ever wanted to know about the subject here, in the Magnetics Design Handbook:

Personally I would have rebuilt the transformer as original (with 2 sections), but it looks like you've been sucessful, so well done!
As far as I can see, if it manages to supply the full load with good regulation and without overheating, then it's working perfectly well. I hope it's still working.


New Member
Personally I would have rebuilt the transformer as original (with 2 sections)

Me too, but I was afraid to mix the primary that runs at 400vcc, with the secondaries
of 12v and 24v.

I am keeping an eye on the transformer temperature, and I see no problems, so far....

Thanks for your help.
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Active Member
Strange fuses in transformer? might be this.

Transformers, mostly iron core than others, well actually can be in any transformer by safety rating, these.
PDF is 2.6 MB in size.

: edit : The core I was referring to as familiar was an iron core that I've seen burn around its housing, that one you have would be ferrite material.

Transformer temperatures can be exceeded by drawing overload current, some can get hot enough when overdrawn to burn wood when placed onto when in this example an iron core would be overheated, and can take quite some time to cool down as well. If an internal winding does not burn and break the winding circuit first before getting to such a thermal mass.

This is but one of many brands.
http://cci-tco.com/wp-content/uploads/product-selection/pdf/NEC SEFUSE 2014 CATALOG.pdf

Noted that the wire had failed in the switching transformer, just if encountering a transformer with a fuse, the above.
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