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Old 21st February 2013, 11:09 PM   #1
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Default Toroidal transformer unpackaging and mounting

I purchased a pair of barely used toroids. There is a 1/2" strip of acetate or vinyl wrapped around the transformer windings. This film strip is falling off and is loose on both so it is deceiving to me whether it should be left intact and secured or if it should be removed. I have never purchased toroids previous to these so I have no experience with them. Mounts were not included so I could really use some tips with mounting as well.
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Old 21st February 2013, 11:53 PM   #2
tomchr is offline tomchr  Canada
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You can see pictures of what a toroidal transformer is supposed to look like here:
Turning a 6.3 V secondary into a 5 V secondary on an Antek 2T350

There is normally a plastic film that is wound around the transformer in the same fashion as the windings. This is a protective film and should remain intact. If it's falling off, I'd wind it back on and secure it with some Kapton tape (or electrical tape if Kapton isn't available and I'm using the transformer for my own hobby purposes).

On smaller toroidal transformers, the center hole is normally filled with epoxy and there's a hole for a mounting bolt through the epoxy. In this case, just get a bolt/screw of the appropriate size and mount the transformer to the chassis. Use a rubber pad between the transformer and the chassis so the windings don't get crushed. I've used pieces of bicycle innertubes for this, but normally the rubber pad is included with the transformer. For bigger transformers, an actual mounting kit is needed. This consists of two rubber pads, a formed metal washer, and bolt+nut+lock washer. You can see examples here: Mounting Washer and Pads for Toroidal Transformers and here: AXXENT - Transformer Accessories - Mounting Kit

One note: Be careful with the length of the mounting bolt/screw. Ensure that no part of the chassis ever touches the end of the mounting bolt that's away from the chassis as this will form a shorted turn (i.e. a short circuit on the secondary).

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Last edited by tomchr; 21st February 2013 at 11:57 PM.
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Old 4th September 2013, 12:04 AM   #3
JdAo is offline JdAo  Portugal
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"One note: Be careful with the length of the mounting bolt/screw. Ensure that no part of the chassis ever touches the end of the mounting bolt that's away from the chassis as this will form a shorted turn (i.e. a short circuit on the secondary)."

Is that in case the insulation breaks ? If not can you explain why ? I mean, aren't most transformers isolated, my Nuvotem has a black coating all over it with a place for a screw in the center, is there still a risk in this case ? If the bolt is not in contact with the inner winding I really don't understand...
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Old 4th September 2013, 08:08 AM   #4
John8 is offline John8  United Kingdom
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The fixing bolt is in contact with the chassis at the bottom of the transformer so if the top of the bolt makes electrical contact with the chassis this constitutes a shorted turn.
Yes it is totally isolated from the other windings but it still goes through the centre of the core and the turn is completed and shorted by the chassis if the bolt is connected in any way to the chassis at the top.
There is no problem with a fixing bolt that screws into a nut in the centre of the transformer but large transformers tend to have a clamping plate and rubber washers.
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Old 4th September 2013, 02:43 PM   #5
Minion is offline Minion  Canada
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Just to expand on when John said ... The transformer puts off a magnetic field and any metal that is in the field that makes a full turn of the transformer (ie: the mounting bolt and chassis ) will cause voltage to develop on that metal , to avoid this you must not let the top of the mounting bolt make contact with metal chassis ....

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Old 5th September 2013, 11:24 AM   #6
AndrewT is offline AndrewT  Scotland
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I usually insulate the top of that bolt with 6 layers of sticky tape.
I then insulate the whole top plate and bolt head with a further sticky tape layer.
Then I add on a second sticky tape layer at right angles to the previous.
I am confident that the metal lid will never break through all that insulation and create an accidental shorted turn.
regards Andrew T.
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