Adding a LV winding to a mains toroidal transformer

[Chris Wilson]

If I add a few turns of 14 swg enamelled wire to get 14v AC  from a new toroidal mains transformer that gives 2 x 35 volt AC outputs as standard, should I spread the windings around the core equally, or keep them close wound on just a section of the core? Thanks.

[amyk]

It shouldn't matter for voltage since the flux circulates in the whole core.

[Fank1]

Spread them out,

[Zero999]

I agree. Spread them out as the heat will dissipate more evenly.

[T3sl4co1l]

Spreading them out also gives lower leakage too.  If you're not drawing much current, neither this nor power dissipation will be a problem though.

Total VA is still limited by the primary wire gauge, of course.

Tim

[SeanB]

Wind evenly around the core, and then insulate with mylar tape (not PVC insulating tape, but silicone self amalgamating tape will do here) to keep them from shorting to the chassis. The mounting kit should have a big dished washer and 2 silicone rubber disks to isolate the transformer. Sleeve the leadouts with glass fibre sleeving to keep them from shorting.

[Chris Wilson]

Thanks Sean, will be doing this tonight, with luck, the transformer did come with dished washers and rubber insulators. I have had Kaptan tape arrived in the post this AM, to protect the new winding. Cheers.

[SeanB]

I rewound a toroid ( it had been cooked rather well by a shorted secondary) and converted it into a current transformer, using only the core and the plastic end caps, then making a new secondary of 12 turns of 2.5mm wire and then simply had a primary pass through it. Connected to a 5A AC ammeter and it did sterling duty to measure incoming mains current. It originally was used for a 50W downlighter, so no problem here of core saturation, it probably would have worked up to 1kA primary current, though I would have had a hard time getting the 200 turns of wire through the centre along with the 35mm primary wire.