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240v toroidal isolation transformer with Pri\Sec Gnd shield

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killingtime:
Hi,

I'm looking for a 240v toroidal isolation transformer (200va) that has an electrostatic screen between the primary and secondary, to reduce noise coupling from the mains side.

Toroidal mains transformers have a high primary-secondary capacitance due to the tightly coupled windings, so any high frequency disturbances on the mains side tend to get coupled to the secondary side quite effectively unless the shield is in place.

I've looked on RS, farnells, Rapid, CPC (UK) and cannot find any manufacturer that offers this feature with their transformers. Hammond does but only on their 115v product line.

Any suggestions?

I could 'roll my own' I suppose by taking a new Xmr apart if all else fails. Is the primary winding on the inside or outside with toroidal transformers? If on the inside, I could just pick any 200va Xmr with a 240v primary and dump the secondaries before winding my own over the primary. Easier to buy one though if they exist.

Thanks.

Gyro:
The only ones I've found have been salvaged from defunct test equipment, but they're smaller.  I don't think they are available from any of the main distributors. You'd probably need to go straight to a transformer manufacturer, some of them do custom ones.

The secondary is (I think) universally on the outside. It is very possible, if tedious, to rewind the secondary without compromising primary - secondary insulation.


EDIT: The other possibility would be to implement external noise filtering. A filtered mains inlet and maybe high frequency capacitors to chassis and series inductors on the secondary too. You ought to be able to swamp the inter-winding capacitance by at least a couple of orders of magnitude without significant losses.

edavid:
toroid.com says that their medical isolation transformers incorporate a "static shield", although they don't give details.

BTW it's "toroidal", not "toroidial".

killingtime:
After some further digging, I've found one retailer that has UK stock. Digikey.

There are manufacturers in the UK that will make you a custom Xmr, but expect to pay many times normal price.

The key search term is 'Medical' in the part description, or 'Med' / 'MD' in the Mfr product code. Talema Group LLC, and Triad Magnetics both offer a line with an earthed interwinding shield. Price is about 50% more than the cheapest toroidal Xmr but if it meets medical standards then I'd expect the quality to be a little better...

I did look at non-medical E core split bobbin Xmrs, much cheaper but the regulation for a 200va unit was specified at 25%. On a 245v supply (which I get) that's over 306v unloaded, assuming 245v at full load (1:1 isolation) . Many universal input switch mode PSUs top out well before that. The medical grade toroidal Xmrs were specified at ~7% which yields ~260v unloaded.

Regards.

Ian.M:
It may be worth considering using two identical 115V ones from Hammond wired in series.

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