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turning transformer into LF choke

Microwave Oven Transformers (MOT's)

- have a high voltage secondary winding, rated at around 2800V and .5A, with resistance well under 150 ohm

- have the sides of the I-section of the core line-welded to the E-section, not meshed with it, and therefore relatively easy to split off by using a hack saw on one side.

I long wanted to experiment with turning transformers into LF chokes for vacuum tube power supplies.

MOT's seem particularly apt and available candidates, but the following considerations should apply to any transformer.

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I plan on

- putting all windings in series (in-phase of course)

- splitting off I-sections to reduce saturation as DC flows

- adding magnetic shielding in the form of a thick ferrous alloy shield belt coplananar with the winding, and extending well beyond its height.

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Things I have doubts about:

- should the core I-section be removed or left in place but insulated and distanced from the E-section? (I saw both options mentioned)

- is the core supposed to be insulated from the chassis? and from the magnetic shield?

- is there an advantage in point-grounding core and shield to chassis, as opposed to bonding them more broadly?

- how do I close the loop in a ferrous alloy mag shield belt? bolt it? spot weld it? line weld it? or are overlapping layers enough?

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I hope in wise advice.

[x-posted onto sci.electronics.design & rec.radio.amateur.boatanchors]
Reply to
spamhog
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OK, so far, albeit I'd think it'd be more fun to leave the two windings independent - you get at least a half-dozen additional ways to experiment. ;-)

Use sheet copper for this, as wide as the winding window, all the way around the whole thing. This provides a shorted turn on the outside of everything, containing the flux leakage.

Hack- or chop-saw them off, then put a layer of thin mylar tape across the parts where they butt together, then put it back together with clamps or so.

No. Ground the core to the chassis; let the magnetic sheild float, but don't worry if it shorts to the core here and there; it doesn't carry any significant current. On second thought, ground it to short out any extraneous components of the signal.

Just ground everything to chassis - the magnetic flux shouldn't have anything to do with your signal/return paths.

Don't use ferrous alloy for that - use copper, and just solder it.

I hope so too. ;-)

Have Fun! Rich

Reply to
Rich Grise

Actually... it seems there are other, massively better materials. I haven't understood much, and I'm shocked by my own ignorance, but it looks like high permeability materials reroute magnetic field lines through and around themselves. Cu and Al don't work that way. Iron and steel have 200 times the permeability of either Cu or Al, and it requires a good, large-surface bond for continuity - so it's riveting, bolting, folding tight or smtg of that ilk.

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Reply to
spamhog

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