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turning transformer into LF choke

S

spamhog

Jan 1, 1970
0
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.

--------------------------------------------------------------------

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.

--------------------------------------------------------------------

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?
 
E

Eeyore

Jan 1, 1970
0
spamhog said:
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.

I don't know about MOTs, but most transformers (except flyback which are
really chokes with 2 windings) don't have an air gap. You'll need an air
gap for a choke, so I'd forget it.

Graham
 
S

spamhog

Jan 1, 1970
0
Thank you Graham! :)
Please read again at least these lines of my message:
[MOT's]
- 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.
[...]
- should the core I-section be removed
or left in place but insulated and distanced
from the E-section? (I saw both options mentioned)
<<

These quotes refer to
- MOTs NOT having an air gap to start with
- the EASE of separating the two sections
of the core of a MOT, as to introduce the air gap
- my question as to whether the air gap should be
small or large but FINITE, or infinite,
i.e. with the I-section removed.

Were you writing about THAT air gap? If you know more than me about it
(as I hope) would you care to elaborate? :)

And, any thoughts about shielding?

TIA,

Filippo



most transformers [...] don't have an air gap.
You'll need an air gap for a choke, so I'd forget it.
 
E

Eeyore

Jan 1, 1970
0
spamhog said:
Thank you Graham! :)
Please read again at least these lines of my message:
[MOT's]
- 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.
[...]
- should the core I-section be removed
or left in place but insulated and distanced
from the E-section? (I saw both options mentioned)
<<

These quotes refer to
- MOTs NOT having an air gap to start with
- the EASE of separating the two sections
of the core of a MOT, as to introduce the air gap
- my question as to whether the air gap should be
small or large but FINITE, or infinite,
i.e. with the I-section removed.

Were you writing about THAT air gap? If you know more than me about it
(as I hope) would you care to elaborate? :)

Well if you can easily make an air gap you're in business.

And, any thoughts about shielding?

Shielding from what ? The stray field will be greatest in the vicinity of
the air gap.

Graham
 
R

Rich Grise

Jan 1, 1970
0
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.

I plan on
- putting all windings in series (in-phase of course)
- splitting off I-sections to reduce saturation as DC flows

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

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.
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)

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

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

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

Don't use ferrous alloy for that - use copper, and just solder it.
I hope in wise advice.

I hope so too. ;-)

Have Fun!
Rich
 
G

George Herold

Jan 1, 1970
0
I don't know about MOTs, but most transformers (except flyback which are
really chokes with 2 windings) don't have an air gap. You'll need an air
gap for a choke, so I'd forget it.

Graham- Hide quoted text -

- Show quoted text -

"You'll need an air
gap for a choke"

Wow!!! I didn't know that. What if I close one side with a resistor?
There's still some magnetic field to store energy, (though without a
gap all the fields are wimpy fringing fields.) Hmm, does this mean it
might be easier to calculate(estimate) inductance in terms of "volume"
of field strength rather than number of turns times some area. I'm
thinking of Helmholtz coils which is the inductance I'm most
interested in at the moment, but I assume it could apply to other
configurations.

George
 
S

spamhog

Jan 1, 1970
0
Don't use ferrous alloy for that - use copper, and just solder it.

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.

-----> http://tinyurl.com/3kyr7v
 
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