Hi

We commonly see a flux band made of copper foil around coil of
transformer but I have not seen this kind of band used around chokes.
Should this kind of flux band be used around filter chokes also?
Thanks

"jeff kapp" wrote in message
ups.com...
Hi

We commonly see a flux band made of copper foil around coil of
transformer but I have not seen this kind of band used around
chokes.
Should this kind of flux band be used around filter chokes also?
Thanks

With transformers it's always around the coil AND core, i.e. outside
both, as a "shorted" turn. This cuts the external AC magnetic field.
If it is around the coil only inside the core it must be open circuit
(of course!). Then it would only provide an electrostatic shielding
effect.
I have never seen an outside copper band on chokes and I, too, don't
know why.
Also, I have never seen the shorted band around OPT's to cut any
notionally induced 60 Hz hum (already very small from my experiments
locating OPT's and power transformers.)
Cheers,
Roger

in article , Engineer at
wrote on 11/23/05 9:11 PM:

"jeff kapp" wrote in message
ups.com...
Hi

We commonly see a flux band made of copper foil around coil of
transformer but I have not seen this kind of band used around
chokes.
Should this kind of flux band be used around filter chokes also?
Thanks

With transformers it's always around the coil AND core, i.e. outside
both, as a "shorted" turn. This cuts the external AC magnetic field.
If it is around the coil only inside the core it must be open circuit
(of course!). Then it would only provide an electrostatic shielding
effect.
I have never seen an outside copper band on chokes and I, too, don't
know why.
Also, I have never seen the shorted band around OPT's to cut any
notionally induced 60 Hz hum (already very small from my experiments
locating OPT's and power transformers.)
Cheers,
Roger



One would expect the change in flux to be much less in a choke vs. A/C
transformer; perhaps that is why no bands are used?

Jon Yaeger wrote:

in article , Engineer at
wrote on 11/23/05 9:11 PM:

"jeff kapp" wrote in message
ups.com...
Hi

We commonly see a flux band made of copper foil around coil of
transformer but I have not seen this kind of band used around
chokes.
Should this kind of flux band be used around filter chokes also?
Thanks

With transformers it's always around the coil AND core, i.e. outside
both, as a "shorted" turn. This cuts the external AC magnetic field.
If it is around the coil only inside the core it must be open circuit
(of course!). Then it would only provide an electrostatic shielding
effect.
I have never seen an outside copper band on chokes and I, too, don't
know why.
Also, I have never seen the shorted band around OPT's to cut any
notionally induced 60 Hz hum (already very small from my experiments
locating OPT's and power transformers.)
Cheers,
Roger



One would expect the change in flux to be much less in a choke vs. A/C
transformer; perhaps that is why no bands are used?

Most chokes we are considering are dc carrying filter chokes, and indeed
the
ac flux isn't large because the ac is relatively low compared to a power
tranny.

But consider the choke used in a choke input type of rectifier;
the amount of ac voltage across the choke is huge, so the flux must be
high.
I normally pot such items in steel boxes with tar because they are usually
noisy.
The box also zaps the flux.

Patrick Turner.

"Patrick Turner" wrote in message
...


Jon Yaeger wrote:

in article , Engineer at
wrote on 11/23/05 9:11 PM:

"jeff kapp" wrote in message
ups.com...
Hi

We commonly see a flux band made of copper foil around coil of
transformer but I have not seen this kind of band used around
chokes.
Should this kind of flux band be used around filter chokes also?
Thanks

With transformers it's always around the coil AND core, i.e.
outside
both, as a "shorted" turn.

(snip)


One would expect the change in flux to be much less in a choke vs.
A/C
transformer; perhaps that is why no bands are used?

Most chokes we are considering are dc carrying filter chokes, and
indeed
the
ac flux isn't large because the ac is relatively low compared to a
power
tranny.

But consider the choke used in a choke input type of rectifier;
the amount of ac voltage across the choke is huge, so the flux must
be
high.
I normally pot such items in steel boxes with tar because they are
usually
noisy. The box also zaps the flux.

Patrick Turner.

Good point about the choke input flux, Patrick. Not only is it large
but very rich in harmonics - the higher ones might more easily get
onto the low signal stages. I am very tempted to wind a few turns of
thick (bare or enameled, it doesn't matter which) copper wire around
the outside of the choke and short the ends. It might be fun and
instructive to monitor the amp's S/N ratio (with NFB disabled) while
opening and closing this wire short (for enameled wire only.) If
significant, it would be a very cheap fix.
Cheers,
Roger