Re: Current Limiting and Impedence

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

Hi Paul,

On 13 May 2005, at 11:53, Tesla list wrote:

> Original poster: "Paul B. Brodie" <pbbrodie@xxxxxxxxxxxxx>
>
> Malcolm,
> You mention N27 ferrites but the ferrites I have access to are surplus
> and completely unmarked and of unknown origin. The good thing is that
> I have a bucketload of them that have accumulated from various sources
> over many years. I believe that most of them are ferrite beads that
> were on power cords. Would these work in use as you suggested, crushed
> up in a PVC tube? I like this idea as I have ready access to so many
> ferrite beads. Since it sounds so good, I just know you are going to
> tell me that they are completely different from the ferrite used for
> toroids and transformers and therefore useless for this application.
> {:-( Regards. Paul Think Positive

Indeed. Just about anything which reduces the amount of copper you
have to use in the choke is good. My advice to anyone who has
suspicions as to whether something is good or bad is to do what We
all had to do in the good old days before the internet - suck it and
see. Then it was all about ideas and trying them out, working (in my
case) in isolation from the rest of the world. In your case, build a
core, wind 30 or 40 turns over the length you would like to wind a
choke, measure its inductance and derive its Al value so you can
design you inductor. You can slide the core in and out of the coil to
vary the inductance so the choke should be designed such that with
the core fully in, it has at least a bit more inductance than you
will need in use.

Malcolm

> ----- Original Message -----
> From: "Tesla list" <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
> To: <<mailto:tesla@xxxxxxxxxx>tesla@xxxxxxxxxx>
> Sent: Thursday, May 12, 2005 4:46 PM
> Subject: Re: Current Limiting and Impedence
>
>  > Original poster: "Malcolm Watts"
> <<mailto:m.j.watts@xxxxxxxxxxxx>m.j.watts@xxxxxxxxxxxx>
>  >
>  > On 12 May 2005, at 10:27, Tesla list wrote:
>  >
>  > > Original poster: "Gerald  Reynolds"
> <<mailto:gerryreynolds@xxxxxxxxxxxxx>gerryreynolds@xxxxxxxxxxxxx>
>  > >
>  > > Hi Malcolm,
>  > >
>  > > Where can you get Silicon steel?? Do they make welding rods out
>  of > > this stuff or is there another alloy that would work?? and
>  what > > diameter rod should we be looking for?? > > > > Gerry R. > >
>  Hi Gerry, >           Silicon steel is the material used in
>  transformer > laminations. I can't suggest any shortcuts to getting
>  it any other > way sorry. I'm suggesting making the core using
>  discarded transformer > laminations. You just want whatever material
>  you are using to have as > small a cross-sectional area as small
>  possible to minimize eddy > currents. A tale was recounted some years
>  ago of someone who made a > ballast core from varnished welding rods
>  which got so hot that the > varnish melted. It is questionable
>  whether low-frequency type > transformer iron is really that suited
>  to the job anyway as the gap > is going to throw step functions at
>  it. A PVC tube full of smashed up > N27 ferrites would be a better
>  option. I did this once and built two > such cores. > > Malcolm > > >
>  > > > > >Original poster: "Malcolm Watts"
> <<mailto:m.j.watts@xxxxxxxxxxxx>m.j.watts@xxxxxxxxxxxx>
>  > > >
>  > > >Hi Paul,
>  > > >           Whatever rods you purchase for the core, they have to
>  have > > >a very small x-sectional area. They should also not retain
>  much if > > >any magnetism after being de-energized (check with a
>  magnet) or they > > >will have large hysteresis losses resulting in
>  lots of heating. > > >Silicon steel such as used in transformer cores
>  is preferred if you > > >can get them. > > > > > >Malcolm > > > > > >
>  > > > > > >
>
>
>