Re: Current Limiting and Impedence

["Tesla list" <tesla@xxxxxxxxxx>]

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

Hi Gerry,

On 14 May 2005, at 1:12, Tesla list wrote:

> Original poster: "Gerald  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Malcolm,
>
> My CRC chemistry and physics book shows the following properties of
> silicon iron:
>
> mu = 500 @ 20 gauss
> mu_max = 7000
> Bsat = 19700 gauss
> Wh = 3500 ergs/cm^2  @ saturation (hysteresis loss)
>
> Interesting to compare to insulated powder - 2-81 permalloy (I assume
> your N27 ferrites are in this catagory???)
>
> mu = 125 @ 20 gauss
> mu_max = 130
> Bsat = 8000 gauss
> Wh = ---  don't know what this means, maybe negligible and if so =>
> little or no core loss??

Bsat is more like 500mT (== 5000 Gauss?) for N27 (3C85) material. The
material exhibits far lower losses than iron in hard switching use up
to about 100kHz. It is most commonly used in the older SMPS. The
newer ones switching in the 300kHz+ range are most likely 3Fx type
material.

> The smaller mu might be good if one wants a small inductance but the
> lower B saturation may mean larger core area.

Quite true. I thought at the time it was an idea worth exploring as I
had a bundle of old SMPS components rescued from rubbish skips on
hand. It works well given that a typical primary side ballast is
often in the 10 - 30mH inductance range.

Malcolm

> It looks like purified iron may even be a better choice than silicon
> iron much higher mu, about the same saturation point, and very low
> core loss. Don't know anything about availability or cost.


> Any comments???  and/or more info about N27 ferrites
>
> Gerry R
>
>
>
> >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
> >
> >----- 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
> > > >
> > > >
> > > >
> > > >
> > > >
> > >
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