Re: [Amps] toroid filament choke?

[R.Measures <r@somis.org>]

On Oct 27, 2004, at 9:36 AM, Will Matney wrote:

> k7fm wrote:
>
> > Will said:
> >
> > "You have to keep in mind the current surge there until the heater 
> > warms up.
> > The colder the heater is, the more current it draws. I would use a 
> > conductor
> > with at least 750 cir mils per ampere. For 3 amperes, that equals out 
> > to be
> > about 16 AWG wire for the minimum"
> >
> > The filament choke should not take into account any surge current, 
> > and there
> > is an advantage in limiting the wire size to limit surge current.
>
> Why shouldn't one take into consideration the surge current?

Because the hot-cold R-change in a heater is insignificant, whereas the 
hot-cold R-change in a typical filament is >8-to 1 -- partly because of 
the greater hot-cold delta-T in filament type tubes.  Also, 
tungsten/wolfram (W) has a high temp-coefficient, while the nickel (Ni) 
& chromium (Cr) alloy wire  used in heaters has a low temp-coefficient.
.
> It takes up to one minute + for the heater to reach operating 
> temperature. A cold heater is only a few ohms, and that's a lot of 
> current. Matter of fact, the toroid manuals specify wire at 500 or 750 
> cir mils per ampere for this type of choke, I went and took a look. 
> Though this is a RF choke, the current it sees is different than one 
> like a plate choke who has a more constant current (surge). At 500 
> cir. mils. per ampere (about 19 AWG wire), might work. This because 
> there's no insulation over the winding. But, if a piece of large ID 
> heat shrink is put around it like on some, this goes out the door. In 
> my opinion, it should be wound almost as heavy as the heater winding 
> in the transformer. I'd use other surge preventative measures than 
> using the choke for it. The reason being, it doesn't take long to boil 
> the varnish on formvar magnet wire unless it's high temp insulation,

Modern electric-motor wire is coated with an incredibly tough, 
180ºC-rated silicone-varnish.  Forvar went the way of the tubed 
automobile tire and the passenger pigeon.
 //  note - an okay way to remove silicone varnish from Cu wire is with 
a flame.  After the silicone-varnish burns, clean the ash residue off 
of the wire with steel wool.

> and heating of the core over time can cause a failure in it 
> (cracking). Here's a good experiment. Take a choke like this, 20 AWG 
> or smaller wire, with a smaller core. Over one minutes time, slowly 
> decrease the current equivelant to what the tube(s) heater is. See how 
> hot the wire and core become. One wound correctly shouldn't get much 
> over about 80 degrees F.. Of course I've always been guilty of 
> building things like a Sherman Tank, but I'm just following the set 
> rules by the manufacturers.
>
> > By
> > limiting the wire size, the smaller wire will limit the surge current 
> > and
> > protect the tube.  Since the choke is presumably in the slipstream of 
> > air, a
> > significantly smaller size of wire can be used.  If there is no other 
> > means
> > to protect the filament surge current, then this is an opportunity to
> > protect the tube and wind a smaller choke.
> >
> > Since chokes always seem to be just large enough to not fit where you 
> > want
> > them to, take advantage of downsizing.
> >
> > Colin  K7FM
> >
> >
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> Will Matney
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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org

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