[Amps] toroid filament choke?

[Will Matney]

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? 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, 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