[Amps] toroid filament choke?
R.Measures
r at somis.org
Wed Oct 27 14:57:55 EDT 2004
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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