Amps
[Top] [All Lists]

Re: [Amps] toroid filament choke?

To: Will Matney <craxd1@ezwv.com>
Subject: Re: [Amps] toroid filament choke?
From: R.Measures <r@somis.org>
Date: Wed, 27 Oct 2004 11:57:55 -0700
List-post: <mailto:amps@contesting.com>

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


--- Outgoing mail is certified Virus Free. Checked by AVG anti-virus system (http://www.grisoft.com). Version: 6.0.783 / Virus Database: 529 - Release Date: 10/25/04


__________ NOD32 1.880 (20040928) Information __________


This message was checked by NOD32 antivirus system.
http://www.nod32.com




Will Matney
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps



Richard L. Measures, AG6K, 805.386.3734. www.somis.org


_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps

<Prev in Thread] Current Thread [Next in Thread>