Need to wind a toroid with a center-tap. Have seen this somewhere, but
can't remember where and can't find it. Anyone know how to easily wind a
tap into a toroid? Am thinking maybe put a loop at that point with a twist
in the wire to keep it from slipping back onto the toroid and loosening the
other windings. Anybody have any ideas on this?

TIA

Dave

Need to wind a toroid with a center-tap. Have seen this somewhere, but
can't remember where and can't find it. Anyone know how to easily wind a
tap into a toroid? Am thinking maybe put a loop at that point with a twist
in the wire to keep it from slipping back onto the toroid and loosening the
other windings. Anybody have any ideas on this?
=======================
A bi-filar winding A-a and B-b . Connect a to B (that's the centre)
Number of turns is half the total required. A and b are obviously the
end connections.
So a toroid requiring 10 turns with a centre needs 5 bi-filar turns.

Frank KN6WH / GM0CSZ

On Feb 27, 1:10�am, "Dave" wrote:
Need to wind a toroid with a center-tap. *Have seen this somewhere, but
can't remember where and can't find it. *Anyone know how to easily wind a
tap into a toroid? *Am thinking maybe put a loop at that point with a twist
in the wire to keep it from slipping back onto the toroid and loosening the
other windings. *Anybody have any ideas on this?

Besides a bifilar winding, which has already been suggested, try this:

Start at the middle and work towards the ends.

Say you need 10 turns center tapped. Take a piece of
wire long enough for 10 turns, find the exact halfway point,
scrape off the insulation there and make a twist. Then put the twist
on the outside of the core, and wind one end 5 turns one way around
the core, and the other end 5 turns the other way.

Or, use two pieces of wire long enough for 5 turns, Wind the first
one, bring its end out, twist/splice to the second one, wind 5 more
turns.

73 de Jim, N2EY

wrote in message
oups.com...
On Feb 27, 1:10?am, "Dave" wrote:
Need to wind a toroid with a center-tap. Have seen this somewhere, but
can't remember where and can't find it. Anyone know how to easily wind a
tap into a toroid? Am thinking maybe put a loop at that point with a twist
in the wire to keep it from slipping back onto the toroid and loosening
the
other windings. Anybody have any ideas on this?

Besides a bifilar winding, which has already been suggested, try this:

Start at the middle and work towards the ends.

Say you need 10 turns center tapped. Take a piece of
wire long enough for 10 turns, find the exact halfway point,
scrape off the insulation there and make a twist. Then put the twist
on the outside of the core, and wind one end 5 turns one way around
the core, and the other end 5 turns the other way.

Or, use two pieces of wire long enough for 5 turns, Wind the first
one, bring its end out, twist/splice to the second one, wind 5 more
turns.

73 de Jim, N2EY

Gotcha. I am new enough at winding toroids that I am probably going t use
the bifilar winding technique, which seems like it might be more forgiving
if I need to make adjustments afterward (correct me if I'm wrong.) But I
understand what you are saying here, and appreciate your input. Thanks
much. Once I get more adept this is what I will do.

Dave

"Highland Ham" wrote in message
...
Need to wind a toroid with a center-tap. Have seen this somewhere, but
can't remember where and can't find it. Anyone know how to easily wind a
tap into a toroid? Am thinking maybe put a loop at that point with a
twist in the wire to keep it from slipping back onto the toroid and
loosening the other windings. Anybody have any ideas on this?
=======================
A bi-filar winding A-a and B-b . Connect a to B (that's the centre)
Number of turns is half the total required. A and b are obviously the end
connections.
So a toroid requiring 10 turns with a centre needs 5 bi-filar turns.

Frank KN6WH / GM0CSZ

Gotcha! (I think.) So I should wind half my turns with A to a, then
connect a to B and wind the other half. Have I got that right? Or do I
Wind A to a and then wind B to b over that (not actually connecting one to
the other), using B as my CT? (When I first read this that's what I thought
you were talking about, but now am wondering...)

Thanks for help,

Dave

In article ,
"Dave" wrote:

Gotcha! (I think.) So I should wind half my turns with A to a, then
connect a to B and wind the other half. Have I got that right? Or do I
Wind A to a and then wind B to b over that (not actually connecting one to
the other), using B as my CT?

Dave-

How critical is your center tap? If it important that the tap be
perfectly centered, it would be better to use one bi-filar winding
instead of the two separate windings you describe.

If possible, spread the turns out over the toroid so the end is next to
the beginning, so the center tap can be easily connected.

If there are a lot of turns using small wire, you can first wind the
wire pair onto a shuttle (a long, skinny flat spool). Then unwind it
from the shuttle while passing it through the toroid. With lots of
turns, it might help if the two wires were different colors. Otherwise
you would need an Ohmmeter to determine which is which.

Fred
K4DII

"Fred McKenzie" wrote in message
...
In article ,
"Dave" wrote:

Gotcha! (I think.) So I should wind half my turns with A to a, then
connect a to B and wind the other half. Have I got that right? Or do I
Wind A to a and then wind B to b over that (not actually connecting one
to
the other), using B as my CT?

Dave-

How critical is your center tap? If it important that the tap be
perfectly centered, it would be better to use one bi-filar winding
instead of the two separate windings you describe.

If possible, spread the turns out over the toroid so the end is next to
the beginning, so the center tap can be easily connected.

If there are a lot of turns using small wire, you can first wind the
wire pair onto a shuttle (a long, skinny flat spool). Then unwind it
from the shuttle while passing it through the toroid. With lots of
turns, it might help if the two wires were different colors. Otherwise
you would need an Ohmmeter to determine which is which.

Fred
K4DII

Hey Fred,

Actually, I realized after I posted that last one that it is not actually a
"center" tap. It is tap about 20 to 25 % up from the start, in a 2 uH coil.
The original plan called for an 8-turn (on a 1.5 in form) air core coil with
the tap on the second turn, but I am wanting to modify that to use two
switchable coils with as little interference between them as possible, thus
the toroids rather than air-core coils. My calculations tell me that a 2 uH
coil (on an Amidon T-50-2 toroidal core) would have 22+ turns, so I am
guessing the tap would be around 5 or 6 turns. Question: could I wind the
first coil (for 2 uH) and then overlay it with the second (for the five or
six turns), connecting them at the beginning and hooking up their respective
ends to the appropriate components? If desired, I can upload the schematic
I am wanting to modify. It is the oscillator described in the
Hands-on-Radio column of QST in the Aug/Sep/Oct/Nov issues (experiments
#43/44/45/46 in the series.) The original idea was a Hartley osc. that
ended up becoming a crystal-controlled transmiter for the 40 meter band, but
I am wanting to use the osc to build a makeshift signal generator for
testing another project. (I don't have a signal generator currently, thus
the attempt at building a makeshift one.)

Apologies if this is too confused. I am obviously a newbie. If all of this
is so far off-base as to be hopeless, just tell me so and I'll try not to
bother again.

Thanks,

Dave

On Mar 1, 7:11�am, "Dave" wrote:

*I am new enough at winding toroids that I am probably going t use
the bifilar winding technique, which seems like it might be more forgiving
if I need to make adjustments afterward (correct me if I'm wrong.)

IMHO, the bifilar technique is more difficult and harder to
adjust.

Also, a bifilar winding used as a center-tapped coil will have
higher inter-winding capacitance and a lower self-resonant
frequency.

One thing to remember about toroids is that you can always take
all the wire off and start over.

Perhaps the best idea is to try all the methods and see
which works best for you.

*But I
understand what you are saying here, and appreciate your input. *Thanks
much. Once I get more adept this is what I will do.

You're welcome. Good luck.

73 de Jim, N2EY

On Mar 1, 4:11�am, "Dave" wrote:

*I am new enough at winding toroids that I am probably going t use
the bifilar winding technique, which seems like it might be more forgiving
if I need to make adjustments afterward (correct me if I'm wrong.) *But I
understand what you are saying here, and appreciate your input. *Thanks
much. Once I get more adept this is what I will do.

Here's a suggestion that's an old one when I first tried it
(successfully) 30 years ago on multi-filar windings: Get
an old hand-operated drill (electric if it has slow speed),
lay out equal lengths of magnet wire, tape one end of
the bundle securely, chuck the other end in the drill
and do the twisting. Pause several times while doing so
to make sure there aren't any sharp twists along the
way, "iron" those out with fingers. You can get an idea
of the number of twists per inch (or meter or whatever)
as you pause...for wideband RF transformers the number
of twists per unit length need only be approximate. What
you wind up with (no pun intended) is the equivalent of
multi-wire balanced transmission lines of relatively low
impedance and the total length would be roughly a
quarter-wavelength at the highest frequency of such a
transformer.

However, if you are doing a tap for a lower-impedance
point such as an oscillator feedback or impedance in/out,
the physical tap position doesn't have to be precise or
absolute (lots of other factors will be involved there).
Just get as close as you can to published data/article
on tap position and you'll be okay. Winding toroids may
be new to lots of folks but all it takes is more patience
than with old-style cylindrical forms.

I might add that, once a circuit is working with a toroid,
it can be secured by using some petroleum-based
varnish lightly brushed over it. Most "boat" or "spar"
varnish types are good for that but one is stuck with
having to buy quart cans of it (rather expensive). The
reason I mention marine varnish is that it stands up to
moisture in the air. If you use "Q Dope" (liquid form
polystyrene) it looks very nice when done but a year
from now the moisture normally in room air can "lift"
it from non-porous surfaces. On testing with MF to
UHF Q Meters, there is no more degredation of Q
using varnish than with liquid polystyrene or lacquers.
I use the McCloskey brand "Gym-Seal" varnish because
that is also good for wood surfaces around the house;
it was originally formulated as a surface varnish for
gymnasium floors and the like. Takes at least an
over-night drying time with a complete cure in two
days.