Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire are
on the core. The core is open, so that it's uncovered and most of the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

I have measured the inductor with an inductance meter, so I know what
the inductance and other parameters are.

Suppose I take some wire, say roughly small if the inductor is small,
and wind it around the inductor, over the existing windings so that it's
within the magnetic field. I wind enough wire onto the inductor so that
I get about 1/9, or 1/16 or 1/25 the inductance in the new coil.

Since the inductance is the square of the turns, I can say that if I
have wound 10 turns and the inductance is 1/16th that of the original
coil, then the turns ratio is 4 to 1, so the original coil is about 40
turns.

Obviously the Real WOrld kicks in, and things may not always be exactly
as they should be. But I haven't tried this, and I'm wondering if any
other person has, and if it's a not unreasonably accurate[1] way to
guesstimate the turns, or if it is prone to a large amount of error. I
guess it would also apply to a toroid if there is enough room to loop
some wire thru the center hole, but this hole may be filled or covered
up.

So has anyone played around with this contrivance?

[1] A not uncommon journalistic contrivance nowadays; seems like these
authors just uncan stop not undoing this, and have unremembered to not
undo it the old fashioned way, and just say "common".

--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@ h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/e...s/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 at hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@ u@e@n@t@@

"Watson A.Name - "Watt Sun, the Dark Remover"" wrote
in message ...
Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire are
on the core. The core is open, so that it's uncovered and most of the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

I was thinking a solenoid type...obviously you cracked it in half then?

I have measured the inductor with an inductance meter, so I know what
the inductance and other parameters are.

Ok.

Since the inductance is the square of the turns, I can say that if I
have wound 10 turns and the inductance is 1/16th that of the original
coil, then the turns ratio is 4 to 1, so the original coil is about 40
turns.

Obviously the Real WOrld kicks in, and things may not always be exactly
as they should be.

If you snapped the core back together, the existing winding and your test
winding would share a good proportion of the flux, as a result it will act
as a good transformer. However, being open to the air, much of the field
lines will be lost and you'll have a less than unity coupling coefficient.
Depending on the frequency, size and turns you may also encounter trouble
measuring it accurately due to parasitic capacitance in the windings.

I guess it would also apply to a toroid if there is enough room to loop
some wire thru the center hole, but this hole may be filled or covered
up.

This would be much better because you can get a few turns around the core
evenly in most cases. Donno about coupling but I imagine it's worse farther
from the core, even though the turns still circle it fully.

So has anyone played around with this contrivance?

No, but it's a good idea if you can work around the coupling problems.
If only I had an L meter...

Tim

--
Just remember, Man was made in God's image. Woman was created out
of a rib, which, quite honestly, is a cheaper cut of meat." - toon
Website: http://webpages.charter.net/dawill/tmoranwms

On Fri, 4 Jun 2004 19:54:21 -0700, "Watson A.Name - \"Watt Sun, the
Dark Remover\"" wrote:

Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire are
on the core. The core is open, so that it's uncovered and most of the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

I have measured the inductor with an inductance meter, so I know what
the inductance and other parameters are.

Suppose I take some wire, say roughly small if the inductor is small,
and wind it around the inductor, over the existing windings so that it's
within the magnetic field. I wind enough wire onto the inductor so that
I get about 1/9, or 1/16 or 1/25 the inductance in the new coil.

Since the inductance is the square of the turns, I can say that if I
have wound 10 turns and the inductance is 1/16th that of the original
coil, then the turns ratio is 4 to 1, so the original coil is about 40
turns.

Obviously the Real WOrld kicks in, and things may not always be exactly
as they should be. But I haven't tried this, and I'm wondering if any
other person has, and if it's a not unreasonably accurate[1] way to
guesstimate the turns, or if it is prone to a large amount of error. I
guess it would also apply to a toroid if there is enough room to loop
some wire thru the center hole, but this hole may be filled or covered
up.

So has anyone played around with this contrivance?

[1] A not uncommon journalistic contrivance nowadays; seems like these
authors just uncan stop not undoing this, and have unremembered to not
undo it the old fashioned way, and just say "common".

That is a not-unworthy observation.

Given a common ferrite bobbin type inductor, you could apply a
reasonably high frequency sinewave to the inductor, then wind a
single-turn (or a few, maybe) sense winding over the existing winding,
then measure the voltage ratio (excitation/sense) to get the turns
ratio. This only works if the sense winding encompasses as much flux
as the main winding, which won't be entirely true for a bobbin with
air return path. It gets better if you can artificially close the gap
between the ends of the bobbin with some sort of ferrite or
transformer steel path, sort of a high-permeability c-clamp.

Hmmm... maybe it's better to apply an external magnetic field to the
thing to get the ratio. That may make it more likely that the sense
coil encompasses the same flux as the main coil. Probably so.

For a torroid of non-silly permeability, this voltage ratio thing just
works.

John

"Watson A.Name - "Watt Sun, the Dark Remover"" wrote
in message
Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire are
on the core. The core is open, so that it's uncovered and most of the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

Wind turns around it, as you've said. Then drive the unknown core
with some voltage at a high enough frequency that it can actually
develop some voltage (so you can measure it); detect and measure
the voltage at the secondary, (I say detect - depending on what
freq. you use. I don't know the freq. response of a typical DVM),
and the ratio is the ratio. :-)

It shouldn't matter if it's a little lossy, because the turns
ratio is the turns ratio, and the DVM is hi-impedance, right?

Cheers!
Rich

"Watson A.Name -in message

Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire are
on the core. The core is open, so that it's uncovered and most of the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

I have measured the inductor with an inductance meter, so I know what
the inductance and other parameters are.

Suppose I take some wire, say roughly small if the inductor is small,
and wind it around the inductor, over the existing windings so that it's
within the magnetic field. I wind enough wire onto the inductor so that
I get about 1/9, or 1/16 or 1/25 the inductance in the new coil.


** One you have got that far you have constructed a transformer. Drive some
AC current into the original inductor's winding ( from an audio generator or
similar) and measure the AC voltage on it and on the overwind you created.

The turns ratio and the (unloaded) voltage ratio you measure are in exact
proportion.

The same method can be used to discover the number of turns in the windings
of a toroidal transformer or any transformer where you can place a small
overwind.



............. Phil

"Watson A.Name - "Watt Sun, the Dark Remover"" wrote
in message

In fact, I'm willing to bet real money that if you just do the turns
ratio by volts, that you'll get an integer answer. Or 1/integer,
don't be a smartass. ;-) I'll bet $100.00 it's within +- 20% of
the nearest integer (or reciprocal, if you're doing it upside
down), $10.00 that it's with +-10%, $5.00 for +- 5%,
and if it's within 1%, we should both win. :-)

Cheers!
Rich

So my first obvious question is, why would you care? If you want to
duplicate the inductor, you already know the inductance, and you can
measure saturation effects and even loss, with some ingenuity.

But playing along with your request, if you can wind turns around the
existing coil, you also have made a transformer. To the extent the
two windings share a common magnetic field, they will be coupled. You
can, in fact, measure the leakage inductances and come up with quite a
good model, and I suppose from that you can deduce the number of turns
fairly accurately, especially if the coupling is good (and the leakage
inductance small compared with the coupled inductance) as it would be
with a ferrite toroid or a pot core or such.

Cheers,
Tom

"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote in message ...
Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire are
on the core. The core is open, so that it's uncovered and most of the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

I have measured the inductor with an inductance meter, so I know what
the inductance and other parameters are.

Suppose I take some wire, say roughly small if the inductor is small,
and wind it around the inductor, over the existing windings so that it's
within the magnetic field. I wind enough wire onto the inductor so that
I get about 1/9, or 1/16 or 1/25 the inductance in the new coil.

Since the inductance is the square of the turns, I can say that if I
have wound 10 turns and the inductance is 1/16th that of the original
coil, then the turns ratio is 4 to 1, so the original coil is about 40
turns.

Obviously the Real WOrld kicks in, and things may not always be exactly
as they should be. But I haven't tried this, and I'm wondering if any
other person has, and if it's a not unreasonably accurate[1] way to
guesstimate the turns, or if it is prone to a large amount of error. I
guess it would also apply to a toroid if there is enough room to loop
some wire thru the center hole, but this hole may be filled or covered
up.

So has anyone played around with this contrivance?

[1] A not uncommon journalistic contrivance nowadays; seems like these
authors just uncan stop not undoing this, and have unremembered to not
undo it the old fashioned way, and just say "common".

--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@ h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
http://users.pandora.be/educypedia/e...s/databank.htm
My email address is whitelisted. *All* email sent to it
goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 at hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
that will give you a choice and save you money(up to half).
http://www.everybookstore.com You'll be glad you did!
Just when you thought you had all this figured out, the gov't
changed it: http://physics.nist.gov/cuu/Units/binary.html
@@t@h@e@@a@f@f@l@u@e@n@t@@m@e@e@t@@t@h@e@@E@f@f@l@ u@e@n@t@@

If it is an air-cored inductor, calculate the number of turns from its
measured dimensions.

This won't work with a ferrite core because its material permeability is not
known. Although if the core is a simple rod the effective permeability is
roughly 25 regardless of material permeability.

With a high permeability core, 100 or more, effective permeability becomes a
function only of the very long 'air gap'. So inductance stops increasing
with increasing core material permeabilty.

But why would you want to know the number of turns if the coil is already
wound!
----
Reg.

On a sunny day (Fri, 4 Jun 2004 19:54:21 -0700) it happened "Watson A.Name -
\"Watt Sun, the Dark Remover\"" wrote in
:

Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire are
on the core. The core is open, so that it's uncovered and most of the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

I have measured the inductor with an inductance meter, so I know what
the inductance and other parameters are.

Suppose I take some wire, say roughly small if the inductor is small,
and wind it around the inductor, over the existing windings so that it's
within the magnetic field. I wind enough wire onto the inductor so that
I get about 1/9, or 1/16 or 1/25 the inductance in the new coil.

Since the inductance is the square of the turns, I can say that if I
have wound 10 turns and the inductance is 1/16th that of the original
coil, then the turns ratio is 4 to 1, so the original coil is about 40
turns.

Obviously the Real WOrld kicks in, and things may not always be exactly
as they should be. But I haven't tried this, and I'm wondering if any
other person has, and if it's a not unreasonably accurate[1] way to
guesstimate the turns, or if it is prone to a large amount of error. I
guess it would also apply to a toroid if there is enough room to loop
some wire thru the center hole, but this hole may be filled or covered
up.

So has anyone played around with this contrivance?

[1] A not uncommon journalistic contrivance nowadays; seems like these
authors just uncan stop not undoing this, and have unremembered to not
undo it the old fashioned way, and just say "common".
If you can add turns, put ten turns, and 400Hz 1V for example.
Measure voltage on original winding.
If 30V it is 10 x 30 = 300 turns.
JP

What am I missing here? If you know the inductance of the original
coil, there are formulas that will tell you the number of turns. Wind a
coil according to the formula, measure the inductance, and tweak the
number of turns to get as close as you need to be.

Bill
====================


Watson A.Name - "Watt Sun, the Dark Remover" wrote:
Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire are
on the core. The core is open, so that it's uncovered and most of the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

I have measured the inductor with an inductance meter, so I know what
the inductance and other parameters are.

Suppose I take some wire, say roughly small if the inductor is small,
and wind it around the inductor, over the existing windings so that it's
within the magnetic field. I wind enough wire onto the inductor so that
I get about 1/9, or 1/16 or 1/25 the inductance in the new coil.

Since the inductance is the square of the turns, I can say that if I
have wound 10 turns and the inductance is 1/16th that of the original
coil, then the turns ratio is 4 to 1, so the original coil is about 40
turns.

Obviously the Real WOrld kicks in, and things may not always be exactly
as they should be. But I haven't tried this, and I'm wondering if any
other person has, and if it's a not unreasonably accurate[1] way to
guesstimate the turns, or if it is prone to a large amount of error. I
guess it would also apply to a toroid if there is enough room to loop
some wire thru the center hole, but this hole may be filled or covered
up.

So has anyone played around with this contrivance?

On Sat, 05 Jun 2004 03:16:28 GMT, "Rich Grise"
wrote:


It shouldn't matter if it's a little lossy, because the turns
ratio is the turns ratio,

As long as the same flux traverses all the turns.

John

On a sunny day (Sat, 05 Jun 2004 08:55:27 -0700) it happened John Larkin
wrote in
:

On Sat, 05 Jun 2004 03:16:28 GMT, "Rich Grise"
wrote:


It shouldn't matter if it's a little lossy, because the turns
ratio is the turns ratio,

As long as the same flux traverses all the turns.

John

pepepepepepepedantic

"Phil Allison" wrote in message
...

"Watson A.Name -in message

Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire
are
on the core. The core is open, so that it's uncovered and most of
the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

I have measured the inductor with an inductance meter, so I know
what
the inductance and other parameters are.

Suppose I take some wire, say roughly small if the inductor is
small,
and wind it around the inductor, over the existing windings so that
it's
within the magnetic field. I wind enough wire onto the inductor so
that
I get about 1/9, or 1/16 or 1/25 the inductance in the new coil.


** One you have got that far you have constructed a transformer.
Drive some
AC current into the original inductor's winding ( from an audio
generator or
similar) and measure the AC voltage on it and on the overwind you
created.

The turns ratio and the (unloaded) voltage ratio you measure are in
exact
proportion.

The same method can be used to discover the number of turns in the
windings
of a toroidal transformer or any transformer where you can place a
small
overwind.

After reading several followups so far, I'm getting the picture that it
would be easier to measure the voltage ratio. Rich suggested using a
DVM, but IIRC their AC bandwidth is limited, and drops off above a few
kHz or so. Rectifying the AC is an alternativce, but then it's not
accurate if the .6V diode drop is a considerable part of the rectified
DCV. An O'Scope seems the best way to measure, if it can be calibrated.
Actually, come to think of it, all that's needed is the ratio, not the
absolute V values.

One thing that I had in mind when I originated this idea was that, say
for instance, I'm measuring a trigger transformer for a xenon tube,
where the number of turns could be thousands. If I wound a few tens of
turns on it, the V ratio could be a hundred or more. That might be a
bit more difficult to measure than the inductance.

Thanks to all for the thoughtful responses. I'm going to have to try a
few experiments to see how these work.

............ Phil

"Reg Edwards" wrote in message
...
If it is an air-cored inductor, calculate the number of turns from its
measured dimensions.

This won't work with a ferrite core because its material permeability
is not
known. Although if the core is a simple rod the effective permeability
is
roughly 25 regardless of material permeability.

With a high permeability core, 100 or more, effective permeability
becomes a
function only of the very long 'air gap'. So inductance stops
increasing
with increasing core material permeabilty.

Thanks for the interesting info. I would expect the core to be more of
a bobbin. But when it's covered, it's not always certain.

But why would you want to know the number of turns if the coil is
already
wound!

If I don't know the number of turns to begin with, do you expect me to
UNwind the coil to find the number of turns?

As I said, the coil is usually covered or potted in epoxy.

----
Reg.

"Bill Jeffrey" wrote in message
...
What am I missing here? If you know the inductance of the original
coil, there are formulas that will tell you the number of turns. Wind
a
coil according to the formula, measure the inductance, and tweak the
number of turns to get as close as you need to be.

Bill
====================

Okay, I have two identical adjustable core coils, one with the slug all
the way in and the other all the way out. The Out one measures 100 uH
and the In one measures 180 uh. I put both into a box, each with
terminals to the outside, so that the physical coil can't be seen. Then
I give them to you along with the inductance of each, and you tell me
that, by your formulas, the Out one has a different number of turns than
the In one????


Watson A.Name - "Watt Sun, the Dark Remover" wrote:
Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire
are
on the core. The core is open, so that it's uncovered and most of
the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

I have measured the inductor with an inductance meter, so I know
what
the inductance and other parameters are.

Suppose I take some wire, say roughly small if the inductor is
small,
and wind it around the inductor, over the existing windings so that
it's
within the magnetic field. I wind enough wire onto the inductor so
that
I get about 1/9, or 1/16 or 1/25 the inductance in the new coil.

Since the inductance is the square of the turns, I can say that if I
have wound 10 turns and the inductance is 1/16th that of the
original
coil, then the turns ratio is 4 to 1, so the original coil is about
40
turns.

Obviously the Real WOrld kicks in, and things may not always be
exactly
as they should be. But I haven't tried this, and I'm wondering if
any
other person has, and if it's a not unreasonably accurate[1] way to
guesstimate the turns, or if it is prone to a large amount of error.
I
guess it would also apply to a toroid if there is enough room to
loop
some wire thru the center hole, but this hole may be filled or
covered
up.

So has anyone played around with this contrivance?

One way might be to in-case the coil in epoxy resin and saw it in half and simply count the windings.

"Watson A.Name - "Watt Sun, the Dark Remover""
wrote in message ...
Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire
are
on the core. The core is open, so that it's uncovered and most of
the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

The voltage ratio of a transformer is the same as the turns ratio.
Therefore, wind ten turns around the inductor and measure the voltage
at the output when a known voltage is applied to the inductor.

Norm Strong

The voltage ratio of a transformer is the same as the turns ratio.
Therefore, wind ten turns around the inductor and measure the voltage
at the output when a known voltage is applied to the inductor.

Norm Strong

====================

Agreed. That's about the best he can manage. But what is not known is the
coefficient of coupling between the two coils. They are not wound in the
same volume of space or anywhere near to it. One is entirely outside the
other.

If the outside coil has a coefficient of coupling of 0.5 with the inside
coil then it is equivalent to a coil with only half the number of turns.

The arithmetic is simple. But what the coeff of coupling might be is
anybody's guess without knowledge of ALL dimensions of BOTH coils. Ask your
dentist if you could borrow his X-ray machine for the day. Even then a
hefty treatise involving higher mathematics on how to calculate the
coefficient of coupling between two coils would be essential.

All one knows is that the turns error, possibly very large, must lie on the
low side of the true value.

Its just occurred to me that with access to a precision X-ray machine or
electron microscope it may be possible actually to count the number of
turns. Try NASA.

How many Henrys is the thing anyway?
===
Reg.

On Sat, 5 Jun 2004 21:11:09 +0000 (UTC), "Reg Edwards"
wrote:

The voltage ratio of a transformer is the same as the turns ratio.
Therefore, wind ten turns around the inductor and measure the voltage
at the output when a known voltage is applied to the inductor.

Norm Strong

====================

Agreed. That's about the best he can manage. But what is not known is the
coefficient of coupling between the two coils. They are not wound in the
same volume of space or anywhere near to it. One is entirely outside the
other.

If the outside coil has a coefficient of coupling of 0.5 with the inside
coil then it is equivalent to a coil with only half the number of turns.

The arithmetic is simple. But what the coeff of coupling might be is
anybody's guess without knowledge of ALL dimensions of BOTH coils. Ask your
dentist if you could borrow his X-ray machine for the day. Even then a
hefty treatise involving higher mathematics on how to calculate the
coefficient of coupling between two coils would be essential.

All one knows is that the turns error, possibly very large, must lie on the
low side of the true value.

Its just occurred to me that with access to a precision X-ray machine or
electron microscope it may be possible actually to count the number of
turns. Try NASA.

How many Henrys is the thing anyway?
===
Reg.


A lot of our larger battery charger transformer designs have primaries wound
outside with the secondaries closest to the core. It is quite a common
technique, even on some of the smaller stuff we use.

Peter

--
Peter & Rita Forbes

Engine pages for preservation info:
http://www.oldengine.org/members/diesel

A lot of our larger battery charger transformer designs have primaries
wound
outside with the secondaries closest to the core. It is quite a common
technique, even on some of the smaller stuff we use.

Peter

=================================

Of course they are. They do it all the time. Ever since the Victorian Age.
But the primary-to-secondary coefficient of coupling, with the leakeage
reactances, is accurately KNOWN from the start of the design. Certainly not
the last. It's fundamental. But being at least acquainted with the things,
I would have thought you already knew that.

But students should not take me too seriously. I'm really a kindly person.
===
Reg

"Reg Edwards" wrote in message
...
Even then a
hefty treatise involving higher mathematics on how to calculate the
coefficient of coupling between two coils would be essential.

I bet I could dig up some Fraday's law stuff from my physics textbook. It'd
be a nasty integral to evaluate but would get you there.

Tim

--
"Just remember, Man was made in God's image. Woman was created out
of a rib, which, quite honestly, is a cheaper cut of meat." - toon
Website: http://webpages.charter.net/dawill/tmoranwms

On Sat, 5 Jun 2004 13:17:08 +1000, "Phil Allison"
wrote:


** One you have got that far you have constructed a transformer. Drive some
AC current into the original inductor's winding ( from an audio generator or
similar) and measure the AC voltage on it and on the overwind you created.

The turns ratio and the (unloaded) voltage ratio you measure are in exact
proportion.


As long as the same flux traverses all the turns.


John

"John Larkin"
On "Phil Allison"


** One you have got that far you have constructed a transformer. Drive
some
AC current into the original inductor's winding ( from an audio generator
or
similar) and measure the AC voltage on it and on the overwind you
created.

The turns ratio and the (unloaded) voltage ratio you measure are in exact
proportion.


As long as the same flux traverses all the turns.


** That is not a very helpful remark.

The suggestion was that the overwind be around the existing coil of the
inductor * PLUS * there is no load on the overwind so leakage inductance
is irrelevant.



................ Phil

"Reg Edwards"


But the primary-to-secondary coefficient of coupling, with the leakeage
reactances, is accurately KNOWN from the start of the design.


** Leakage reactance is irrelevant with a no load test.




............ Phil

As has been pointed out in other postings to the thread, the
coefficient of coupling is important. Whatever flux from the primary
(driven winding) does not couple to the secondary will not induce
voltage in the secondary, and the measured turns ratio will be low as
a result. However, by measuring the inductance of the primary when
the secondary is open and again when it is shorted, and doing the same
with the secondary, you can find the leakage inductances and therefore
the coefficient of coupling, fairly accurately. (The second
measurement is really a check for consistency.) No need for xrays.
You could further improve the accuracy, I suppose, by including a
resistance value for each winding; ideally it would be the AC
resistance at the operating frequency. It will probably make for
easier calculations if you load the secondary very lightly for the
measurement.

But I'm still not seeing any need to know the number of turns, other
than for idle curosity. "I need to know because I want to"??

Cheers,
Tom

"Watson A.Name - \"Watt Sun, the Dark Remover\"" wrote in message ...
....
After reading several followups so far, I'm getting the picture that it
would be easier to measure the voltage ratio. Rich suggested using a
DVM, but IIRC their AC bandwidth is limited, and drops off above a few
kHz or so. Rectifying the AC is an alternativce, but then it's not
accurate if the .6V diode drop is a considerable part of the rectified
DCV. An O'Scope seems the best way to measure, if it can be calibrated.
Actually, come to think of it, all that's needed is the ratio, not the
absolute V values.

One thing that I had in mind when I originated this idea was that, say
for instance, I'm measuring a trigger transformer for a xenon tube,
where the number of turns could be thousands. If I wound a few tens of
turns on it, the V ratio could be a hundred or more. That might be a
bit more difficult to measure than the inductance.

Thanks to all for the thoughtful responses. I'm going to have to try a
few experiments to see how these work.

............ Phil

"Jan Panteltje" wrote in message
s.com...
On a sunny day (Sat, 05 Jun 2004 08:55:27 -0700) it happened John Larkin
wrote in
:

On Sat, 05 Jun 2004 03:16:28 GMT, "Rich Grise"
wrote:


It shouldn't matter if it's a little lossy, because the turns
ratio is the turns ratio,

As long as the same flux traverses all the turns.

John

pepepepepepepedantic

Well, it does make a difference.

I learned something today! Guess I can go back to bed. :-)

Cheers!
Rich

On Sun, 6 Jun 2004 13:23:28 +1000, "Phil Allison"
wrote:

The turns ratio and the (unloaded) voltage ratio you measure are in exact
proportion.


As long as the same flux traverses all the turns.


** That is not a very helpful remark.


But it's true. There's not a lot of sense pretending you can measure
something if you can't.

The suggestion was that the overwind be around the existing coil of the
inductor * PLUS * there is no load on the overwind so leakage inductance
is irrelevant.

Leakage inductance means exactly that the same flux does *not* thread
all turns. So the unloaded voltage induced into the sense winding will
be less volts/turn than the main coil. This is the likely situation
for a drum core with a large air return path; some of the return flux
will sneak back *inside* the sense coil.

As the sense winding gets bigger in diameter, its signal level tends
to zero, loaded or not.

John

"John Larkin"
"Phil Allison"

The turns ratio and the (unloaded) voltage ratio you measure are in
exact
proportion.

As long as the same flux traverses all the turns.

** That is not a very helpful remark.

But it's true.


** It is *unhelpful* because it is so damn ambiguous.


There's not a lot of sense pretending you can measure
something if you can't.


** It makes less sense to scorn a perfectly practical test method.


The suggestion was that the overwind be around the existing coil of the
inductor * PLUS * there is no load on the overwind so leakage
inductance
is irrelevant.


Leakage inductance means exactly that the same flux does *not* thread
all turns. So the unloaded voltage induced into the sense winding will
be less volts/turn than the main coil. This is the likely situation
for a drum core with a large air return path; some of the return flux
will sneak back *inside* the sense coil.



** The overwind is to be around the existing coil, wound in parallel and on
top of it, touching it - is that hard to comprehend ?

A further ( rather obvious) condition is that the inductor coil current for
the test be low enough to not generate a significant voltage drop across the
coil's resistance - or you calculate that drop and take it into account.


As the sense winding gets bigger in diameter, its signal level tends
to zero, loaded or not.



** I just took a small mains toroidal ( 30VA) and with the primary
energised at 230 volts passed a one turn loop through the core and measured
0.102 volts rms across the ends. The loop could be made as open as you
liked or tight wrapped as you liked with NO change in the measured voltage.

The primary magnetising current was only 1.5 mA and the primary resistance
was 94 ohms - so a negligible primary drop of 140 mV.

So I make the primary turns to be 2255 ( +/- the AC voltmeter's 0.3 %
error, or about 7 turns)



............. Phil

In article ,
Tom Bruhns wrote:
As has been pointed out in other postings to the thread, the
coefficient of coupling is important. Whatever flux from the
primary (driven winding) does not couple to the secondary will
not induce voltage in the secondary, and the measured turns ratio
will be low as a result. However, by measuring the inductance of
the primary when the secondary is open and again when it is
shorted, and doing the same with the secondary, you can find the
leakage inductances and therefore the coefficient of coupling,
fairly accurately.

That method of measuring the leakage inductance
(by shorting windings) gives a hint towards a
possible experimental method.... Short the sec
with an ammeter and treat the thing as a CT.

After all, CT's have a current-ratio that is quite
close to the turns-ratio, even though the coupling
can be poor (as in a CT with a bar primary). This
is because the leakage inductance (and R-primary)
can be regarded as being in series with a constant
current stimulus source. The major source of error
is then the sideways current due to the shunt loss.

So perhaps do a short-circuit current-ratio test,
then measure the sideways shunt-current taken by
just the primary, at the same equivalent voltage.

--
Tony Williams.

"Phil Allison" wrote in message
...
"John Larkin"

Leakage inductance means exactly that the same flux does *not* thread
all turns. So the unloaded voltage induced into the sense winding will
be less volts/turn than the main coil. This is the likely situation
for a drum core with a large air return path; some of the return flux
will sneak back *inside* the sense coil.

** I just took a small mains toroidal ( 30VA) and with the primary
energised at 230 volts passed a one turn loop through the core and
measured
0.102 volts rms across the ends. The loop could be made as open as you
liked or tight wrapped as you liked with NO change in the measured
voltage.

The difference, of course, is that in a toroid, all the flux is constrained
to the core, so it'll work every time. As Mr. Larkin pointed out, since the
winding in question is on a bobbin, and would go in a cup core or pot core,
you would, in fact, lose leakage flux. So the problem does become kinda
non-trivial.

But I'm thinking some kind of temporary core, a la amprobe or some UI
core from the junk box, but then you're getting into Rube Golberg
stuff.

Cheers!
Rich

Watson A.Name - "Watt Sun, the Dark Remover" wrote:
"Bill Jeffrey" wrote in message
...

What am I missing here? If you know the inductance of the original
coil, there are formulas that will tell you the number of turns. Wind

a

coil according to the formula, measure the inductance, and tweak the
number of turns to get as close as you need to be.

Bill
====================


Okay, I have two identical adjustable core coils, one with the slug all
the way in and the other all the way out. The Out one measures 100 uH
and the In one measures 180 uh. I put both into a box, each with
terminals to the outside, so that the physical coil can't be seen. Then
I give them to you along with the inductance of each, and you tell me
that, by your formulas, the Out one has a different number of turns than
the In one????

No, I'm saying that you take the slug all the way out, and the bobbin
off the pot core/cup core, so you have an air core coil. Measure the
inductance and plug it into the formula. (You did say that it's wound
on a bobbin, which usually implies that you can get the bobbin off the
ferrite.)

There are many formulas for calculating inductance. All of them admit
to being approximations - but that's all you need. For example:

"For a coil of rectangular cross-section, of thickness t inches, length
l inches and mean diameter (average of inside and outside) d inches,
Hazletine's formula is L = 0.8d^2N^2 /(12d + 36l + 40t) uH"

Now if your entire coil, including the ferrite, is potted in epoxy, it
is a different situation. But I don't see that in any of your posts.

Bill

"Phil Allison" wrote in message ...
"Reg Edwards"


But the primary-to-secondary coefficient of coupling, with the leakeage
reactances, is accurately KNOWN from the start of the design.


** Leakage reactance is irrelevant with a no load test.

You'll get a lot of disagreement with that; even experiments will
disagree with you. (One way to think about it is that the applied
primary voltage is split between the leakage inductance and the
perfectly-coupled inductance of the model. There's no drop across the
secondary's leakage inductance, but there for sure is across the
primary's leakage inductance.) But as Tony W. so kindly pointed out,
it's much less important if you use a short-circuit (current ratio)
test.

Cheers,
Tom

On Sun, 6 Jun 2004 15:31:40 +1000, "Phil Allison"
wrote:


"John Larkin"
"Phil Allison"

The turns ratio and the (unloaded) voltage ratio you measure are in
exact
proportion.

As long as the same flux traverses all the turns.

** That is not a very helpful remark.

But it's true.


** It is *unhelpful* because it is so damn ambiguous.


There's not a lot of sense pretending you can measure
something if you can't.


** It makes less sense to scorn a perfectly practical test method.


The suggestion was that the overwind be around the existing coil of the
inductor * PLUS * there is no load on the overwind so leakage
inductance
is irrelevant.


Leakage inductance means exactly that the same flux does *not* thread
all turns. So the unloaded voltage induced into the sense winding will
be less volts/turn than the main coil. This is the likely situation
for a drum core with a large air return path; some of the return flux
will sneak back *inside* the sense coil.



** The overwind is to be around the existing coil, wound in parallel and on
top of it, touching it - is that hard to comprehend ?


Not a bit. But the outer coil has - surprise! - a bigger diameter than
the inner, so more of the return flux is flowing *inside* the sense
coil, in the direction that reduces the induced voltage. Given a
typical drum/bobbin type inductor, I'd guess that the resulting error
might be in the 50% sort of turf; the actual error depends on the
geometry of the windings, and how close the sense winding can actually
get, given the insulation or epoxy or whatever cited.

A further ( rather obvious) condition is that the inductor coil current for
the test be low enough to not generate a significant voltage drop across the
coil's resistance - or you calculate that drop and take it into account.

Up to saturation - and an drum core will usually vaporize before it
saturates - the voltage ratio, whatever it is, will be independent of
drive level; the coupling is linear. A high drive *frequency* will
limimize the effects of copper loss, although it can be approximately
accounted for.


As the sense winding gets bigger in diameter, its signal level tends
to zero, loaded or not.



** I just took a small mains toroidal ( 30VA) and with the primary
energised at 230 volts passed a one turn loop through the core and measured
0.102 volts rms across the ends. The loop could be made as open as you
liked or tight wrapped as you liked with NO change in the measured voltage.

With a closed, high-permeability core, voltage ratios can track turns
ratios to a part per million, as in a precision AC ratio box. Since
virtually all the flux is concentrated in the steel, any loop of any
size pretty much slices the same amount of flux. A torroid is ideal
for close coupling. That's not the case with a system dominated by air
gap, because the flux is scattered all over in space.


The primary magnetising current was only 1.5 mA and the primary resistance
was 94 ohms - so a negligible primary drop of 140 mV.

So I make the primary turns to be 2255 ( +/- the AC voltmeter's 0.3 %
error, or about 7 turns)


Transformar manufacturers routinely use DVM-looking gadgets that
indicate turns ratio, and can easily and accurately resolve whole or
half turns. But only when leakage inductance is low, as for a closed,
high-mu core with tightly-coupled windings.

John

On 6 Jun 2004 10:32:44 -0700, (Tom Bruhns) wrote:

"Phil Allison" wrote in message ...
"Reg Edwards"


But the primary-to-secondary coefficient of coupling, with the leakeage
reactances, is accurately KNOWN from the start of the design.


** Leakage reactance is irrelevant with a no load test.

You'll get a lot of disagreement with that; even experiments will
disagree with you. (One way to think about it is that the applied
primary voltage is split between the leakage inductance and the
perfectly-coupled inductance of the model. There's no drop across the
secondary's leakage inductance, but there for sure is across the
primary's leakage inductance.) But as Tony W. so kindly pointed out,
it's much less important if you use a short-circuit (current ratio)
test.

Cheers,
Tom

Leakage inductance will diminish both voltage and current ratios. If
not, a 1" diameter 1-turn coil driven by 1 amp could induce an amp
into another 1" ring a mile away. Tesla would have liked that.

John

It's interesting, when I learned this stuff ( I won't tell you when, but my
then-new text was published in 1935!), albeit in the context of
utility/power engineering, about the LAST thing we learned was the tricks
and conventions about turns ratios, etc.

Just looking at a question in my book:

"Assuming a coil of thus & so dimensions surrounding a core of this &
that dimension & type of material, calculate: 1) The flux in the core, 2)
the flux in the air," etc.

Follow-up question:

"Assuming a second identical coil placed elsewhere on the core, calculate
induced voltage if only the flux in the iron passes through the second
coil," etc.

The whole text was written like that. The concept of a "perfect
transformer" was introduced much later, and only in certain contexts. For
utility purposes, perfect transformers are undesirable!

Kind of strange by today's standards; we were taught all the painful details
right up front, and later allowed to throw out the ones that didn't apply. I
think it's done the other way around today.

Oh, if anyone cares, book cited is "Alternating Current Machinery," Bryant &
Johnson, 1935.



"John Larkin" wrote in
message ...
On Sun, 6 Jun 2004 15:31:40 +1000, "Phil Allison"
wrote:


"John Larkin"
"Phil Allison"

The turns ratio and the (unloaded) voltage ratio you measure are
in
exact
proportion.

As long as the same flux traverses all the turns.

** That is not a very helpful remark.

But it's true.


** It is *unhelpful* because it is so damn ambiguous.


There's not a lot of sense pretending you can measure
something if you can't.


** It makes less sense to scorn a perfectly practical test method.


The suggestion was that the overwind be around the existing coil of
the
inductor * PLUS * there is no load on the overwind so leakage
inductance
is irrelevant.


Leakage inductance means exactly that the same flux does *not* thread
all turns. So the unloaded voltage induced into the sense winding will
be less volts/turn than the main coil. This is the likely situation
for a drum core with a large air return path; some of the return flux
will sneak back *inside* the sense coil.



** The overwind is to be around the existing coil, wound in parallel and
on
top of it, touching it - is that hard to comprehend ?


Not a bit. But the outer coil has - surprise! - a bigger diameter than
the inner, so more of the return flux is flowing *inside* the sense
coil, in the direction that reduces the induced voltage. Given a
typical drum/bobbin type inductor, I'd guess that the resulting error
might be in the 50% sort of turf; the actual error depends on the
geometry of the windings, and how close the sense winding can actually
get, given the insulation or epoxy or whatever cited.

A further ( rather obvious) condition is that the inductor coil current
for
the test be low enough to not generate a significant voltage drop across
the
coil's resistance - or you calculate that drop and take it into account.

Up to saturation - and an drum core will usually vaporize before it
saturates - the voltage ratio, whatever it is, will be independent of
drive level; the coupling is linear. A high drive *frequency* will
limimize the effects of copper loss, although it can be approximately
accounted for.


As the sense winding gets bigger in diameter, its signal level tends
to zero, loaded or not.



** I just took a small mains toroidal ( 30VA) and with the primary
energised at 230 volts passed a one turn loop through the core and
measured
0.102 volts rms across the ends. The loop could be made as open as you
liked or tight wrapped as you liked with NO change in the measured
voltage.

With a closed, high-permeability core, voltage ratios can track turns
ratios to a part per million, as in a precision AC ratio box. Since
virtually all the flux is concentrated in the steel, any loop of any
size pretty much slices the same amount of flux. A torroid is ideal
for close coupling. That's not the case with a system dominated by air
gap, because the flux is scattered all over in space.


The primary magnetising current was only 1.5 mA and the primary
resistance
was 94 ohms - so a negligible primary drop of 140 mV.

So I make the primary turns to be 2255 ( +/- the AC voltmeter's 0.3 %
error, or about 7 turns)


Transformar manufacturers routinely use DVM-looking gadgets that
indicate turns ratio, and can easily and accurately resolve whole or
half turns. But only when leakage inductance is low, as for a closed,
high-mu core with tightly-coupled windings.

John

In
sci.electronics.design,sci.electronics.repair,sci. electronics.components,alt.binaries.schematics.ele ctronic,
"Watson A.Name - \"Watt Sun, the Dark Remover\""
wrote:


"Reg Edwards" wrote in message
...

But why would you want to know the number of turns if the coil is
already
wound!

If I don't know the number of turns to begin with, do you expect me to
UNwind the coil to find the number of turns?

You didn't answer the question. WHY do you want to find the number
of turns?

Okay, I'll answer for you. Reverse engineering. You want to make
one or more coils exactly like it. Of course, not only do you need the
number of turns (and the exact layout of the turns), you also need to
know exactly what the magnetic core material is - you can either ask
the manufacturer (of either the core or the coil), or measure its
physical size and test all its magnetic properties.

An inductor is one of the easier components to make in the "home
laboratory" and it's good to know you can look up formulas and stuff
(the ARRL handbook, at least older editions, has some useful coolbook
formulas for cylindrical coils) for those times when you need it ASAP
and can't wait for overnight delivery, but otherwise it's still
cheaper to buy than to build.

As I said, the coil is usually covered or potted in epoxy.

----
Reg.

-----
http://mindspring.com/~benbradley

OK, let's suggest something different.

1. Measure the diameter of the wire in the existing coil.

2. Measure the DC resistance of the existing coil, and calculate the length
of the wire needed to generate that resistance.

3. Figure out how many turns will use up that length of wire.

You did say ESTIMATE, did you not?

??!!??

"Watson A.Name - "Watt Sun, the Dark Remover"" wrote
in message ...
Suppose that I have an inductor that's covered with epoxy or similar
that prevents me from seeing or finding out how many turns of wire are
on the core. The core is open, so that it's uncovered and most of the
magnetic field is outside outside of the inductor. Obviously it's a
bobbin type core.

I have measured the inductor with an inductance meter, so I know what
the inductance and other parameters are.

Suppose I take some wire, say roughly small if the inductor is small,
and wind it around the inductor, over the existing windings so that it's
within the magnetic field. I wind enough wire onto the inductor so that
I get about 1/9, or 1/16 or 1/25 the inductance in the new coil.

Since the inductance is the square of the turns, I can say that if I
have wound 10 turns and the inductance is 1/16th that of the original
coil, then the turns ratio is 4 to 1, so the original coil is about 40
turns.

Obviously the Real WOrld kicks in, and things may not always be exactly
as they should be. But I haven't tried this, and I'm wondering if any
other person has, and if it's a not unreasonably accurate[1] way to
guesstimate the turns, or if it is prone to a large amount of error. I
guess it would also apply to a toroid if there is enough room to loop
some wire thru the center hole, but this hole may be filled or covered
up.

So has anyone played around with this contrivance?

[1] A not uncommon journalistic contrivance nowadays; seems like these
authors just uncan stop not undoing this, and have unremembered to not
undo it the old fashioned way, and just say "common".

--
@@F@r@o@m@@O@r@a@n@g@e@@C@o@u@n@t@y@,@@C@a@l@,@@w@ h@e@r@e@@
###Got a Question about ELECTRONICS? Check HERE First:###
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goes directly to the trash unless you add NOSPAM in the
Subject: line with other stuff. alondra101 at hotmail.com
Don't be ripped off by the big book dealers. Go to the URL
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On Sun, 6 Jun 2004 16:30:39 -0400, "BFoelsch"
wrote:

It's interesting, when I learned this stuff ( I won't tell you when, but my
then-new text was published in 1935!), albeit in the context of
utility/power engineering, about the LAST thing we learned was the tricks
and conventions about turns ratios, etc.

Just looking at a question in my book:

"Assuming a coil of thus & so dimensions surrounding a core of this &
that dimension & type of material, calculate: 1) The flux in the core, 2)
the flux in the air," etc.

Follow-up question:

"Assuming a second identical coil placed elsewhere on the core, calculate
induced voltage if only the flux in the iron passes through the second
coil," etc.

I had to take a year of Electrical Machinery in college, including
labs with big transformers and motors and stuff. I learned a lot from
it.

The whole text was written like that. The concept of a "perfect
transformer" was introduced much later, and only in certain contexts. For
utility purposes, perfect transformers are undesirable!

Is that because they conduct short circuits too well?

John

On Sun, 06 Jun 2004 14:32:46 -0700, John Larkin
wrote:

[snip]
I had to take a year of Electrical Machinery in college, including
labs with big transformers and motors and stuff. I learned a lot from
it.

[snip]
John


Same here. Now-a-days I think they just point at a motor and grunt,
judging from what little today's engineering graduates seem to know
:-(

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice480)460-2350 | |
| E-mail Address at Website Fax480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.

"John Larkin" wrote in
message ...
On Sun, 6 Jun 2004 16:30:39 -0400, "BFoelsch"
wrote:

It's interesting, when I learned this stuff ( I won't tell you when, but
my
then-new text was published in 1935!), albeit in the context of
utility/power engineering, about the LAST thing we learned was the tricks
and conventions about turns ratios, etc.

Just looking at a question in my book:

"Assuming a coil of thus & so dimensions surrounding a core of this &
that dimension & type of material, calculate: 1) The flux in the core, 2)
the flux in the air," etc.

Follow-up question:

"Assuming a second identical coil placed elsewhere on the core,
calculate
induced voltage if only the flux in the iron passes through the second
coil," etc.

I had to take a year of Electrical Machinery in college, including
labs with big transformers and motors and stuff. I learned a lot from
it.

The whole text was written like that. The concept of a "perfect
transformer" was introduced much later, and only in certain contexts. For
utility purposes, perfect transformers are undesirable!

Is that because they conduct short circuits too well?

Primarily. In olden times transformers were designed for the best possible
"regulation." After power systems got stiff enough, however, the concept was
changed to allow transformers to limit fault current, and the old measure of
"regulation" was replaced with the modern day "impedance."

Extra credit: What is the definition of impedance, in the transformer sense?

Answer: The percentage of rated primary voltage which will produce rated
current through a short-circuited secondary.

John