Estimating the Number of Turns of an Inductor

[Reg Edwards]

The answer to all your problems. The following is an exact solution.

1, Wind a known number of extra turns N2 over the coil.

2. Measure inductance L1 of the coil.

3. Short-circuit the extra number of turns and measure coil inductance L2
again.

4. Calculate the coupling coefficient between the two coils -

K = Sqrt( ( 1 - L2 / L1 )

5. Apply a voltage V1 across the coil and measure the voltage V2 at a light
load across the extra turns N2.

6. The number of turns on the coil is -

N1 = N2 * V1 / V2 / K

===
Reg.

 

[Watson A.Name - \Watt Sun, the Dark Remover\]

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.

I could have two inductors that look the same on their outside, but have
radically different inductance values. I could have two inductors that
are of the same inductance but have radically different core material
and size.

I'm just trying to get some idea of what makes up the inductor. Turns
of wire is just one of them. Another is what kind of core material is
used. I think that if I know the turns, it will give some idea of the
core material.

[snip]

Cheers,
Tom

"Watson A.Name - \"Watt Sun, the Dark Remover\""

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".

--
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[Terry Given]

I could have two inductors that look the same on their outside, but have
radically different inductance values. I could have two inductors that
are of the same inductance but have radically different core material
and size.

if the core has a large air gap (> 1mm), inductance is governed pretty much
by the air gap, and is pretty much independant of the core material.

I'm just trying to get some idea of what makes up the inductor. Turns
of wire is just one of them. Another is what kind of core material is
used. I think that if I know the turns, it will give some idea of the
core material.

cheers
Terry

 

[John Larkin]

** Not the point.

If applying the laws of electromagnetics to solve an electromagnetic
problem is not the point, then what is the point?

John

 

[Tom Bruhns]

The answer to all your problems. The following is an exact solution.

1, Wind a known number of extra turns N2 over the coil.

2. Measure inductance L1 of the coil.

3. Short-circuit the extra number of turns and measure coil inductance L2
again.

4. Calculate the coupling coefficient between the two coils -

K = Sqrt( ( 1 - L2 / L1 )

5. Apply a voltage V1 across the coil and measure the voltage V2 at a light
load across the extra turns N2.

6. The number of turns on the coil is -

N1 = N2 * V1 / V2 / K

That's just what I thought, too, at first, Reg, but then thought about
this case:

Two coils, identical number of turns. Coil L1 has a self-inductance
of 100uH. Coil L2 has a self-inductance of 400uH. The coils are
arranged so that K=0.100. Apply 1V. to coil L1. Based on K and the
known turns ratio and (6) above, I'd expect 0.1V measured across coil
L2. But I believe we'll actually see 0.2V, as the mutual inductance
will be 20uH, and V(L2)=M*V(L1)/L1.

I had actually come around to believing what you first wrote, that
indeed you can't get there from here...short of dissection or xrays or
equivalent.

Cheers,
Tom

 

[Tom Bruhns]

If I want to make a reasonable facsimile of the coil, I have to know a
bit about it, like what kind of ferrite material and how many turns it
has. I thought that getting the number of tuens would be alot of help
with this.

So what's wrong with just measuring the electrical properties of the
coil, if you really can't take it apart? You can measure inductance
as a function of frequency, and as a function of bias current, and you
can measure loss (or Q or ESR). It's also worthwhile knowing from the
circuit in which the inductor is used what the important
characteristics are: that is, from the circuit, you should be able to
specify the inductor.

Certainly you should be able to estimate the number of turns through
transformer turns ratio, in spite of the possible errors, because you
should be able to get pretty good coupling between the coils. If K=1
there can be no difference in inductance between two coils with the
same number of turns. As Reg and others have pointed out, though, the
inductance is determined more by the air gap than by the core
permeability, so knowing the approximate number of turns may not tell
you as much about the core material as you had hoped.

Cheers,
Tom

 

[Reg Edwards]

The answer to all your problems. The following is an exact solution.

That's just what I thought, too, at first, Reg, but then thought about
this case:

Two coils, identical number of turns. Coil L1 has a self-inductance
of 100uH. Coil L2 has a self-inductance of 400uH. The coils are
arranged so that K=0.100. Apply 1V. to coil L1. Based on K and the
known turns ratio and (6) above, I'd expect 0.1V measured across coil
L2. But I believe we'll actually see 0.2V, as the mutual inductance
will be 20uH, and V(L2)=M*V(L1)/L1.

I had actually come around to believing what you first wrote, that
indeed you can't get there from here...short of dissection or xrays or
equivalent.

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

Tom, you may be right when the surrounding field is distorted by a magnetic
core which affects one coil more than the other. It brings yet another
variable into play. But a simple ferrite rod through both (as may be the
present case) will not seriously affect things. Having measured coupling
coefficient K and used it to correct the result I think most of the error
has been eliminated.

You may very well get a different value for the mutual inductance M. But M
is not needed or used in the final calculation. Its value is irrelevant.

Nevertheless, I will concede - replace "Exact solution" with "Approximate
solution" to the problem. ;o)
===
Reg, G4FGQ

 

[Tom Bruhns]

....
=================

Tom, you may be right when the surrounding field is distorted by a magnetic
core which affects one coil more than the other. It brings yet another
variable into play. But a simple ferrite rod through both (as may be the
present case) will not seriously affect things. Having measured coupling
coefficient K and used it to correct the result I think most of the error
has been eliminated.

You may very well get a different value for the mutual inductance M. But M
is not needed or used in the final calculation. Its value is irrelevant.

Nevertheless, I will concede - replace "Exact solution" with "Approximate
solution" to the problem. ;o)

Hi Reg...

Actually, I was thinking in my example (two coils, equal number of
turns but differing inductances) about just simple air-core coils, one
short and larger diameter, the other longer and smaller diameter.
When solutions become "approximate," I like to have some sense of
where the approximations fall apart. If I make the measurement as
suggested, what's the maximum error I'm likely to find? (And of
course I think we both agree that there's not a whole lot of value
determining the number of turns anyway...)

In this case, I think it's interesting to think about what happens
when K approaches unity. Certainly K is measurable. If we know K,
can we put a bound on the error in assuming that the inductance ratio
is the square of the turns ratio? I suppose so. Clearly if K=1, the
coils share 100% of their fields, and the inductance ratio must be
exactly the square of the turns ratio. So, I think practical advice
for the OP would be to wind the test turns tightly over the existing
coil; try to get K as high as you can. If your measured K is less
than 0.5, be a lot more worried about the accuracy of the turns-ratio
result than if K is above 0.9.

Regarding M: of course you don't need to use it but since it's
uniquely determined by M=K*sqrt(L1*L2), you can just as well
reformulate the answer in terms of M instead of K, with nothing lost.
Sometimes thinking in terms of K gives me useful insights that I'd
miss by using M, and sometimes it's the other way around. (Some might
say that K is determined by M rather than the other way around, I
suppose... ;-)

A tad more red wine?

Cheers,
Tom

 

[Phil Allison]

"John Larkin"
"Phil Allison"
"Watson A.Name"

If applying the laws of electromagnetics to solve an electromagnetic
problem is not the point, then what is the point?

** That is a nice straw man.



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

 

[John Larkin]

"John Larkin"
"Phil Allison"
"Watson A.Name"



** That is a nice straw man.

That still doesn't seem to be the point.

John

 

[Terry Given]

That still doesn't seem to be the point.

John

NZ$0.02: the point appears to be providing PA with a platform from which to
conduct a minimum-bandwidth, maximum-obfuscation argument. Perhaps its an
OOS thing, preventing the typing of suitably lucid comments. Beats the hell
out of me how maxwells laws arent the point when discussing an inductor....

oh, and if it was my inductor, I'd gut the damn thing with my trusty
leatherman, and count the number of turns (if I had more than 1). I can
count way past 10,000.....and have done so before. I do this to every
magnetic device I design/work with, to ensure we got what we wanted, or
figure out whats there in the first place - construction is usually pretty
important.

cheers
Terry

 

[Tom Bruhns]

** Strange then that a one turn loop could be up to a metre in diameter
with no change in voltage.

If you maintain the coefficient of coupling close to unity, it
shouldn't be strange. OTOH, if the coefficient of coupling, K, is
low, as with a 1 meter diameter loop coupled to an 0.1 meter diameter
solenoid coil, all in air, expect the voltage ratio to be
significantly different from N*K.

Do you not believe in clamp-on ammeters?

Cheers,
Tom

 

[John Larkin]

NZ$0.02: the point appears to be providing PA with a platform from which to
conduct a minimum-bandwidth, maximum-obfuscation argument. Perhaps its an
OOS thing, preventing the typing of suitably lucid comments. Beats the hell
out of me how maxwells laws arent the point when discussing an inductor....

oh, and if it was my inductor, I'd gut the damn thing with my trusty
leatherman, and count the number of turns (if I had more than 1).

If it was my inductor, I'd measure L and DCR and buy an equivalent
from Renco or somebody.

Roger the obfuscation.


John

 

[Watson A.Name \Watt Sun - the Dark Remover\]

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



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

If I want to know what the core material is, then I need to know how
many turns it has, given a known inductance. But even if I decide to
buy rather than build one, I will have to determine the number of turns
so I can buy a coil like it, with approximately the same number of
turns. Like you said below, it might be cheaper to buy one.

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.

Can't ask the manufacturere if one doesn't know who made the coil to
begin with.

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.

[snip]

 

[Rich Grise]

Roger the obfuscation.

Roger's not an obfuscation! He's a rabbit!

Cheers!
Rich

 

[John Larkin]

Roger's not an obfuscation! He's a rabbit!

Cheers!
Rich

So, let's try it:

Who framed Roger the obfuscation?

Darn, you're right, it doesn't work!

John

 

[Spehro Pefhany]

Roger's not an obfuscation! He's a rabbit!

"Roger" is not being used as a proper name here. It's a transitive
verb.


Best regards,
Spehro Pefhany

 

[Terry Given]

"Roger" is not being used as a proper name here. It's a transitive
verb.


Best regards,
Spehro Pefhany

as in "to Roger" ?

Cheers
Terry

 

[Rich Grise]

as in "to Roger" ?

Yes, and if you've ever had your obfuscation Rogered, you know how
painful that can be!

Cheers!
Rich

 

[Terry Given]

Yes, and if you've ever had your obfuscation Rogered, you know how
painful that can be!

Cheers!
Rich

Bugger

cheers
Terry