Inductor question

[Jay]

Hi everyone. I have a question about inductors I hope someone can help
me with. It is for a physics lab and I am not too familiar with them.
I had imagined that they would have two leads like a resistor. However
I sent out for a variable inductor and received something that had
five leads - two on one side and three on the other. It also had two
larger ones on the side but I suspect they would be used for mounting
on a board. My question is about the function of the five leads here -
what exactly is what?

Thanks!
-J

 

[Jacobe Hazzard]

Jay wrote:

Hi everyone. I have a question about inductors I hope
someone can help me with. It is for a physics lab and I
am not too familiar with them. I had imagined that they
would have two leads like a resistor. However I sent out
for a variable inductor and received something that had
five leads - two on one side and three on the other. It
also had two larger ones on the side but I suspect they
would be used for mounting on a board. My question is
about the function of the five leads here - what exactly
is what?

Thanks!
-J

A straight inductor does only have two leads. Your five lead device contains
some combination of winding taps and coupled windings.

Measure the resistance between the leads with an ohmmeter, you will find
that some leads are connected by a low resistance, and by comparing the
resistances you should be able to determine the order (the pair with the
highest resistance between them are then ends of the winding, any other
wires are taps connected to the middle parts of the winding).

You may find that there are two separate windings, so that the resistance
between wires belonging to different windings is nearly infinite. This means
your inductor is also a transformer (since transformers are just coupled
inductors). It's also possible that some pins on the device aren't connected
to anything, or are connected to the case. Use your ohmmeter to figure this
out.

There should be a ferrite slug in the middle with screw taps in it, turn
this with a non-ferrous screwdriver to adjust the inductance of the windings
and/or the coupling between them.

 

[Jay]

"Jacobe Hazzard" <spamsink@ATeudoramail.DOTcom> wrote in message news:<QXESb.64340$9Ce1.3220@news04.bloor.is.net.cable.rogers.com>...

Jay wrote:
Hi everyone. I have a question about inductors I hope
someone can help me with. It is for a physics lab and I
am not too familiar with them. I had imagined that they
would have two leads like a resistor. However I sent out
for a variable inductor and received something that had
five leads - two on one side and three on the other. It
also had two larger ones on the side but I suspect they
would be used for mounting on a board. My question is
about the function of the five leads here - what exactly
is what?

Thanks!
-J

A straight inductor does only have two leads. Your five lead device contains
some combination of winding taps and coupled windings.

Jacobe - thanks very much for the information!

Take Care -
- Jay

Measure the resistance between the leads with an ohmmeter, you will find
that some leads are connected by a low resistance, and by comparing the
resistances you should be able to determine the order (the pair with the
highest resistance between them are then ends of the winding, any other
wires are taps connected to the middle parts of the winding).

You may find that there are two separate windings, so that the resistance
between wires belonging to different windings is nearly infinite. This means
your inductor is also a transformer (since transformers are just coupled
inductors). It's also possible that some pins on the device aren't connected
to anything, or are connected to the case. Use your ohmmeter to figure this
out.

There should be a ferrite slug in the middle with screw taps in it, turn
this with a non-ferrous screwdriver to adjust the inductance of the windings
and/or the coupling between them.

 

[Robert Monsen]

R.Spinks wrote:

If you have a length (l) of wire coiled to make an inductor and you stretch
the wire (ie. space the coils further) but do not change the diameter (see
attached .bmp) is the inductance the same? I understand inductance to be L =
(mu N^2 A) / l + 0.45d. So if I'm stretching the coils but not changing l
(length) will the inductance change? I guess that technically the cross
sectional area (A) would be changing slightly due to stretching to coils ...
but I'm not sure if that minor variation is negligible - since I'm not
actually changing the diameter. I thought that you compress/decompress
coils (which is what I think this example is) to tune inductance... is this
what's happening? How does a variable inductor work? I have no equipment so
I can't just coil some wire and stretch it myself to see. Insight is
appreciated. Thanks.

No, the 'l' in the formula isn't the length of wire, its the length of
the coil. Thus, stretching a coil increases the length, which decreases
the inductance.

Variable inductors generally work by using a magnetic core which is
adjusted into or out of the coil using the screw.

 

[Rich Grise]

On Saturday 18 September 2004 07:45 pm, Robert Monsen did deign to grace us
with the following:

R.Spinks wrote:
If you have a length (l) of wire coiled to make an inductor and you
stretch the wire (ie. space the coils further) but do not change the
diameter (see attached .bmp) is the inductance the same? I understand
inductance to be L = (mu N^2 A) / l + 0.45d. So if I'm stretching the
coils but not changing l (length) will the inductance change? I guess
that technically the cross sectional area (A) would be changing slightly
due to stretching to coils ... but I'm not sure if that minor variation
is negligible - since I'm not
actually changing the diameter. I thought that you compress/decompress
coils (which is what I think this example is) to tune inductance... is
this what's happening? How does a variable inductor work? I have no
equipment so I can't just coil some wire and stretch it myself to see.
Insight is appreciated. Thanks.




No, the 'l' in the formula isn't the length of wire, its the length of
the coil. Thus, stretching a coil increases the length, which decreases
the inductance.

Yabbut, it also changes the pitch, so it's not linear, or even second-

order, I think.

Good Luck!
Rich