DC Current in Parallel Inductors?

M

Miss_Koksuka

Guest
Hello All,

My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?

Thank you!

Desiree
 
Miss_Koksuka wrote:
Hello All,

My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?
Click to expand...
For extra bonus points, also consider what effect, if any, there would
be to your final answer if there was a current flowing in a loop through
the two inductors at time zero. (Ideal parallel inductors will happily
support a circulating current without loss.)
 
"Miss_Koksuka" <desiree_koksuka@yahoo.com> wrote in message
news:f7f121cd-f823-416e-b821-70608cee0106@x5g2000prf.googlegroups.com...
Hello All,

My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?

Thank you!

Desiree
Click to expand...
Would the troll like a cookie?
Nice troll, come here, gootch gootchy goo

SPLAT
 
Miss_Koksuka wrote:
On Jul 2, 6:17 pm, Dan Coby <adc...@earthlink.net> wrote:
Miss_Koksuka wrote:
Hello All,
My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?
For extra bonus points, also consider what effect, if any, there would
be to your final answer if there was a current flowing in a loop through
the two inductors at time zero. (Ideal parallel inductors will happily
support a circulating current without loss.)

Thanks Dan, but how would I calculate such a thing? And I'm still not
sure whether two different value inductors in parallel will share the
mainline DC current unequally or equally after reaching steady state.
(I feel, but half my class does not, that after steady state is
reached that both parallel inductors could simply be replaced by a
zero ohm piece of wire...).
Click to expand...
Instead of looking at the steady state, you need to look at how you get there.

The remainder of your homework problem is left to the student.
 
On Jul 2, 6:17 pm, Dan Coby <adc...@earthlink.net> wrote:
Miss_Koksuka wrote:
Hello All,

    My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch?  If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?

For extra bonus points, also consider what effect, if any, there would
be to your final answer if there was a current flowing in a loop through
the two inductors at time zero.  (Ideal parallel inductors will happily
support a circulating current without loss.)
Click to expand...
Thanks Dan, but how would I calculate such a thing? And I'm still not
sure whether two different value inductors in parallel will share the
mainline DC current unequally or equally after reaching steady state.
(I feel, but half my class does not, that after steady state is
reached that both parallel inductors could simply be replaced by a
zero ohm piece of wire...).

Thanks!

-Desiree
 
Dan Coby <adcoby@earthlink.net> wrote:

Instead of looking at the steady state, you need to look at how you get there.
Click to expand...
Has anyone got a Digkey part number for some of these inductors? Sounds
like they would be really useful.

--
 
"Dan Coby" <adcoby@earthlink.net> wrote in message
news:p_WdnVVgSNPJw9DXnZ2dnUVZ_vmdnZ2d@earthlink.com...
Miss_Koksuka wrote:
On Jul 2, 6:17 pm, Dan Coby <adc...@earthlink.net> wrote:
Miss_Koksuka wrote:
Hello All,
My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?
For extra bonus points, also consider what effect, if any, there would
be to your final answer if there was a current flowing in a loop through
the two inductors at time zero. (Ideal parallel inductors will happily
support a circulating current without loss.)

Thanks Dan, but how would I calculate such a thing? And I'm still not
sure whether two different value inductors in parallel will share the
mainline DC current unequally or equally after reaching steady state.
(I feel, but half my class does not, that after steady state is
reached that both parallel inductors could simply be replaced by a
zero ohm piece of wire...).
,
Instead of looking at the steady state, you need to look at how you get
there.

The remainder of your homework problem is left to the student.
Click to expand...
Right, this is a theoretical question about ideal inductors, so they
never actually reach "steady state," meaning constant current in this case.
For a circuit made of ideal components, you would have to define steady
state:
in the limit as t increases without bound.
 
Miss_Koksuka wrote:
Hello All,

My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?
Click to expand...
It doesn't. An ideal inductor has zero resistance of course, and you have
two of these lovely devices in parallel.
But ask yourself how long does it take to reach steady state in an ideal
circuit? Tell your teacher you'll give him the answer after "the big
crunch".

Dave.

--
================================================
Check out my Electronics Engineering Video Blog & Podcast:
http://www.alternatezone.com/eevblog/
 
On Thu, 2 Jul 2009 15:49:01 -0700 (PDT), Miss_Koksuka
<desiree_koksuka@yahoo.com> wrote:

Hello All,

My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?

Thank you!

Desiree
Click to expand...

I guess we assume no initial currents before we switch on the supply.

Put the two inductors, in parallel, into a black box. Now you have 10
volts through 1K ohms driving a 0.909 uH inductor.

Calculate the voltage versus time across the black box.

Now consider what would happen if that voltage profile were applied to
the 1 uH inductor, and separately to the 10 uH inductor.

The issue isn't so much what the circuit looks like "after it reaches
its steady state" but the path it took to get there. An inductor
integrates voltage into current, so it remembers everything that ever
happened to it.

What did Spice say?

John
 
On Fri, 03 Jul 2009 02:51:07 +0100, nospam <nospam@please.invalid>
wrote:

Dan Coby <adcoby@earthlink.net> wrote:

Instead of looking at the steady state, you need to look at how you get there.

Has anyone got a Digkey part number for some of these inductors? Sounds
like they would be really useful.
Click to expand...
Yeah, room temperature superconductors will be mighty handy. Finite-Q
filter designs are a nuisance.

John
 
"Miss_Koksuka" <desiree_koksuka@yahoo.com> wrote in message
news:f7f121cd-f823-416e-b821-70608cee0106@x5g2000prf.googlegroups.com...
Hello All,

My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?

Thank you!

Desiree

Given the level of the question steady state means after all the math is
Click to expand...
done. Parallel devices have equal voltages.

Tom
 
"Miss_Koksuka" <desiree_koksuka@yahoo.com> wrote in message
news:4e070800-fa2a-47e1-9982-d053d668794f@v15g2000prn.googlegroups.com...
On Jul 3, 12:26 am, John Larkin
<jjSNIPlar...@highTHISlandtechnology.com> wrote:
On Thu, 2 Jul 2009 15:49:01 -0700 (PDT), Miss_Koksuka



desiree_koks...@yahoo.com> wrote:
Hello All,

My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?

Thank you!

Desiree

I guess we assume no initial currents before we switch on the supply.

Put the two inductors, in parallel, into a black box. Now you have 10
volts through 1K ohms driving a 0.909 uH inductor.

Calculate the voltage versus time across the black box.

Now consider what would happen if that voltage profile were applied to
the 1 uH inductor, and separately to the 10 uH inductor.

The issue isn't so much what the circuit looks like "after it reaches
its steady state" but the path it took to get there. An inductor
integrates voltage into current, so it remembers everything that ever
happened to it.

What did Spice say?

John
Click to expand...
Thanks guys. I'm trying to put all your answers together to
clearly figure this all out, but its tough!
John, here is a clearer explanation, and what I am seeing in
Spice:
In a circuit with a (10V) DC power supply, and a series current
limiting (100 Ohm) resistor, and two ideal* inductors (with no mutual
coupling) that are in parallel with each other -- one being 1uH and
the other 10uH -- why do the DC currents take >>5xL/R to reach
equality in each branch? Why should an ideal inductor of ANY value
have ANY effect whatsoever on the DC current *after* it reaches its
steady state?
My Spice simulator shows that it takes a HUGE amount of time
(25ms) to reach equal current of 50mA in each branch, and until then
the current in the 10uH branch is 9.1mA, and the current in the 1uH
branch is 91mA. Since 25ms is WAY past five time constants, why does
it take so darn long to even-out the currents in each leg?

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

(* Rser=0.001 to make Spice happy.) <=======That explains it
right there

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


The current in ideal inductors would never even out, it would always have a
ratio of 10:1
But you have give the inductors resistance. They are no longer ideal, and
the current
will even out. The time constant for this effect is determined by the
series resistance
of the inductors.
 
On Jul 3, 12:26 am, John Larkin
<jjSNIPlar...@highTHISlandtechnology.com> wrote:
On Thu, 2 Jul 2009 15:49:01 -0700 (PDT), Miss_Koksuka



desiree_koks...@yahoo.com> wrote:
Hello All,

   My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch?  If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?

Thank you!

Desiree

I guess we assume no initial currents before we switch on the supply.

Put the two inductors, in parallel, into a black box. Now you have 10
volts through 1K ohms driving a 0.909 uH inductor.

Calculate the voltage versus time across the black box.

Now consider what would happen if that voltage profile were applied to
the 1 uH inductor, and separately to the 10 uH inductor.

The issue isn't so much what the circuit looks like "after it reaches
its steady state" but the path it took to get there. An inductor
integrates voltage into current, so it remembers everything that ever
happened to it.

What did Spice say?

John
Click to expand...
Thanks guys. I'm trying to put all your answers together to
clearly figure this all out, but its tough!
John, here is a clearer explanation, and what I am seeing in
Spice:
In a circuit with a (10V) DC power supply, and a series current
limiting (100 Ohm) resistor, and two ideal* inductors (with no mutual
coupling) that are in parallel with each other -- one being 1uH and
the other 10uH -- why do the DC currents take >>5xL/R to reach
equality in each branch? Why should an ideal inductor of ANY value
have ANY effect whatsoever on the DC current *after* it reaches its
steady state?
My Spice simulator shows that it takes a HUGE amount of time
(25ms) to reach equal current of 50mA in each branch, and until then
the current in the 10uH branch is 9.1mA, and the current in the 1uH
branch is 91mA. Since 25ms is WAY past five time constants, why does
it take so darn long to even-out the currents in each leg?

(* Rser=0.001 to make Spice happy.)

Confused,

-Desiree

* C:\Program Files\LTC\LTspiceIV\Draft-INDS.asc
L1 N001 N002 1ľ Rser=0.001
R1 N002 0 100
V1 N001 0 10
L2 N001 N002 10ľ Rser=0.001
..TRAN 500us 0.05 UIC
..PLOT TRAN I(L1) I(L2)
..backanno
..end
 
On Thu, 02 Jul 2009 15:49:01 -0700, Miss_Koksuka wrote:

Hello All,

My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k Ohm
resistor, and two (ideal) inductors in parallel with each other, one
being 1uH and the other 10uH, will the DC currents be the exact same in
each inductor branch after reaching steady state, or will they be less
(by 10X) in the 10uH branch? If so, why should an ideal inductor of ANY
value have any effect whatsoever on DC current after it reaches its
steady state?
Click to expand...
Perhaps one of the things your instructor wanted to do was to wean you
away from using Spice -- which is a fine tool for some things -- for
everything.

Try doing it using Laplace domain analysis; John Larkin's suggestion of
finding the voltage and then the current is to the point if you want to
simplify the math.

Remember that Spice is a real-world tool, and an ideal inductor is not a
real-world device. So using Spice and expecting it to cancel out
infinities is inappropriate. OTOH, this is a fairly simple problem with
linear circuit elements -- hence my suggestion of using Laplace analysis.

--
www.wescottdesign.com
 
"Miss_Koksuka" <desiree_koksuka@yahoo.com> wrote in message
news:77758290-02a3-41f6-bc36-905990685674@x6g2000prc.googlegroups.com...
On Jul 3, 8:58 am, "Michael Robinson" <nos...@billburg.com> wrote:
"Miss_Koksuka" <desiree_koks...@yahoo.com> wrote in message

news:4e070800-fa2a-47e1-9982-d053d668794f@v15g2000prn.googlegroups.com...
On Jul 3, 12:26 am, John Larkin



jjSNIPlar...@highTHISlandtechnology.com> wrote:
On Thu, 2 Jul 2009 15:49:01 -0700 (PDT), Miss_Koksuka

desiree_koks...@yahoo.com> wrote:
Hello All,

My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?

Thank you!

Desiree

I guess we assume no initial currents before we switch on the supply.

Put the two inductors, in parallel, into a black box. Now you have 10
volts through 1K ohms driving a 0.909 uH inductor.

Calculate the voltage versus time across the black box.

Now consider what would happen if that voltage profile were applied to
the 1 uH inductor, and separately to the 10 uH inductor.

The issue isn't so much what the circuit looks like "after it reaches
its steady state" but the path it took to get there. An inductor
integrates voltage into current, so it remembers everything that ever
happened to it.

What did Spice say?

John

Thanks guys. I'm trying to put all your answers together to
clearly figure this all out, but its tough!
John, here is a clearer explanation, and what I am seeing in
Spice:
In a circuit with a (10V) DC power supply, and a series current
limiting (100 Ohm) resistor, and two ideal* inductors (with no mutual
coupling) that are in parallel with each other -- one being 1uH and
the other 10uH -- why do the DC currents take >>5xL/R to reach
equality in each branch? Why should an ideal inductor of ANY value
have ANY effect whatsoever on the DC current *after* it reaches its
steady state?
My Spice simulator shows that it takes a HUGE amount of time
(25ms) to reach equal current of 50mA in each branch, and until then
the current in the 10uH branch is 9.1mA, and the current in the 1uH
branch is 91mA. Since 25ms is WAY past five time constants, why does
it take so darn long to even-out the currents in each leg?

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

(* Rser=0.001 to make Spice happy.) <=======That explains it
right there

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

The current in ideal inductors would never even out, it would always have
a
ratio of 10:1
But you have give the inductors resistance. They are no longer ideal, and
the current
will even out. The time constant for this effect is determined by the
series resistance
of the inductors.
Click to expand...
Thanks Mike. So at least I'm not doing anything wrong with the Spice
simulator! But I'll not rest until I find out exactly *why* this
occurs. The mechanism behind it has me completely baffled, since it
is not an LC tank circuit, so energy is not being exchanged back and
forth between an inductor and capacitor. It is merely two inductors
in parallel (I always assumed that such a circuit would simply act
like single, lower value, inductor). So strange, but none of my
(many) school books seems to cover any of this, they only say that an
ideal inductor is a "short" to DC.

Thanks again,

-Desiree

Are you pursuing an engineering degree?
 
On Jul 3, 8:58 am, "Michael Robinson" <nos...@billburg.com> wrote:
"Miss_Koksuka" <desiree_koks...@yahoo.com> wrote in message

news:4e070800-fa2a-47e1-9982-d053d668794f@v15g2000prn.googlegroups.com...
On Jul 3, 12:26 am, John Larkin



jjSNIPlar...@highTHISlandtechnology.com> wrote:
On Thu, 2 Jul 2009 15:49:01 -0700 (PDT), Miss_Koksuka

desiree_koks...@yahoo.com> wrote:
Hello All,

My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?

Thank you!

Desiree

I guess we assume no initial currents before we switch on the supply.

Put the two inductors, in parallel, into a black box. Now you have 10
volts through 1K ohms driving a 0.909 uH inductor.

Calculate the voltage versus time across the black box.

Now consider what would happen if that voltage profile were applied to
the 1 uH inductor, and separately to the 10 uH inductor.

The issue isn't so much what the circuit looks like "after it reaches
its steady state" but the path it took to get there. An inductor
integrates voltage into current, so it remembers everything that ever
happened to it.

What did Spice say?

John

     Thanks guys.  I'm trying to put all your answers together to
clearly figure this all out, but its tough!
     John, here is a clearer explanation, and what I am seeing in
Spice:
     In a circuit with a (10V) DC power supply, and a series current
limiting (100 Ohm) resistor, and two ideal* inductors (with no mutual
coupling) that are in parallel with each other -- one being 1uH and
the other 10uH -- why do the DC currents take >>5xL/R to reach
equality in each branch? Why should an ideal inductor of ANY value
have ANY effect whatsoever on the DC current *after* it reaches its
steady state?
    My Spice simulator shows that it takes a HUGE amount of time
(25ms) to reach equal current of 50mA in each branch, and until then
the current in the 10uH branch is 9.1mA, and the current in the 1uH
branch is 91mA. Since 25ms is WAY past five time constants, why does
it take so darn long to even-out the currents in each leg?

============================
(* Rser=0.001 to make Spice happy.)            <=======That explains it
right there

============================
The current in ideal inductors would never even out, it would always have a
ratio of 10:1
But you have give the inductors resistance.  They are no longer ideal, and
the current
will even out.  The time constant for this effect is determined by the
series resistance
of the inductors.
Click to expand...
Thanks Mike. So at least I'm not doing anything wrong with the Spice
simulator! But I'll not rest until I find out exactly *why* this
occurs. The mechanism behind it has me completely baffled, since it
is not an LC tank circuit, so energy is not being exchanged back and
forth between an inductor and capacitor. It is merely two inductors
in parallel (I always assumed that such a circuit would simply act
like single, lower value, inductor). So strange, but none of my
(many) school books seems to cover any of this, they only say that an
ideal inductor is a "short" to DC.

Thanks again,

-Desiree
 
Miss_Koksuka wrote:
Are you pursuing an engineering degree?

Yes Mike, but I am only in my first year of electronics.
Click to expand...
Okay. Now we need to know a little more about what you have learned.

Have you had calculus? Do you know what this equation means and how
to use it:

v = L di/dt
 
Thanks guys. I'm trying to put all your answers together to
clearly figure this all out, but its tough!
Click to expand...
Your intuition might work better with capacitors rather than inductors.

What would you expect if you had a resistor, small cap, and big cap
in series?


--
These are my opinions, not necessarily my employer's. I hate spam.
 
"Miss_Koksuka" <desiree_koksuka@yahoo.com> wrote in message
news:f31ae1f5-ccce-4fdd-a188-095718b7a8ce@m3g2000pri.googlegroups.com...
On Jul 3, 12:26 pm, "Michael Robinson" <nos...@billburg.com> wrote:
"Miss_Koksuka" <desiree_koks...@yahoo.com> wrote in message

news:77758290-02a3-41f6-bc36-905990685674@x6g2000prc.googlegroups.com...
On Jul 3, 8:58 am, "Michael Robinson" <nos...@billburg.com> wrote:



"Miss_Koksuka" <desiree_koks...@yahoo.com> wrote in message

news:4e070800-fa2a-47e1-9982-d053d668794f@v15g2000prn.googlegroups.com...
On Jul 3, 12:26 am, John Larkin

jjSNIPlar...@highTHISlandtechnology.com> wrote:
On Thu, 2 Jul 2009 15:49:01 -0700 (PDT), Miss_Koksuka

desiree_koks...@yahoo.com> wrote:
Hello All,

My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will
they
be less (by 10X) in the 10uH branch? If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?

Thank you!

Desiree

I guess we assume no initial currents before we switch on the supply.

Put the two inductors, in parallel, into a black box. Now you have 10
volts through 1K ohms driving a 0.909 uH inductor.

Calculate the voltage versus time across the black box.

Now consider what would happen if that voltage profile were applied to
the 1 uH inductor, and separately to the 10 uH inductor.

The issue isn't so much what the circuit looks like "after it reaches
its steady state" but the path it took to get there. An inductor
integrates voltage into current, so it remembers everything that ever
happened to it.

What did Spice say?

John

Thanks guys. I'm trying to put all your answers together to
clearly figure this all out, but its tough!
John, here is a clearer explanation, and what I am seeing in
Spice:
In a circuit with a (10V) DC power supply, and a series current
limiting (100 Ohm) resistor, and two ideal* inductors (with no mutual
coupling) that are in parallel with each other -- one being 1uH and
the other 10uH -- why do the DC currents take >>5xL/R to reach
equality in each branch? Why should an ideal inductor of ANY value
have ANY effect whatsoever on the DC current *after* it reaches its
steady state?
My Spice simulator shows that it takes a HUGE amount of time
(25ms) to reach equal current of 50mA in each branch, and until then
the current in the 10uH branch is 9.1mA, and the current in the 1uH
branch is 91mA. Since 25ms is WAY past five time constants, why does
it take so darn long to even-out the currents in each leg?

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

(* Rser=0.001 to make Spice happy.) <=======That explains it
right there

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

The current in ideal inductors would never even out, it would always
have
a
ratio of 10:1
But you have give the inductors resistance. They are no longer ideal,
and
the current
will even out. The time constant for this effect is determined by the
series resistance
of the inductors.

Thanks Mike. So at least I'm not doing anything wrong with the Spice
simulator! But I'll not rest until I find out exactly *why* this
occurs. The mechanism behind it has me completely baffled, since it
is not an LC tank circuit, so energy is not being exchanged back and
forth between an inductor and capacitor. It is merely two inductors
in parallel (I always assumed that such a circuit would simply act
like single, lower value, inductor). So strange, but none of my
(many) school books seems to cover any of this, they only say that an
ideal inductor is a "short" to DC.

Thanks again,

-Desiree

Are you pursuing an engineering degree?
Click to expand...
Yes Mike, but I am only in my first year of electronics.

Good. I'm going to show you a simple solution. It amounts to a fairly
rigorous demonstration that you can show your instructor.
Assume the inductors have zero current in them at t=0, which is when you
apply the voltage.
Dan Coby gave you formula v=L*(di/dt)
I'm going to call the inductances L1 and L2. Simillarly,
currents in the inductors I1 and I2. I1 and I2 are variables, functions of
t (time).
Now, v is always equal for both inductors becuase they are in parallel.
Therefore L1 (dI1/dt) = L2 (dI2/dt)
Say L2 is the "bigger" inductor. L2 = 10 L1 implies
dI1/dt = 10 dI2/dt which we can integrate over time:
I1 = 10 I2 + C (I hope you've had some basic calculus)
now, C has to be zero because at t=0, I1 = I2 = 0
so I1 = 10 I2
and that's always true, for any value of t
 
On Fri, 3 Jul 2009 06:15:27 -0700 (PDT), Miss_Koksuka
<desiree_koksuka@yahoo.com> wrote:

On Jul 3, 12:26 am, John Larkin
jjSNIPlar...@highTHISlandtechnology.com> wrote:
On Thu, 2 Jul 2009 15:49:01 -0700 (PDT), Miss_Koksuka



desiree_koks...@yahoo.com> wrote:
Hello All,

   My teacher gave us a problem that is driving me absolutely crazy,
and my Spice simulator is supplying odd answers. His question: In a
circuit with a 10V DC power supply, and a series current limiting 1k
Ohm resistor, and two (ideal) inductors in parallel with each other,
one being 1uH and the other 10uH, will the DC currents be the exact
same in each inductor branch after reaching steady state, or will they
be less (by 10X) in the 10uH branch?  If so, why should an ideal
inductor of ANY value have any effect whatsoever on DC current after
it reaches its steady state?

Thank you!

Desiree

I guess we assume no initial currents before we switch on the supply.

Put the two inductors, in parallel, into a black box. Now you have 10
volts through 1K ohms driving a 0.909 uH inductor.

Calculate the voltage versus time across the black box.

Now consider what would happen if that voltage profile were applied to
the 1 uH inductor, and separately to the 10 uH inductor.

The issue isn't so much what the circuit looks like "after it reaches
its steady state" but the path it took to get there. An inductor
integrates voltage into current, so it remembers everything that ever
happened to it.

What did Spice say?

John

Thanks guys. I'm trying to put all your answers together to
clearly figure this all out, but its tough!
John, here is a clearer explanation, and what I am seeing in
Spice:
In a circuit with a (10V) DC power supply, and a series current
limiting (100 Ohm) resistor, and two ideal* inductors (with no mutual
coupling) that are in parallel with each other -- one being 1uH and
the other 10uH -- why do the DC currents take >>5xL/R to reach
equality in each branch? Why should an ideal inductor of ANY value
have ANY effect whatsoever on the DC current *after* it reaches its
steady state?
Click to expand...

As noted, you have to include the entire history of the voltage
applied to the inductor to know its current.

A shorted inductor of unknown history has an indeterminate current.
Ditto two paralleled inductors.


My Spice simulator shows that it takes a HUGE amount of time
(25ms) to reach equal current of 50mA in each branch, and until then
the current in the 10uH branch is 9.1mA, and the current in the 1uH
branch is 91mA. Since 25ms is WAY past five time constants, why does
it take so darn long to even-out the currents in each leg?
Click to expand...
Spice artifact, essentially some minimum (non-zero) resistance
parameter. For ideal inductors, you wouldn't see that. The voltage
waveform is a spike up to 10 volts, exponentially decaying with a time
constant of about 9 nanoseconds.

Spice often lies.


(* Rser=0.001 to make Spice happy.)
Click to expand...

That also shoots down the concept of "ideal inductor."


John
 
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