Torque Coil turns vanish?
I keep coming up with the number of turns of a torque coil vanishing with respect to torque (for a given wire size).
Given that Torque = A x B x I x n where A is area, B is field, I is current and n is number of turns.
But I is inversely proportional to the length of wire (resistance). I = V/R but R is proportional to n.
Hence the number of turns. So I is proportional to V/n.
But V is fixed in the cubesat, (5v) area is fixed (where we can put the coil) and so plugging in,
T = A x B x V/n x n and the n’s cancel.
So I can get the same torque with –any- number of turns for a given size wire.
What changes with turns, of course, is the current. So to save spacecraft power, the more turns, the less current, and thus, the least power for the same torque.
Interesting result.
We ended up with 600 turns of #34 wire to keep the current at 25 mA so we can drive it directly from a CPU pin.
I wonder where’s the limit? Infinitie number of turns… Zero current? I guess you run out of space.
AH HA! Yes, that was it. We started with #30 wire but it took so many turns to get to below the 25 mA, that there wasn’t room for the coil. So we went to smaller wire and ended up with the #34 to give us a ¼” thick coil with 600 turns to give 25 mA.
But I just always found it interesting that once you have chosen the wire size, the number of turns is only determined by the current you want. The Torque is constant.
Bob, WB4APR
That's an interesting technique, but as the CPU pin turns off and the magnetic field collapses, don't you get a huge spike in voltage due to dI/dt that could kill the CPU I/O port?
We ended up with 600 turns of #34 wire to keep the current at 25 mA so we can drive it directly from a CPU pin.
73
David
-----Original Message----- From: Robert Bruninga [email protected] To: amsat-bb [email protected] Sent: Mon, 27 Oct 2014 20:35 Subject: [amsat-bb] Torque Coil turns vanish?
I keep coming up with the number of turns of a torque coil vanishing with respect to torque (for a given wire size).
Given that Torque = A x B x I x n where A is area, B is field, I is current and n is number of turns.
But I is inversely proportional to the length of wire (resistance). I = V/R but R is proportional to n.
Hence the number of turns. So I is proportional to V/n.
But V is fixed in the cubesat, (5v) area is fixed (where we can put the coil) and so plugging in,
T = A x B x V/n x n and the n’s cancel.
So I can get the same torque with –any- number of turns for a given size wire.
What changes with turns, of course, is the current. So to save spacecraft power, the more turns, the less current, and thus, the least power for the same torque.
Interesting result.
We ended up with 600 turns of #34 wire to keep the current at 25 mA so we can drive it directly from a CPU pin.
I wonder where’s the limit? Infinitie number of turns… Zero current? I guess you run out of space.
AH HA! Yes, that was it. We started with #30 wire but it took so many turns to get to below the 25 mA, that there wasn’t room for the coil. So we went to smaller wire and ended up with the #34 to give us a ¼” thick coil with 600 turns to give 25 mA.
But I just always found it interesting that once you have chosen the wire size, the number of turns is only determined by the current you want. The Torque is constant.
Bob, WB4APR _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Amen, one has to have kickback diodes to the rails to absorb that energy. (Else, poof goes the pin)
Bob
*Subject:* Re: [amsat-bb] Torque Coil turns vanish?
That's an interesting technique, but as the CPU pin turns off and the magnetic field collapses, don't you get a huge spike in voltage due to dI/dt that could kill the CPU I/O port?
We ended up with 600 turns of #34 wire to keep the current at 25 mA so we
can drive it directly from a CPU pin.
73
David
-----Original Message----- From: Robert Bruninga [email protected] To: amsat-bb [email protected] Sent: Mon, 27 Oct 2014 20:35 Subject: [amsat-bb] Torque Coil turns vanish?
I keep coming up with the number of turns of a torque coil vanishing with
respect to torque (for a given wire size).
Given that Torque = A x B x I x n where A is area, B is field, I is current
and n is number of turns.
But I is inversely proportional to the length of wire (resistance). I = V/R
but R is proportional to n.
Hence the number of turns. So I is proportional to V/n.
But V is fixed in the cubesat, (5v) area is fixed (where we can put the
coil) and so plugging in,
T = A x B x V/n x n and the n’s cancel.
So I can get the same torque with –any- number of turns for a given size
wire.
What changes with turns, of course, is the current. So to save spacecraft
power, the more turns, the less current, and thus, the least power for the
same torque.
Interesting result.
We ended up with 600 turns of #34 wire to keep the current at 25 mA so we
can drive it directly from a CPU pin.
I wonder where’s the limit? Infinitie number of turns… Zero current? I
guess you run out of space.
AH HA! Yes, that was it. We started with #30 wire but it took so many
turns to get to below the 25 mA, that there wasn’t room for the coil. So
we went to smaller wire and ended up with the #34 to give us a ¼” thick
coil with 600 turns to give 25 mA.
But I just always found it interesting that once you have chosen the wire
size, the number of turns is only determined by the current you want. The
Torque is constant.
Bob, WB4APR
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Bob,
Often the simplest things turn out to be the most interesting. Your post reminded me of something a bit similar that happened while I worked for Motorola. There was 450MHz mobile radio in the product line that had 3.5 Watts of audio power for the speaker. The brain trust in the marketing department decided that the competitors radios had 5.0 watts so we had to re-design the audio section. The design had been optimized for maximum efficiency in the small space available for the speaker but none the less we had to do it.
The speaker had to be re-designed - the wire used for the voice coil was too small to handle the 5 watts. When the wire size was increased to handle the increased current there was room for fewer turns of wire in the voice coil. The end result was the radio now had 5 watts of audio power but actually generated several dB lower sound levels than the previous efficient design with 3.5 watts of audio power. Marketing was happy - we just shook our heads.
W9KE tom...
On Mon, Oct 27, 2014 at 3:35 PM, Robert Bruninga [email protected] wrote:
I keep coming up with the number of turns of a torque coil vanishing with respect to torque (for a given wire size).
Given that Torque = A x B x I x n where A is area, B is field, I is current and n is number of turns.
But I is inversely proportional to the length of wire (resistance). I = V/R but R is proportional to n.
Hence the number of turns. So I is proportional to V/n.
But V is fixed in the cubesat, (5v) area is fixed (where we can put the coil) and so plugging in,
T = A x B x V/n x n and the n’s cancel.
So I can get the same torque with –any- number of turns for a given size wire.
What changes with turns, of course, is the current. So to save spacecraft power, the more turns, the less current, and thus, the least power for the same torque.
Interesting result.
We ended up with 600 turns of #34 wire to keep the current at 25 mA so we can drive it directly from a CPU pin.
I wonder where’s the limit? Infinitie number of turns… Zero current? I guess you run out of space.
AH HA! Yes, that was it. We started with #30 wire but it took so many turns to get to below the 25 mA, that there wasn’t room for the coil. So we went to smaller wire and ended up with the #34 to give us a ¼” thick coil with 600 turns to give 25 mA.
But I just always found it interesting that once you have chosen the wire size, the number of turns is only determined by the current you want. The Torque is constant.
Bob, WB4APR _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Yes, that's right -- it turns out that, given a target dipole (or torque), and volume, you can adjust the coil resistance by selecting the right wire gauge. If you run twice as many turns, you get twice the resistance, and so (for a fixed voltage) half the current. That gives you the same dipole, regardless. Fundamentally, the dipole is proportional to the mass of copper that's in the coil.
It also turns out that the power required to generate a given dipole varies as the square of the dipole, given constant geometry. Let that guide your ADCS sizing!
-- Luke Stras (VA3LNS)
On Mon, Oct 27, 2014 at 4:35 PM, Robert Bruninga [email protected] wrote:
I keep coming up with the number of turns of a torque coil vanishing with respect to torque (for a given wire size).
Given that Torque = A x B x I x n where A is area, B is field, I is current and n is number of turns.
But I is inversely proportional to the length of wire (resistance). I = V/R but R is proportional to n.
Hence the number of turns. So I is proportional to V/n.
But V is fixed in the cubesat, (5v) area is fixed (where we can put the coil) and so plugging in,
T = A x B x V/n x n and the n’s cancel.
So I can get the same torque with –any- number of turns for a given size wire.
What changes with turns, of course, is the current. So to save spacecraft power, the more turns, the less current, and thus, the least power for the same torque.
Interesting result.
We ended up with 600 turns of #34 wire to keep the current at 25 mA so we can drive it directly from a CPU pin.
I wonder where’s the limit? Infinitie number of turns… Zero current? I guess you run out of space.
AH HA! Yes, that was it. We started with #30 wire but it took so many turns to get to below the 25 mA, that there wasn’t room for the coil. So we went to smaller wire and ended up with the #34 to give us a ¼” thick coil with 600 turns to give 25 mA.
But I just always found it interesting that once you have chosen the wire size, the number of turns is only determined by the current you want. The Torque is constant.
Bob, WB4APR _______________________________________________ Sent via [email protected]. AMSAT-NA makes this open forum available to all interested persons worldwide without requiring membership. Opinions expressed are solely those of the author, and do not reflect the official views of AMSAT-NA. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
participants (4)
-
David G0MRF -
Luke Stras -
Robert Bruninga -
Thomas Doyle