# [amsat-bb] Torque Coil turns vanish?

Robert Bruninga bruninga at usna.edu
Tue Oct 28 12:28:11 UTC 2014

```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 <bruninga at usna.edu>
To: amsat-bb <amsat-bb at amsat.org>
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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