PSN-L Email List Message

Subject: coil winding
From: sean@...........
Date: Mon, 4 Sep 2000 13:39:49 -0500 (CDT)


I haven't made a formless coil for a speaker magnet because I haven't
found a temporary form of the exact dimensions to wind it on. (see my 
write up on making the formless coil.... or do you need a repeat of it).
But I haven't tried either, since I can make a reliable, reproducible 
rare earth magnet assembly with McMaster parts for $30, rather than 
dismanteling speakers.  (also written and posted).

But when you play with the formulas, the output is a function of
the total winding length. This means that the output increases directly 
as the number of turns increases. However, to get more turns into the 
same winding volume (the cylinder of wire in the magnet gap), requires
smaller wire, obviously.  Wire of 1/2 the cross section area, like
# 38 is half the area of #32 (from wire tables), will result in twice
the number of turns so twice the wire length and output. But half the
area is twice the resistance per length, so the winding resistance
increases by a factor of 4.

For example, increasing the turns conveniently by using wire of 1/2 the 
diameter with the same coil dimensions, if I use #42  enameled wire, which
is about half the diameter of #36, I will get twice the turns per layer, 
and twice the number of layers, for 4 times the turns and output. With 
the same mean length per turn, this will be four times the wire length, 
but it will measure 16 times the resistance. So I get only 4 times the 
output at 16 times the resistance, BUT I also have 16 times the Johnson 
noise due to the resistance, and it may be difficult to damp (as a moving
coil sensor) with a resistor if the magnet is not strong enough.

size diameter resistance         NEWARK    roll  length, ft  cost
#32  0.0088  164.1 
#34  0.0069  260.9
#36	 0.0055	 414.8 ohms/1000 ft.  #36E1321  1/2lb   6400     $27.33
#38	 0.0044	 659.6                #36F779   1 lb    19300     $73.49
#40	 0.0034	 1049.0
#42	 0.0028	 1659
#44	 0.0023	 2593

And another consideration for a coil used for a broadband feedback
sensor, high coil resistance leads to instability. Anything much over
100 ohms is a problem. This became quickly evident when manufacturers
tried to add feedback to existing seismometers with constants of 50
to 200 Newtons/Ampere (Volts/meter/second) but thousands of ohms.

Increasing the coil magnet output is an advantage to a point
for a simple moving coil sensor, but for feedback it does NOT increase
the output: in fact it decreases it, where k=M/(G*C). However, it
does improve the high frequency response of the feedback loop, so
there are tradeoffs.



Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]

Larry Cochrane <cochrane@..............>