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Solenoid



Hi all,
I would apreciate your help with my following
problem:
I'm making two solenoids whose inductance I
want to measure. The way I do it is connecting a
solenoid in series with a resistor and then feeding
this circuit with a pulse signal(i.e. 1-0, 1-0,etc.)
Then I visualize the exponential deacay of the potential
at the ends of the resistor. The total amplitude of this
decay divided by e(=2.72...) gives the decay time.

The problem is to get a clean and neat decay curve.

Both solenoids are 11cm long, one has 2cm diameter while
the other has 5cm. The smaller one has aprox. 250+-3 turns
of a 0.8mm cupper wire, set up in two layers. The other,
1200+-(50,70,100?more or less) turns and a 0.2mm cupper
wire.

The small one seems to give a neat decay curve with a time
constant of 2microsec. using a 100 Ohms resistor. Well that means
L=t*R =0.2 mH..Disregarding the increase on L due to the different
layers with increasing section, I expect a value of
muo(pemeabilty of vacuum=4*pi*e-7 Wb/A)
n (density of turns)
S (Section of the solenoid)

L=muo*n*S=muo*(300/11*1e+2)*(pi*1e-4)=8e-7 H !!

This is the first problem.Huge discrepancy, isn't it?

The second is that I cannot get a clean decay curve with
the second solenoid. It is usual to see damped-like oscillations
but here indeed I get a more abrupt oscillations intead of the
smooth ones. And this with any freq.(50-2k Hz range scanned.
Although most measurments were taken around 1K;may high-freq.
effects already be important?)

Any Idea of how coul I get a smoother decay?
Should I try to get L via phase-differences between the
current through R and L,rather than measuring decay time?

Any one else tried to measure L? What do you recommend.

There are hopes that someone will give some hints, but
references-only answers wil also be wellcome.

Thaks a lot
Regards

M.A.Santos
msantos@etse.urv.es
msantos@tanit.etse.urv.es