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Re: More Coupling...



Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <free0076-at-flinders.edu.au>



On Tue, 26 Dec 2000, Tesla list wrote:

> Original poster: "Ed Phillips by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <evp-at-pacbell-dot-net>
> 
> 	One note on estimating secondary turns if you are too lazy to sit and
> count them one by one, without making a single error.  If you have a
> good ohmmeter you can measure the secondary resistance, and then measure
> the resistance of a known length of the same wire and calculate the
> length required to give the same resistance, and from that compute the
> number of turns.  Of course, for this to be really accurate you need to
> know the effective diameter of the turns to at least the same precision
> as your desired answer.  The counting method is better, of course, but
> tedius.
> 
> Ed
> 

Note that your average digital multimeter might only have an accuracy of
1% despite having four and a half digits. So with this meter you could be
1% out, or in a 1500 turns coil that's 15 turns out. On the off
chance that you score a really expensive meter, it must be calibrated
regularly to do any better than your cheap DMM. In addition the connection
would need to be extremely good or you introduce resistance between the
probes and the wire even if you can zero the meter. You would probably
need a Kelvin bridge circuit in order to measure such a low resistance
with enough accuracy.

The next problem is the temperature dependence of the resistance that you
are measuring. To be able to use this method you need to have the coil at
a uniform temperature and the known piece of wire needs to be at the same
temperature. This might not seem important but remember you are chasing
0.1% accuracy if you want to be within 1.5 turns on a 1500 turn coil.

Another problem once these are overcome is the measurement of length. 0.1%
accuracy in length, for example, would be measuring a 1 m length of wire
to within 1 mm which is not easy especially since it likes to stretch. The
stretching of the wire as it is wound onto the coil will of course throw
up more errors... Don't forget that the former is not perfectly round.

To make matters worse, if you strive to maintain 0.1% accuracy over, say 5
parameters, you end up with an overall limiting error of 0.5% if the
terms are multiplied or divided. By the time that you finish, you end up
knowing the number of turns to, say within plus or minus 10 turns if you
were amazingly careful, or perhaps plus or minus 50 turns if you were
doing it quickly. You would probably get that accuracy measuring the
length of the coil and dividing by the thickness as stamped on the side of
the spool/reel, without needing any fancy equipment.

Please don't think that I am having a go at you, I just don't like the
method.

Darren Freeman