From: Spehro Pefhany
Subject: Re: Current Sensing Schematic
X-Newsreader: Forte Agent 1.92/32.572
Date: Fri, 29 Nov 2002 16:32:09 GMT
NNTP-Posting-Date: Fri, 29 Nov 2002 11:32:09 EST
On Fri, 29 Nov 2002 06:17:01 +0000, the renowned John Woodgate
>But the output of a Rogowski with low frequency sine-waves is very
>small, isn't it?
The reference I mentioned gives this formula for the output
of a Rogowski coil:
EMF = ((u(air) * N*L )/(2*pi)) * ln (c/b) * di/dt
= M * di/dt
Where N is the number of turns in the rectangular coil
L is the length of the turns (along the wire)
b is the distance from the center of the conductor to
the inner edge of the coils
c is the distance from the center of the conductor to
the outer edge of the coils
So, lets say the coil has 200 turns, is 50mm long, and
c and b are 15mm/10mm.
M = (((4 * Pi * 10E-7) * 200 * 50E-3)/2 * pi)
* ln(1.5) * di/dt
M = 8.1E -7
So, for this application, say we have a current of
500mA and peak di/dt of, say, 50* 2*pi * 0.5
= 157 A/s
So, if I got the units consistent and made no errors,
the output would be 125uV peak. Not crazy low for on/off
detection, but a small signal. With a longer coil or
with more turns (like 1000) we could get it up to
a mV or so.
Tony Williams did some work with ferrite cores, and,
IIRC, got more like a dozen mV under similar conditions.
"it's the network..." "The Journey is the reward"
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