From: mustafa@seas.smu.edu (Mustafa Kocaturk)
Subject: DC Clamp-on Ammeters, Hall Effect
Keywords: Hall effect galvanometer
Organization: SMU School Of Engineering and Applied Science
Date: Tue, 24 Nov 1992 03:25:47 GMT

In article <1992Nov23.035008.1@fnalo.fnal.gov> khartman@fnalo.fnal.gov writes:
>
>        I understand how the AC clamp-on ammeters (eg. Amprobe) work,
>but could someone explain how some models can measure DC current as well.
>This puzzles me, since the ammeter is basically a clamp-on transformer, with
>the clamp being the core.  If the current in the wire under test doesn't 
>alternate, the transformer won't work.  Am I missing something obvious here?
> 

Hardly ever, I guess. A hint: Deflection in TV sets.
You probably know that the force deflecting the CRT beam is
called the "Lorentz force" in physics. It is the force
that acts on charges moving in a magnetic field.

It works interestingly in semiconductors placed in a B-field.
Charge carriers in motion in a semiconductor experience
a force ~ q(v X B). They drift in the direction of this force,
setting up a potential difference (the Hall potential) across the
semiconductor slab. Measuring this potential, one can extrapolate
the B field strength. ("X" stands for vector multiplication and
"~" means "is proportional to"). A Hall effect sensor looks like
the following:
                       +-------------------------o
                       |                         +'ve
            +----------------------+
Current     |  semiconductor slab  |             Hall voltage 
   i ------>|      inside a        +------> i    ~ (i X B) for N type
            |B-field towards reader|             ~ (-i X B) for P type
            +----------------------+
                       |                         -'ve
                       +-------------------------o

The polarity of the Hall voltage depends on the
type of the semiconductor material used, i. e. whether
the majority carriers are electrons or holes.

The DC clamp-on ammeters probably incorporate a Hall-effect sensor
embedded in an air gap somewhere in the magnetic clamp material.
A mechanical design like that of a moving-coil galvanometer,
using the magnetic field set up inside the magnetic clamp material
by the current being measured for deflecting a coil carrying a
predetermined current, is also possible. The mechanical
constructions of the two types would possibly look like

Galvanometer style            Clamp-on DC ammeter
clamp-on DC ammeter            using Hall effect

        v---- undesired air gaps ----v       O == cross-sectional
     ___,___                      ___,___   view of the conductor
    /       \    Clamps around   /       \
    |   O   |  current carrying  |   O   |
    |       |      conductor     |       | h == hinge   
    |       |                    |       | (an undesired air gap) 
 ...l.......h.........       ....l.......h.......
 :   \     / known i :       :    \     /       :
 :    \(O)/ in coil  :       :     \|H|/   Hall effect sensor
 :     /             :       :       |     with proper orientation
 :  ../......  meter :       :       |     and four terminals 
 :  ./.......  dial  :       : connected to a current source
 :                   :       : and a voltmeter  :
 :...................:       :..................:

The semiconductor version would be more sensitive,
versatile and robust, I presume.

The undesired (variable) air gaps cannot be eliminated owing to
the openability of the clamp jaws. These air gaps are major
potential sources of error in measurement. Another source
of error is hysteresis, or residual magnetism in the
magnetic material clamp jaws are made of.

These are just my guesses as to how they *might* be built,
I wish my drawings could look as pretty as some peoples'
signatures, but I hope most of the readers will get the idea :-)

In closing, may I ask the following question, the answer to which
I think many readers and I already know, but some readers may like
to think about:

   We know that like charges repel each other.
   How do the electrons in an electron beam stay together despite the
   repulsion, such that the beam produces a tiny spot on a CRT display ?

Kind regards,
-- 
Mustafa Kocaturk   mustafa@seas.smu.edu  EE Dept., Room 305A, Caruth Bldg.
Home: 214-706-5954  Office: 214-768-1475  SMU Box 753190, Dallas, TX 75275