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Thread: [EE] Current is Flowing
face BY : Mark Peterson email (remove spam text)

>From:    Gabriel Caffese <gabrielsdispamIMPSAT1.COM.AR>
>Subject: How to know it there a current is flowing...

>        I need to know if a small current, that falls between 0.2ma and
>flows over a pair of copper wires of printed circuit boards.
>        Those printed circuit boards cannot be touched, and I cannot
>any standard measurement equipment, because there is no available space as
>to do that.
>        The only way (I think) , is to use something like a Hall effect
>transistor, wich can be hardly put (but can at last), below the printed
>circuit boards that are being inserted.
>        So, I would like to know if someone has ever done something like
>this before, and, if I am on the right way, and this can be done.
>            Hope to receive news from you !!!!

You could use a Hall effect, either open loop or closed loop.  In open
loop, the amplified output of a Hall element is directly used as the
measurement value.  Offset and drift are determined by the Hall element and
the amplifier.  These sensors have low price but also have low sensitivity.
For closed loop, the Hall voltage developed across the sensor, that is
placed in the gap of a magnetic core that surrounds one of your conductors,
is highly amplified and the amplifier's output current then flows through a
compensation coil wound on the same magnetic core.  It generates a magnetic
field that has the same amplitude but opposite direction as the field due
to current flow in the monitored conductor.  The result is that the
magnetic flux in the core is compensated to zero.  Closed loop sensors are
more sensitive and precise, but they also require more cost and effort to

Another option is the use of a magnetoresistive sensor.  These are devices
that have ferromagnetic materials deposited as thin films in narrow strips
while in a magnetic field.  The resistance of this material varies in
relation to the magnitude of a magnetic field they are placed in.  They are
usually used in a bridge arangement and offer good sensitivity.  Their
linearity is not very high so the same compensation principle used with the
closed loop Hall sensors should be used with these.

Mark P


See also: www.piclist.com/techref/power.htm?key=current
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