[Amps] RE : Led display problem

[Mark Moss]

On Tue, 2005-04-19 at 16:08, hermans wrote:
> Hall effect!!.... would you make clear how you would proceed with a hall
> effect Francis. 
> 
> Jos on4kj

I'll fill in a bit more detail here:

As Francis Carcia wrote:

> Hall effect is an active current transformer device 
> so it would be used to measure current.

More specifically, Hall-effect sensors convert magnetic flux density
into voltage.  I can think of at least two common ways hall-effect
sensors are used to measure current through a wire:

1. Take a toroidial core (or other suitable shape) and cut an air-gap in
the core large enough to accomodate the hall-effect sensor (typically
1mm-2mm will do.  Then wind the toriod with a number of turns of wire
which will carry the current to be measured.  As the current through the
wire increases, so will the magnetic flux density in the air-gap.  The
hall-effect sensor will convert the flux density to a voltage which can
be used to drive a display of some sort.

2. Take the gapped toroidal core described above and wind a large number
of turns of a small-guage wire around the core.  (I've seen commercially
made current sensors of this type where where 1000 turns are used.)  Now
pass a wire which will carry the current to be measured through the
center of the toroid.  Use an op-amp to control the current through the
toroid winding so that the flux density measured in the air-gap is 0
gauss.  Multiply the current in the toroidial winding by the number of
turns in the toroidial winding to determine the amount of current on the
wire passing through the center of the toroid.

Method 1 is probably the easiest, since in the simplest case no
electronics beyond the hall-effect sensor and a meter to display the
current are required.  You can also sense AC current (up through low
audio frequencies, anyway) this way without any additional parts. 

Method 2 is a bit more complex as you need additional electronics to
control the current through the toroid's winding to keep the flux in the
air-gap at 0 gauss.  However, as long as your control circuit works
correctly, the accuracty should be pretty good and shouldn't drift much
over temperature.

Something else to consider:  A number of manufacturers produce
hall-effect sensors that can be programmed/adjusted/calibrated in
circuit.  I believe Melexis, Infineon, Micronas, and Allegro each sell
this type of sensor (some might even be availaible through places like
Digi-Key).  I don't know about all of these, but I've used Melexis'
MLX90251 part before (for measuring position, not current), and it is
nice in that it is cheap (~$1.25) and you can set the sensitivity (mV
out per gauss in), offset (0 gauss = ? volts out) and maximum output (so
you don't peg your meter, perhaps?) in-circuit.  My understanding is you
can do the same with parts from the other manufacturers listed above.

73,

Mark Moss, KC8DEI