homemade gaussmeter

[[email protected]]

I'm hoping someone can help me out since I'm a real newbie at all this.
I made a homemade gaussmeter using the following site:
http://my.execpc.com/~rhoadley/magmeter.htm

For my meter I used the Allegro A3516LUA Hall Effect Device.

What I want is to modify this design so that I can read changes in the
voltage (via the multimeter display) that corresponds to an EMF change
of zero to ten milliGauss. I toyed with the idea of using a wheatstone
bridge but I do not think it will work. Anyone have any ideas?

Please email me directly! Thanks!

 

[John Woodgate]

I read in sci.electronics.design that [email protected] wrote (in

I'm hoping someone can help me out since I'm a real newbie at all this.
I made a homemade gaussmeter using the following site:
http://my.execpc.com/~rhoadley/magmeter.htm

For my meter I used the Allegro A3516LUA Hall Effect Device.

What I want is to modify this design so that I can read changes in the
voltage (via the multimeter display) that corresponds to an EMF change
of zero to ten milliGauss. I toyed with the idea of using a wheatstone
bridge but I do not think it will work. Anyone have any ideas?

Please email me directly! Thanks!

No, you post here, you are answered here.

These Hall things have sensitivities of millivolts per gauss. good luck
with detecting milligauss changes, giving microvolt changes of d.c.!

 

[[email protected]]

Sorry for asking to be emailed directly, guess that was a big faux pas!
Consider my wrist officially slapped. I understand that detecting
microvolt changes in DC is difficult but did not think it was
impossible. The Hall device I have is rated at 2.5mV/G. I could go
with a Hall Device that is rated at 5mV/G but I'm not sure that will
help much.

Anyone with any ideas?

 

[John Woodgate]

I read in sci.electronics.design that "[email protected]"

Sorry for asking to be emailed directly, guess that was a big faux pas!
Consider my wrist officially slapped. I understand that detecting
microvolt changes in DC is difficult but did not think it was
impossible. The Hall device I have is rated at 2.5mV/G. I could go
with a Hall Device that is rated at 5mV/G but I'm not sure that will
help much.

Anyone with any ideas?

The simplest way is probably to connect your sensor to a chopper op-amp.
Chop at audio frequency. That gets you an a.c. signal, which is still
microvolts but is much easier to amplify. Then amplify it to some useful
level (200 mV?) to work your meter. Restrict the bandwidth of your
amplifier to cover just a narrow band around the chop frequency.

 

[Dave]

Consider my wrist officially slapped. I understand that detecting

The simplest way is probably to connect your sensor to a chopper op-amp.
Chop at audio frequency. That gets you an a.c. signal, which is still
microvolts but is much easier to amplify. Then amplify it to some useful
level (200 mV?) to work your meter. Restrict the bandwidth of your
amplifier to cover just a narrow band around the chop frequency.

Also consider using a pair of devices 'back to back' in a bridge arrangement
to cancel out offset effects - IIRC these sensors are temp dependent. You
need to avoid self-heating effects, and also need to watch out for
'thermocouple' effects at the joints. Microvolts are hard to measure !

Your life will be much easier if you can modulate the magnetic field you are
sensing, by (for inst) rotating source or sensor assembly - then you can use
AC gain and synchronous detection, as above.

Dave

 

[[email protected]]

I read in sci.electronics.design that "[email protected]"


The simplest way is probably to connect your sensor to a chopper op-amp.
Chop at audio frequency. That gets you an a.c. signal, which is still
microvolts but is much easier to amplify. Then amplify it to some useful
level (200 mV?) to work your meter. Restrict the bandwidth of your
amplifier to cover just a narrow band around the chop frequency.

This may be a simple approach, but it isn't elegant.

What is really required is a simple Hall effect sensor with four
electrodes, one on each of all four sides of a square plate of
semiconductor material. You drive a current across the square between
one pair of electrodes, and measure the Hall voltage generated at right
angles to the current path across the second set of electrodes.

Drive an alternating current across the plate, and you can
synchronously detect an alternating voltage across the sense
electrodes, giving you your chopping at source.

The Hall effect is temperature dependent, so for added extra
sex-appeal, measure the resistance in the direction of current flow to
make the plate its own resistance thermometer.
This wasn't patentable back in 1991, so it isn't patentable now, and
any patent that might have existed back in 1991 will probably have
expired by now.

Getting hold of such a Hall plate might not be easy - Siemens (now
Infineon) used to sell a lovely - if rather expensive - part for a few
hundred dollars each, but it had dropped out of their catalogue when I
last looked.

 

[[email protected]]

Thanks everyone. Those are great ideas. I think I may have gotten in
a bit over my head on this one. It sounds like a fun project to figure
out but I think it's well above my level of knowledge right now. I'll
probably play with it sometime in the near future when I think I can
handle it! Thanks again all!

 

[[email protected]]

These Hall things have sensitivities of millivolts per gauss. good luck
with detecting milligauss changes, giving microvolt changes of d.c.!

The proper solution would be to concentrate the magnetic field *before*
it gets to the sensor.

Imagine a pair of ordinary kitchen funnels, but instead of being hollow
and made from plastic or glass, they're solid in cross section and made from
iron or a ferrite material. The two funnels are placed with the small ends
together and just far enough apart for the sensor to be placed between them.
The large ends have an area much larger than the small ends, so the magnetic
lines of force will be concentrated in proportion the ratio of the areas. You
get amplification of the field that makes the sensor output its self greater
with no electronics needed. A thousand to one should be easily obtained.

Jim

 

[Ken Smith]

I'm hoping someone can help me out since I'm a real newbie at all this.
I made a homemade gaussmeter using the following site:
http://my.execpc.com/~rhoadley/magmeter.htm

For my meter I used the Allegro A3516LUA Hall Effect Device.

Why not use a magnetoresistive device? They don't cost much and are way
more sensitive.

 

[John Woodgate]

I read in sci.electronics.design that Ken Smith

Why not use a magnetoresistive device? They don't cost much and are way
more sensitive.

So is an inductor on a ferrite bobbin; the 33 mH and 47 mH types make
excellent magnetic receiving antennas. But the OP wanted to use his Hall
device.

 

[E. Kappos]

I read in sci.electronics.design that "[email protected]"


The simplest way is probably to connect your sensor to a chopper op-amp.
Chop at audio frequency. That gets you an a.c. signal, which is still
microvolts but is much easier to amplify. Then amplify it to some useful
level (200 mV?) to work your meter. Restrict the bandwidth of your
amplifier to cover just a narrow band around the chop frequency.

Would this chopper technique really work given that these Hall devices have
a
noise specification typically 10 mV p-p or even more?

With some, like the Allegro A1301 the noise spec is 150 uV.
With many others, the noise performance is not even mentioned on the
datasheet.

Regards,

Euthymios Kappos

 

[Ken Smith]

Why not use a magnetoresistive device? They don't cost much and are way
more sensitive.

So is an inductor on a ferrite bobbin; the 33 mH and 47 mH types make
excellent magnetic receiving antennas. But the OP wanted to use his Hall
device.[/QUOTE]

Yes, but people in hell *want* ice water.

If the Hall device won't do the job, he should look for something that
will. I don't think he's going to get mG out of a hall device unless he
averages for a long time.