Measuring Differential Voltage (hall device - current sensor)

[kermado]

Hi,

I am designing a current sensor that measures current through a wire using a hall device. I have run into a few problems. The hall device only varies its voltage output in small quantities with the current I want to measure.

When using a standard voltmeter, 1-7A only reads a change of 0.01v on the voltmeter. Therefore, the precision of the device is very limited. I have a voltmeter that measures mV, however, it cannot measure mV when dealing with a typical output voltage of 4V (value too big). (I.e. I need to measure 4.001V or 4.0001V).

Can anybody advise me of a simple circuit design that can perhps measure a change in voltage (say when the input is 4v, and the change is 0.001V from 4V)


--------(futher info.)-------------------

You can see a typical hall device here (

http://www.adtek.co.kr/product/images/hall_sensor_02.gif).

To measure the flux density (and calculate current), I would measure the potential difference beween the voltage input and output)

Thanks and Kind Regards!

 

[IRstuff]

Presumably, your 4V comes from somewhere in the circuit, so your measurement should be referenced to that.

TTFN

 

[VEBill]

As stated by IRstuff, it's called differential voltage meaurement.

You could also coil the primary to increase the magnetic field.

It's far more common to use a transformer approach than the Hall effect approach. I suspect that there's good reasons behind that choice.

PS: Here a good Hall effect website:

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

 

[VEBill]

...of course the Hall effect works even with DC whereas the transformer only works with AC.

Link:

http://www.metersandinstruments.com/images/AEMC_CurrentProbes.pdf

The above webpage mentions some interesting points just below Figure 7 on page 3.

 

[itsmoked]

of course the Hall effect works even with DC whereas the transformer only works with AC

AND... Hall effects are highly temperature sensitive.

AND... can be affected by the position in the earths DC magnetic field.

SO... may often need to be calibrated.

This is why my $400 Fluke clamp on has an annoying, fussy, thumb wheel pot, that is used to constantly re-zero it.

 

[nbucska]

Before we start to solve an undefined problem, let's understand it first:

What is the current min/max?
What is the accuracy you need?
If AC, freqency range? If not sine, waveform?
How fast do you have to measure?
Is this for lab or for production?
Type of selected HALL sensor?


<nbucska@pcperipherals DOT com> subj: eng-tips
read FAQ240-1032

 

[Skogsgurra]

As you know, the voltage is measured across the Hall effect device, perpendicular to the control current (not the measured current).

This voltage is small - yes. And you shouldn't measure any large DC component. The 4 volts are probably part of the voltage drop that the control current causes in the bulk of the probe. So your first thing to do is to measure between the right points. If you do that and still have a large DC component, then I'm afraid that the Hall effect device is defect - probably from abuse before you got all connections right. get a new one and be more careful this time.

There are lots of standard circuits for measuring differential voltage. The Differential Amplifier is one such circuit and the Instrumentation Amplifier is another. Google both for diagrams and ready-to-run parts.


With one of these circuits, you will be able to resolve down to microvolts without any problem. You will then notice that Itsmoked is correct when he tells you about temperature sensitivity. One way of avoiding that problem is to use a Flux Gate instead to measure the magnetic field caused by the current. There are effective shielding and compensation techniques that eliminate the influence of the Eart's magnetic field.


Gunnar Englund

http://www.gke.org

 

[kermado]

Thanks guys for the info!

 

[bill318]

Kermando,

Have a look at amploc sensors

http://www.amploc.com/

look at the AMP and ZAP series. These sensors use a torroid core with a slit in them for the hall-effect sensor. They have a simple 5Vdc supply and the output is 1/2 VCC @ 0 amps and is ratiometric with VCC. We have used the AMP200 and AMP50 for several years with very good luck in both AC and DC current measurement. Temperature stability is good and we have no problems with stray magnetic fields.

As others have mentioned, it really depends on the current levels you are trying to measure and to what accuracy.

Bill

 

[Comcokid]

Actually for DC a multi-turn coil works well to increase the senitivity of a Hall. Slot a metal washer, wind 10 turns on the washer, and place the Hall in the slot will yield (idealy) a 10x increase.

Lots of technical information for using Hall effect devices can be found at the Allegro Micro website.

http://www.allegromicro.com/techpub2/techpub2.htm#B

 

[BobM3]

Although physically larger than most hall effect devices, the Allegro ACS75x current sensors are very easy to use. There are several sensitivities offered.

 

[WorkedVYSV8]

I used a product called a CSAV1 for measing current in a car application. Very impressive product.