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Gauss meter circuit

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GMCullan

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Any suggestions for a gauss meter circuit? I've been experimenting with a linear Hall effect sensor. I can get the darn thing to work, but I've been trying to use an op-amp to get a "0" reading with no input; a "+" reading on one pole, and a "-" reading from the other pole. A preference would be to use a single-supply op-amp. I've got the circuit to work with a dual-supply op-amp. But the whole thing would be so much simpler with one 9 VDC battery rather than two.

In advance, thanks for your help.
 
So use 1 9VDC and use a virtual ground on the opamp.
 
Thanks for your reply. Been trying to figure this one out on my own. I'll research "virtual ground" and go from there.

Thanks again for pointing me in the right direction.
 
Let me know if you get the thing working. I would love to give this one a shot, I have been in the market for a gaus meter for a while, but could never justify the cost, This might make a fun project though.

Are you just trying to get a meter that tells you polarity, or are you eventually going for one that gives you field strength readings?
 
GMCullan said:
Any suggestions for a gauss meter circuit? I've been experimenting with a linear Hall effect sensor. I can get the darn thing to work, but I've been trying to use an op-amp to get a "0" reading with no input; a "+" reading on one pole, and a "-" reading from the other pole. A preference would be to use a single-supply op-amp. I've got the circuit to work with a dual-supply op-amp. But the whole thing would be so much simpler with one 9 VDC battery rather than two.

In advance, thanks for your help.

Which OPA are you using?
How close to '0' output would be acceptable, [using a single supply]
 
I've used a couple of different quad op-amps that require +/- power supplies, as well as a couple of +/ground supply quad op-amps. Don't remember the part numbers off the top of my head. The linear Hall effect sensor is an Allegro 3 lead unit. I've been powering it with a 5 VDC regulator circuit. In this instance, it has a no-input output of approximately 2.5 VDC with one magnet pole pulling the voltage up and the other pulling the voltage down.

What I'm trying to do is to compare this output voltage to a "ground" or reference point and then measure the resulting +/- voltage. I've been using a generic LCD panel voltmeter set such that +/- 2 VDC gives full scale readings.

The end use of the meter will be to measure and grade ceramic, polymer, and Neo based slot car motor and traction magnets.
 
GMCullan said:
I've used a couple of different quad op-amps that require +/- power supplies, as well as a couple of +/ground supply quad op-amps. Don't remember the part numbers off the top of my head. The linear Hall effect sensor is an Allegro 3 lead unit. I've been powering it with a 5 VDC regulator circuit. In this instance, it has a no-input output of approximately 2.5 VDC with one magnet pole pulling the voltage up and the other pulling the voltage down.

What I'm trying to do is to compare this output voltage to a "ground" or reference point and then measure the resulting +/- voltage. I've been using a generic LCD panel voltmeter set such that +/- 2 VDC gives full scale readings.

The end use of the meter will be to measure and grade ceramic, polymer, and Neo based slot car motor and traction magnets.

hi,
As you require a +/-Vout, you must have dual power supply to the opa.

The +2.5V offset/centre voltage of the sensor needs to be offset at the opa inputs, such that a +2.5Vsig is 0.0V at the output of the opa.

Any displacement from the sensor centre voltage, say a -Vsig drives the opa output negative and any +Vsig drives the opa output positive.

Is this what you mean?

If so, I can post a opa circuit with offset correction.

Eric
 
Hi Eric, I appreciate your help. What I'm trying to do is to make the project as small as reasonable so as to have it track-side portable. If I could run it off of one 9 DVC battery rather than two, all the better. As mentioned, I have had the circuit working on an Op-amp that required +/- VDC power with a central ground. Then I came across some op-amps with essentially similar characteristics but required only + VDC and - (ground) power inputs. I tried using the first circuitry with the second type op-amp, but it would not work.
 
GMCullan said:
Hi Eric, I appreciate your help. What I'm trying to do is to make the project as small as reasonable so as to have it track-side portable. If I could run it off of one 9 DVC battery rather than two, all the better. As mentioned, I have had the circuit working on an Op-amp that required +/- VDC power with a central ground. Then I came across some op-amps with essentially similar characteristics but required only + VDC and - (ground) power inputs. I tried using the first circuitry with the second type op-amp, but it would not work.

Hi,
The requirement thats causing you grief is the need to swing the opa output negative when the sensor signal is less than +2.5V [centre point].

You can buy an ic that can convert say +9V to about -9V, this would give you the +/-9V required for your project.

If thats an acceptable solution we can post a circuit diagram?:rolleyes:

Lets know.

Download the datasheet for the ICL7660 from www.datasheetarchive.com
 
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Yes, that would be a possible solution.

All this just to measure and grade slot car motor and traction magnets!
 
There's not need for a dual supply or any op-amps. I would power the hall effect from a 5V supply (I'd use the LM2936Z-5) and use the +/- 2V meter to measure the potential difference between the hall effect's output and a 2.5V reference (all 2V meters I've seen have differential inputs) created by a potential divider.
 
I've made one a while ago using a Melexis MLX90215 linear HS with a 10mV/mT output. That way I can measure magnetic strength up to 250mT.
-250mT = 0V
0mT = 2.5V
+250mT = 5.0V
Just connect a voltmeter to the output of the Melexis and do the math yourself :D:D

What HS are you using anyway??
 
My first experiments used the Allegro Hall-effect sensor, a 7805 5 VDC regulator, and a voltmeter. Worked fine, but as noted, you needed to "do the math" if you wanted an actual gauss reading.

The reason for using the op-amp is to scale the output such that I can get a direct gauss reading. Yes, I know it won't be dead-on accurate, but it will work well for my intended purposes. Units are commercially available that will do what I want to do, ranging from $200 to $400.

But it is just so much fun, as well as much less expensive, to make my own.
 
If you want to read +xxx and -yyy mT (Gauss) you need a negative power rail. Like EricGibbs already said, use a ICL7660 to make one from a positive rail.
Go to the TI website and look for the Java script "Resistor Calculator for Op Amp Gain and Zero Shift". That script will show you OpAmp configuration and resistors needed to convert any x-y input range to a x1-y1 output range.

The easiest way to find it is search for any opamp type and scroll down to the "Tools & Software" section of the new page. There you can find the "Resistor Calculator for Op Amp Gain and Zero Shift" link.
 
What scaling factor do you require?

Perhaps you could post the datasheet for the hall-effect you're using.

Also, you can get op-amps that will run from a single rail supply or use potential divider and an ordinary op-amp.
 
Hero999 said:
What scaling factor do you require?
Scaling from mV sensor output to magnetic field strength in Gauss :D:D

Since most voltmeters have a 3 1/2 digits display, he can display from 0 to 1999 Gauss + and -
So he needs a conversion from whatever Gauss range to 0mV to 1999mV

y = m*x + b with y = magnetic strength (Gauss), x = sensor output (mV)

Something is well explained on the TI site :eek:


Hero999 said:
Also, you can get op-amps that will run from a single rail supply or use potential divider and an ordinary op-amp.
Since he wants +xxxx Gaus and -yyyy Gaus on his voltmeter readout accoring to Noth and South pole of the magnet, I'm pretty sure he needs an OpAmp with +xx and -yy output voltage, meaning dual power supply :eek:
 
mcs51mc said:
Scaling from mV sensor output to magnetic field strength in Gauss :D:D

Since most voltmeters have a 3 1/2 digits display, he can display from 0 to 1999 Gauss + and -
So he needs a conversion from whatever Gauss range to 0mV to 1999mV

y = m*x + b with y = magnetic strength (Gauss), x = sensor output (mV)

Something is well explained on the TI site :eek:
If the scaling factor is <1 he needs a potential divider not an op-amp.

Since he wants +xxxx Gaus and -yyyy Gaus on his voltmeter readout accoring to Noth and South pole of the magnet, I'm pretty sure he needs an OpAmp with +xx and -yy output voltage, meaning dual power supply :eek:
He still doesn't need an op-amp with a +/- output and it doesn't require a dual power supply.

Most meters have differential inputs (they measure this potential difference between the + input and - input), for example if the + input is 2V and the - input is 3V, it will read -1V.

It's possible to bias op-amps at half the supply voltage using a potential divider. The idea is you treat the 2.5V as 0V and pretend it's a 2.5V bipolar power supply. If the resistors in the potential divider cause a problem, it's easilly solved by adding an non-inverting amplifer to the potential divider to create a low impedance 2.5V reference.
 
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I don't know if this will help but one can easily make a "GROUND" connection at half the supply voltage - in this case 4.5V. I have made many 9V powered devices that used a few op amps and I simply make a voltage divider out of 2 equal resistors (say 10K) and then strap one of the single rail op amps as a voltage follower by connecting its output directly to its inverting input(-) while connecting its non-inverting input (+) to the center of the voltage divider. The output of that op amp will then be at 4.5V DC (or always half the current battery voltage) but the output will also be at or near ZERO impedance AC and thus work as a Ground point. Thus the negative rail of the op amps all go to GROUND while the positive rails are all connected to +9 V and the "ground" connection for all mid voltage DC and AC purposes is connected to that op amp output that is sitting at 4.5V
 
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