Custom probe needed for MI Model 904 Gaussmeter - Page 1
Forum section: electronics / projects

author: enut11
author_rank: Super Contributor
timestamp: June 08, 2020, 08:26:16 pm
content: **Discussion moved from REPAIR section **I have a Magnetic Instrumentation (MI) model 904 Gaussmeter that came without a probe and would like to get it working.Have not been able to find any info or manual online.I believe this type of instrument uses a Hall Effect chip. These chips normally have 3 pins: power, ground and output.You can see from the photos that the probe socket has 4 wires so maybe the fourth is a shield?Our local supplier (Jaycar) has a UGN3503U sensor that I will try. It needs a nominal 5v supply.Ebay has the following linear chips on offer:https://www.ebay.com.au/itm/10-PCS-AH49E-TO-92S-AH-49E-LINEAR-HALL-EFFECT-IC/122490059728https://www.ebay.com.au/itm/4PCS-HAL401SF-A-Encapsulation-SOT89-3-Linear-Hall-Effect-Sensor-IC/372247259357I have never used a Hall chip so any feedback from the Forum would be appreciated.

author: enut11
author_rank: Super Contributor
timestamp: June 08, 2020, 08:27:47 pm
content: More MI Gaussmeter 904 pix

author: enut11
author_rank: Super Contributor
timestamp: June 08, 2020, 08:28:37 pm
content: What I have been able to determine so far referring to the Gaussmeter input socket:Front switch set to READ:Black wire is grounded.Brown wire is signal to the PMI OP02 opamp pin 2.Orange wire is +15v (from the LM317 connected as constant current source).Yellow wire is ground for CC source only.Front switch set to CAL:Black wire is open.Brown wire is open.Orange wire is grounded.Yellow wire is ground for CC source.A constant current 60-120mA, variable by front panel pot, is applied to a precision 0.38ohm resistor generating a CAL voltage fed to the PMI OP02 opamp.The second opamp LM384 is used for meter drive and North/South field indication.From this I conclude that input connector:Pin 1 (Brown) is Hall sensor output.Pin 2 (Black) is Hall sensor ground.Pin 3 (Orange) is Hall sensor supply.Pin 4 (Yellow) only used for calibration.I found it odd that the Hall sensor was powered by an unregulated supply.enut11

author: enut11
author_rank: Super Contributor
timestamp: June 08, 2020, 10:38:04 pm
content: Part schematic around the input socket.I measured the sensitivity of the input opamp via R37 and it is 100uV to 100mV over the full front panel switching.It appears that power for the probe must come from the LM317 current source?I think this would rule out conventional Hall Sensor ICs which need a regulated 5v supply and output a +/- signal centered on 2.5vNote also the two power supply grounds are not connected, unless this is done via the (missing) probe plug.

author: enut11
author_rank: Super Contributor
timestamp: June 08, 2020, 10:52:59 pm
content: When in CAL mode the input opamp is connected across a precision 0.38ohm 0.1% resistor but only sees a voltage if input socket pins 3-4 are shorted.The 36ohm pot adjusts the current source from about 60mA to 120mA. This presents a CAL voltage of about 25-50mV via R37 to the opamp.When input socket Pin4 is not connected, an unregulated +15v is available at input socket Pin3.So, what sort of magnetic sensor would be connected to the MI Gaussmeter 904?

author: enut11
author_rank: Super Contributor
timestamp: June 09, 2020, 08:03:13 am
content: This TL173C Hall sensor from Texas Instruments may be a contender. It has a max voltage of 13v and provides internal regulation. The differential opamp neutralises the nominal 6v output of the LM173C chip with no flux present using the R1 null pot.

author: enut11
author_rank: Super Contributor
timestamp: June 10, 2020, 05:47:50 am
content: Some more info has come in, a manual and circuit diagram from Magnetic Instrumentation USA. Has helped me check the basic circuit operation, butNo info on the probe or sensor except that it is 4-wire and 2ohm resistance. Still no part numberA simplified schematic shows the sensor interface, much the same as my own sketch.So, I am looking for a 4-wire 2ohm Hall sensor that operates with 50-100mA constant current and outputs up to 100mV at 30KGauss

author: Kleinstein
author_rank: Super Contributor
timestamp: June 10, 2020, 06:47:57 am
content: The high current and isolated drive for the current suggest that the Probe is an old style "passive" hall sensor. So no more than a plate of thin metal with likely 4 (or 5) contracts. With 5 contacts there would be an additional trimmer for the zero.These sensor are usually not made by the usual semiconductor companies and may not be found at the usual distributors. They are more lime "scientific" sensors.This site seem to have at least a similar sensor, though still higher resistance and thus some modification (e.g. lower current or voltage reduction) needed.https://www.sonnecy-shop.com/de/insb-hall-effekt-elemente-cyty300b-max.-sensitivitaet-7.8-12.8-mv/mt.htmlAFAIK the modern Si (or other semiconductor) based hall sensors use some kind if FETs and much smaller current, as the resistance is higher. One may still be able to adapt a modern sensor chip, by regulating the voltage with a zener or similar and provide the 4th contact with a precision divider. The scale factor may be off (stronger signal).p.s. with the low resistance it could be made for a metallic sensor. AFAIR this was something like bismuth.Not sure such sensors are still available as the semiconductor ones are more sensitive and may have taken over the market.

author: enut11
author_rank: Super Contributor
timestamp: June 10, 2020, 07:48:42 pm
content: This information from Lakeshore Cryotronics USA provides more info on how a current Hall probe operates and interfaces with a readout device.It also explains why the excitation current and read paths must have separate ground.