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Goal:

I've got a set of EC and pH water sensors that need to sit in the same batch of water / capture values real-time for a Arduino project.

However my sensors cancel each other out when both are submerged cause it's creating a circuit in said water. So I've been trying to utilize these MOSFETs (JEDEC TO-220AB - RFP30N06LEs) to turn power on and off (sequentially) between both sensors and capture their values.

Issue

As a test, I've setup two MOSFETs (one for positive, one for ground lines of sensor) and have turned off one sensor, while leaving the other sensor turned on. However even though one sensor is off, I'm still getting a circuit between sensors while both are submerged. If I remove the ground and positive from said MOSFETs, indeed the circuit in the water goes away and my on sensor begins reading correctly.

As results of the above test, I think its safe to assume there is a small current leak from the MOSFETs that is only really apparent when measuring a highly conductive medium such as water.

I've created a video to outline the issue a little quicker:

MOSFET help video question!

Question

Does anyone have any advice on the best approach to completely kill a current to a sensor within in a conductive medium like water? Are MOSFETs indeed the correct way to go? Any advice would be greatly appreciated!

Thank you.

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    \$\begingroup\$ Can you put a schematic on here? \$\endgroup\$
    – Hearth
    Commented Apr 28, 2017 at 2:24
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    \$\begingroup\$ Also, "JEDEC TO-220AB" is the package, not the FET itself--TO-220 is the name of that particular shape of device, to put it simply. The FETs are RFP30N06LEs. \$\endgroup\$
    – Hearth
    Commented Apr 28, 2017 at 2:27
  • \$\begingroup\$ Ah, thanks for the heads up :) Schematic View found here (sorry not the best at doing them): imagebin.ca/v/3KaIb8qKBIPT Additional breadboard view: imagebin.ca/v/3KaFSJkda9Wc \$\endgroup\$
    – Devin Olsen
    Commented Apr 28, 2017 at 3:37
  • \$\begingroup\$ Hi, please can you edit these links into your question so everyone finds them. Thanks. \$\endgroup\$
    – TonyM
    Commented Apr 28, 2017 at 7:14
  • \$\begingroup\$ I can't unfortunately cause I'm limited to how many links I can add to my original positing due to being a new member, sorry :( \$\endgroup\$
    – Devin Olsen
    Commented Apr 28, 2017 at 11:14

2 Answers 2

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looking at everything so far, it appears to be an EMI with total disregard for stray line noise on high impedance sensors with water acting as a large capacitive antenna for line noise.

Use all the usual methods of shielded twisted pairs , and check signals on scope or DMM for DC and AC using your good central ground. Active guarding may be the best solution, which you can look up. But first measure/verify input signals.

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  • \$\begingroup\$ Thank you very much for your reply Tony! I've since made a poor mans shield for my wires with no result change. So I then went out and bought a better multi-meter this AM, and here is where it gets interesting. I flipped the multi-meter over to mA and connected it to the positive drain of the positive MOSFET and vola, the issue is gone and indeed is reading 0 mA. Unhook multi-meter and issue comes back. Something in this multi-meter device fixing the issue :) \$\endgroup\$
    – Devin Olsen
    Commented Apr 28, 2017 at 16:24
  • \$\begingroup\$ Blah sorry, false alarm on the meter hookup. Looks like I didn't have as solid connection and that is why the issue went away and was reading 0. With meter hooked up properly and MOSFETs turned off, still reads 0, but issue remains true still. So I can confirm I don't have any kind of leak happening from the MOSFETS, yet while sensor is still plugged into these turned off MOSFETS it is canceling out the other sensor in the water. \$\endgroup\$
    – Devin Olsen
    Commented Apr 28, 2017 at 18:46
  • \$\begingroup\$ So that got me to thinking maybe you were correct after all and the sensor that is turned off by the MOSFETs is indeed acting like an antenna for the powered on sensor (but from the water side). So I got two 25 feet of 22 gauge solid copper wires out, hooked positive to the positive drain MOSFET, negative to the negative drain MOSFET; dropped the other (exposed) ends into the water and bingo issue remains! So looks like indeed when one sensor is turned off it begins acting like a antenna and draining needed voltage from the remaining powered on sensor. \$\endgroup\$
    – Devin Olsen
    Commented Apr 28, 2017 at 18:46
  • \$\begingroup\$ 25ft of wire is also a huge antenna, this needs a better design with specs on differential and common mode impedances everywhere!!! then define all signals and we can look at where interference is with diagram on circuit with grounding and voltage results ac/dc you give, and results you expect with DMM when ok and not ok \$\endgroup\$
    – D.A.S.
    Commented Apr 28, 2017 at 19:33
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Needed power isolators is all.

https://core-electronics.com.au/gravity-analog-signal-isolator.html allows water probes to be truly power isolated from one another, thus function correctly within the same water source.

Thanks!

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    \$\begingroup\$ If this solved the problem then accept it as the correct answer. That way anyone who reads the question will find it. \$\endgroup\$
    – Transistor
    Commented Nov 3, 2017 at 0:16

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