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I have a simple circuit that reads a temperature and toggles a valve. The thermocouple functions when the dotted ground wire is unplugged, but fails otherwise.

TC Diagram

Minor edit: The 3.3V going into the AD595 is actually 5V.

I'm not looking for workarounds. As a quick fix, I replaced the MOSFET with an optocoupler. It works fine, but I'm trying to learn from this experience.

My understanding:

  • The 24VDC source is a switching regulator, and has a ~50mV wiggle. The thermocouple is a µV-level sensor before amplification, so the regulator easily drowns out the signal.
  • Power and signal grounds are usually built separately, and then joined at one point near the voltage sources. This minimizes high-fluctuating power current traveling through signal currents.

What I've tried:

  • Placing the dotted wire between the negative power and a separate MC ground pin.
  • Adding a large cap to the 24VDC power to reduce (assumed) 50Hz switching noise.

So, my question is how do you handle sharing a reference voltage between signal and power?

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  • \$\begingroup\$ So you've got three power supplies in the circuit? (3.3, 5.0 and 24V) You can try putting a bit of resistance in the ground between the 24V and the others (your dotted line).. Say ten ohms or so. \$\endgroup\$ Aug 6, 2015 at 16:07
  • \$\begingroup\$ It looks like your 24V may be floating with respect to the microcontroller until you connect at the dotted line. \$\endgroup\$ Aug 6, 2015 at 16:31
  • \$\begingroup\$ Is this layed out on a PCB or is it a breadboard? \$\endgroup\$ Aug 6, 2015 at 16:51
  • \$\begingroup\$ Can you show a schematic including more details about the power supply situation? \$\endgroup\$ Aug 6, 2015 at 17:06
  • \$\begingroup\$ Can you give a part number for the 24V regulator, and details on how its used? Often such parts require caps on the output and maybe even a minimum load current. Make sure its being used to spec \$\endgroup\$ Aug 6, 2015 at 17:15

2 Answers 2

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The amplifier has a common mode range that goes from slightly (-150mV) below -Vs to some other voltage above ground (the datasheet has an error in it).

There may be other grounds in the circuit that you are not showing. Often the thermocouple junction is grounded. If not, then it's almost floating with only the 1M resistor taming it.

Chances are the amplifier common mode range is being exceeded, either from a ground that is not shown or from noise and lack of a ground.

One thing you can try is to parallel the 1M resistor with a capacitor such as 100nF (and you can reduce that resistor to 10K with little practical effect- T/C circuits are usually << 100 ohms so even if the junction is grounded it will have no significant effect).

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    \$\begingroup\$ Thanks for the advice! Adding the parallel capacitor brought the noise down to measurable levels, and adding a ~100Ω resistor between the ADC connection and ground cleaned up the rest. \$\endgroup\$ Aug 6, 2015 at 18:08
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It looks like your 24V supply is unreferenced until you ground it to the micontroller circuit. This may have nothing to do with grounding the thermocouple, but a side effect of doing that is providing a reference point for the 24V supply

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  • \$\begingroup\$ That was my first thought looking at the schematic, but it only works when the wire is removed. \$\endgroup\$ Aug 6, 2015 at 16:51
  • \$\begingroup\$ I need to learn to read \$\endgroup\$ Aug 6, 2015 at 16:59
  • \$\begingroup\$ Right, the 24V isn't referenced with the wire removed. This floats the MOSFET and disables valve control. \$\endgroup\$ Aug 6, 2015 at 18:06
  • \$\begingroup\$ @PattimusPrime Doesn't that mean the circuit DOESN'T work unless the wire is connected? \$\endgroup\$ Aug 6, 2015 at 19:07
  • \$\begingroup\$ The valve worked / TC failed when the wire was connected, and the TC worked / valve failed when the wire was disconnected. I can see the ambiguity. \$\endgroup\$ Aug 6, 2015 at 19:46

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