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I have an experimental setup that I am building to, among other things, measure the flow rate of water through a tube. The flow rate is measured by a sensor that is powered by a 24 VDC source and provides a 4-20 mA DC signal. I am currently seeing an interesting issue where there is some amount of noise in the signal, but when I open the door of the electrical box by about an inch (the point to which it opens naturally when unlocked), the amount of noise drops by about a factor of 2.

Possibly relevant information (I am not an electrical engineer, so some of this may be irrelevant):

  • The electrical box is made of painted stainless steel
  • The 24 VDC power supply has a maximum current of 2.1 A (so a power rating of 50 W)
  • The flow rate signal is generated by an FTI LinearLink LA-5-C-MAF
  • The current is being detected by a NI cDAQ 9154 with a NI 9203 current card
  • I am using DIN blocks to make my electrical connections

This is what the electrical box looks like Electrical Box

What could be causing the change in noise when the door on the electrical box is opened or closed?

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  • \$\begingroup\$ Can you provide photos? \$\endgroup\$
    – DKNguyen
    Jul 26, 2019 at 22:38
  • \$\begingroup\$ Is the electrical box connected to earth? And is the 24v negative connected to earth somewhere? \$\endgroup\$
    – Drew
    Jul 26, 2019 at 22:39
  • \$\begingroup\$ @Drew The electrical box has a DIN rail connected to AC ground. Connecting this to the AC ground terminal in the electrical box had no effect. The DC negative comes out of a power supply that has the AC ground connected to it. Do the DC negative and AC ground need to be connected more directly? \$\endgroup\$
    – Aliden
    Jul 26, 2019 at 22:43
  • \$\begingroup\$ @DKNguyen Possibly. What photos would be helpful? \$\endgroup\$
    – Aliden
    Jul 26, 2019 at 22:44
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    \$\begingroup\$ @Aliden a lot of control enclosures will have a ground wire bonded on each side of the hinge. One to the box, one to the door. \$\endgroup\$
    – user199402
    Jul 27, 2019 at 1:47

1 Answer 1

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the door is a reflector, if there is electrical noise produced inside the box closing the door will somewhat prevent its escape and increase noise levels inside the box

Try fitting ferrite noise suppresors on the power wires running in and the DC wires running out of the power supply, (put both wires through each)

enter image description here

alternatively you could add a window in the side of the box to let the noise out.

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