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I want to know why the high capacitance of metal oxide varistors affects sensors, signal sensitivity.

How does capacitance affect sensitivity ?

If we add a low capacitor in series will the total capacitance decrease so sensitivity will be better?

How does inductance affect sensitivity?

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    \$\begingroup\$ The bigger question: Why does adding capacitance between a signal wire and its return wire attenuate the high frequency portion of the signal more than the lower frequency portion. Do you know why this is true irrespective of whether the capacitance is a regular capacitor or that inside an MOV @Ahmed \$\endgroup\$
    – Andy aka
    Commented Dec 12, 2019 at 11:32
  • \$\begingroup\$ Sensitivity? You mean bandwidth? \$\endgroup\$
    – DKNguyen
    Commented Dec 12, 2019 at 14:32
  • \$\begingroup\$ @Andy aka actually i dont know really im really interested to know \$\endgroup\$
    – Ahmed elmenshawie
    Commented Dec 12, 2019 at 14:34
  • \$\begingroup\$ Forget about sensors and MOVs, and just understand how an RC lowpass filter works instead. You can look that up and come back to ask if you need help. And then an RC highpass filter (for what a series capacitance does). LC and LR high pass and lowpass filters should quickly fall into place after you understand RC to answer your inductance question. \$\endgroup\$
    – DKNguyen
    Commented Dec 12, 2019 at 14:41
  • \$\begingroup\$ The point I'm making is that you've asked a question that requires an understanding of basic principles and, if you understood those basic principles, you would have no need to ask the question. You need to do some research on the impedance of capacitors. \$\endgroup\$
    – Andy aka
    Commented Dec 12, 2019 at 14:42

1 Answer 1

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I want to know why the high capacitance of metal oxide varistors affects sensors, signal sensitivity.

Basically, this question boils down to how would moderate to high capacitance affect signal levels from a sensor. And the simple answer is found in the humble RC low pass filter. It has nothing really to do with MOV varistors - any component that has significant capacitance to ground (or the return wire) can reduce the high frequency spectrum of a sensor output.

A capacitor when placed a cross an AC source will take more current when the AC source frequency is higher. This is because of the fundamental formula for a capacitor is: -

$$I = C\cdot\dfrac{dV}{dt}$$

And clearly, for the same RMS level of signals, higher frequencies have a greater value of dV/dt.

If we add a low capacitor in series will the total capacitance decrease so sensitivity will be better?

Then you will lose the main protection offered by the MOV except for very, very fast transient surges.

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  • \$\begingroup\$ Thanks for your answer , adding series capacitor decreases total capacitance which varistor needs to do its job? \$\endgroup\$
    – Ahmed elmenshawie
    Commented Dec 13, 2019 at 13:54
  • \$\begingroup\$ Anything that prevents the MOV directly connecting to the circuit it intends to protect is preventing the MOV from doing its job. \$\endgroup\$
    – Andy aka
    Commented Dec 13, 2019 at 13:58
  • \$\begingroup\$ thats very useful information , so if i wanna put bypass capacitor, the capacitor will be nearer to the power supply then varistor then load \$\endgroup\$
    – Ahmed elmenshawie
    Commented Dec 13, 2019 at 14:02
  • \$\begingroup\$ A bypass capacitor is not a capacitor in series (with the MOV) hence it can go where you want it to go. \$\endgroup\$
    – Andy aka
    Commented Dec 13, 2019 at 14:06
  • \$\begingroup\$ Oh yes u are truly right . You are so helpful . Thanks alot \$\endgroup\$
    – Ahmed elmenshawie
    Commented Dec 13, 2019 at 14:07

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