This question just came up during the review of a design.

Should the TVS rather be placed before the feedthrough-cap or after?

I would place the TVS before the cap (see A in schematic) because TVS is there to protect against fast transients, which requires the shortest return path/lowest inductance possible. On the other hand though the feedthrough-cap is mostly there to get rid of conducted EMI, which also requires placement as close as possible to the connector and a short connection to case/chassis.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ 1N4148 is no TVS diode. If the EMI cap can handle the high voltage from burst/surge, then it should be placed first. \$\endgroup\$ Mar 26, 2020 at 7:56
  • \$\begingroup\$ I know 4148 is no TVS, its a concept drawing, should have deleted the partnumber. The Source is at the connector pin/socket (left). \$\endgroup\$
    – Jogitech
    Mar 26, 2020 at 8:55
  • \$\begingroup\$ What type of surge are you protecting against (amplitude, impedance and duration) and what is the voltage rating on the feed-through capacitor? Are the diodes to remain in position? \$\endgroup\$
    – Andy aka
    Mar 26, 2020 at 9:04
  • \$\begingroup\$ IEC level 4: +-8kv, 150pF discharged over 330R. The caps are rated 50V. But the question was ment on a more general level: lets assume the caps can withstand the surge, what should be the first component seen by the source? But probably there is no standard answer to tjat 😕. What do you mean by “remain in position“? That they wont get damaged? \$\endgroup\$
    – Jogitech
    Mar 26, 2020 at 9:42

1 Answer 1


There is no general answer to your question. But from your comments i get that you need to protect from ESD events and EMI.

In that case you should place the ESD protection first, i.e directly at the connector, because in general, an ESD event has much more harmful potential than EMI.

BTW, ESD protection diodes have some internal capacitance which provides filtering for high frequency EMI sources with high source impedances.


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