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I've been working on a LCR/Impedence meter and want to protect my inputs from possible damage if I forget to discharge a large cap.

I know I've seen various types of diodes and relays used for that purpose. But from what I've read and experienced, the larger diodes can cause degradation and relays would be a pain in the ass.

Then I remembered using a TVS array from TI(TPD2E2U06-Q1) to protect my usb signal lines on a prior project. With a minimum break down voltage of 6.5VDC and a stand off voltage of 5.5VDC, I thought it would be perfect. It has what looks like a 0Db insertion loss out past 100Mhz(I'll be using a max of 500Khz, and more likely 100Khz and below). The line capacitance is only 15pF, I'll have more than this in parasitics. This thing is mostly being built to deal with small impedances/resistances, miliohms and the like(I have a VNA that can handle larger stuff/higher testing signals) and a 5.5VDC cutoff will be plenty for this. I will also be putting two of them in parallel.

Now the question, did I interpret the data sheet correctly? Meaning will it protect my inputs(my front end AD630's are powered at +/- 12VDC, so they are safe until 13VDC) from a live cap and not greatly effect the measurements? If it comes down to it, I don't care if these things act like popcorn, at $.60, they are a LOT cheaper than my AD630s...

Here is another question, would adding a 2 or 5 watt resister in the TVS's ground leg help out here? If They

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Those are ESD protection diodes. They can't absorb a large amount of energy, so it depends on how much energy you have stored in your caps and what the series impedance from the cap to the TVS diode is. A big electrolytic charged to a high enough voltage would probably cause the TVS diode to short, then blow open followed by blowing your input stage.

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  • \$\begingroup\$ Thanks for bringing that up. It forced me to go looking, but like I said, this tester is for the smaller stuff. The only high voltage of any type would be the PFC input caps of a SMPS, and those I would not test in place anyway. The highest votlage I might hit, if I'm unlucky would be 50V. Nonetheless, I've decided to bite the bullet and put shorting resister in place with 2 HexFETs(they will be IRFP260N's) in parallel controlling the leg. \$\endgroup\$ – GB - AE7OO Jul 10 at 3:00

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