I recently purchase a used Tripp-Lite IS250 isolation transformer off of eBay. Here is the datasheet. It's a fairly open secret that these are not true isolation transformers suitable for a tech bench, in that they bond neutral to ground off the secondary winding and also run the mains ground straight through to the output ground pin. These are typically marketed as "power conditioners" that remove noise (via a Faraday shield & the mentioned neutral-to-ground bonding) and protect sensitive equipment (typically medical) against surges.

I am modifying mine into a tech-bench isolation transformer that provides true ground isolation by removing the neutral-to-ground bonding, installing an isolated ground receptacle, and adding a switch to flip on/off the mains ground to output ground depending on my use case. Upon opening the device up, I noticed that there were two capacitor and a MOV installed across the back of the outlet receptacle (one in the top outlet and another in the bottom). The capacitors are MPP film rated at 2 µF 400V, while the MOV is a 130VAC varistor. What function are these serving, and how are they going about that? Photos below.

It seems fairly obvious to me that the varistor is there for surge suppression/protection, but wasn't 100% sure why, as it seems to be in addition to a 2-amp circuit breaker installed in the case. Why would they need both? Regarding the capacitors, I'm much less sure. I'm assuming they're for some kind of filtering - but of what? It was my understanding that AC "passes through" capacitors. Are these to prevent some sort of DC bias in the AC signal, or filtering out noise on the AC signal itself? I thought the noise would be taken care of by the Faraday shield.

Any response that explains what & how these capacitors are filtering, and what is causing the signal they are filtering would be greatly appreciated. I've not been able to find much on the topic besides generic answers. Thanks.

Open Transformer with back of outlet receptacle exposed

Back of outlet receptacle with capacitors and MOV

Open transformer

  • \$\begingroup\$ Please provide a link to the product data sheet or manual to try and figure out what specifications they are trying to be compliant with. \$\endgroup\$
    – Andy aka
    Commented Feb 5, 2023 at 19:52
  • \$\begingroup\$ @Andyaka the data sheet can be found on the product website I linked too, but here is an direct link to it. I’ll update my question with this link \$\endgroup\$
    – riptusk331
    Commented Feb 5, 2023 at 21:04

1 Answer 1


The parts are there for noise filtering, spike suppression, and surge protection.

Spikes and surges are two different things, and MOVs are not the best part for either.
Among other things, their clamping ability decreases significantly with each clamping event.

Consider replacing them with tranzorbs. A tranzorb for AC use is two zener diodes connected back-to-back in the same package. Very fast (as in nanoseconds) response time, and they do not age. TVS is another term for them, but I see that as a generic term that covers both MOVs and tranzorbs. Vishay calls their part a TVS Diode.

Update: So does Littlefuse


$4.43 each in ones.

  • 1
    \$\begingroup\$ Beware, TVS rated for mains surge are big, and very expensive. They do work well though; if you need longevity (10yr+?) or freedom from servicing (i.e. MOVs can just be replaced on schedule), or you have extremely noisy/hazardous mains (frequent surges), they can be worthwhile. Regarding MOVs, an average size (say 14mm) part is good for, I think on the order of 1000s? of events at typical IEC 61000-4-5 surge conditions, which are generally worst-case levels as I understand it. They're not as fragile as people make them out to be. \$\endgroup\$ Commented Feb 5, 2023 at 21:07
  • \$\begingroup\$ See Update..... \$\endgroup\$
    – AnalogKid
    Commented Feb 6, 2023 at 1:26
  • \$\begingroup\$ Hi, 5KP is rated for 10/1000µs waveform, not 8/20; Fig.2 shows we should assume 6x or 30kW peak rating for a 20µs pulse; 5KP200A would be about equivalent to the 130VAC MOV on the original (120VAC service only), which is rated for 15.5A (1000µs), so we expect 93A for 8/20µs. But closer to 750A is required, so it will fail in across-the-line service. For suitable ratings, see: eaton.com/content/dam/eaton/products/electronic-components/… \$\endgroup\$ Commented Feb 6, 2023 at 11:16
  • \$\begingroup\$ Which are cheaper and better rated than I remember (I never really looked into them closely, just teared up at the $100+ tag the biggest ones have 😅 ), but still, ~$30 is quite a bit more than the <$1 you'll spend on an equivalent MOV. Of course these lower voltage ratings are still susceptible to open-neutral conditions (see UL 1449 I think?) and a higher rating or thermal fuse might be desirable for best results. \$\endgroup\$ Commented Feb 6, 2023 at 11:23
  • \$\begingroup\$ This post and tangential comment discussion doesn’t answer my question, and the part that does address my question just restates what I said. I’m aware they’re for filtering. I was asking what they’re filtering and how, not your recommendation for replacement parts. \$\endgroup\$
    – riptusk331
    Commented Feb 6, 2023 at 13:23

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