# What is this schematic symbol that is a circled capacitor?

I am trying to determine what this symbol is that looks like a circled capacitor and is labeled as "Z1" that I circled in blue below.

Here is the full schematic with this component being near the top left at the input.

In my search I found this question with a circled capacitor and a dot but I'm not sure if its the same. I also found websites saying Z is for zener diodes which this obviously is not or this website that says Z could be for a balun, general network, or phase shifter but I did not find circuit symbols that match this one when googling those.

• circled capacitor and a dot - I don't see the dot. – Andy aka Dec 13 '19 at 13:56
• With a dot I would assume a neon/gas-dicharge tube. Without I would not know. (Neon tube without gas????) – Oldfart Dec 13 '19 at 14:07
• @Andyaka the dot is in the question in the link. For clarification mine does not have a dot which is why I don't think its the same as the one in the link. – orange1234 Dec 13 '19 at 14:34

They are indeed varistors like others are (educatingly) guessing.

Varistors Z1 and Z2 have overvoltage protect function on the line input.

You can find this source by googling on ATX power supply using the website on the schematic.
Search term: ATX POWER SUPPLY site:www.pavouk.org

It's likely an MOV (Metal Oxide Varistor) used for surge suppression. Context guides this answer - the schematic is from the late 1990's when this was a common use and it is across the hot/neutral lines and in the earth ground path.

The "Z" reference designator strongly suggests that it is some kind of voltage-clamping device. It could be anything from a spark gap to a gas-dicharge tube to a semiconductor device such as a MOV or TVS. What does the parts list (bill of materials) say about it?

• My guess from the symbol alone would be a gas discharge tube of some sort, since that's the closest to the symbol. – Hearth Dec 13 '19 at 13:53
• I don't see a BOM but the website I got the schematic from is this hardwaresecrets.com/anatomy-of-switching-power-supplies/2 – orange1234 Dec 13 '19 at 13:55
• Well, since that site doesn't talk any further about that particular schematic -- the section on transient filtering uses a different schematic -- there's no way to know exactly what Z1 is. – Dave Tweed Dec 13 '19 at 13:59