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Author Topic: Wiring help with computer surge protector  (Read 9514 times)

Posts: 34

« on: August 16, 2013, 08:01:22 AM »

Let me preface that I am a bit embarrassed that I can't figure this out...  Embarrassed

I recently picked-up a monitor stand/surge protector made in the 80s by SOS computers; it will be paired with a vintage computer (Kaypro) I am using for CW operation--yes, I collect vintage computers.  Anyway, the built-in surge protector and switches were not wired at all (appears someone disassembled the stand at some point and unplugged the wiring at the surge protector circuit). I'm not entirely sure how to plug in the hot, neutral, and ground spade connectors in combination with the Varistor on the board.

 - There are 5 hot, 5 neutral, and 5 ground wires (2 switches, 2 outlets, and one AC plug)
 - 15 spade connectors are on the board; connectors are connected in pairs or groups of three and some of these pairs/groups are also connected via traces
 - Two screw holes are connected to a group of three spade connectors in the center
 - Two pair of spade connectors are "floating" and simply soldered to the board

The Varistor goes to a pair of spade connectors on one side and three spade connector on the opposite side.  Here are some pics of the set-up

Pushing the "computer" or "monitor" button shorts neutral and hot (I believe these are wired correctly). I am assuming the two screw holes and spades connected via traces to the holes go to ground, but I wind up having two extra ground wires not connected.

Thank you,


Posts: 14491

« Reply #1 on: August 16, 2013, 09:14:00 AM »

Chris, you need to be really careful with this thing because any mis-wiring could wind up causing overheated wiring and potential fire. I can't really figure out what's going on by the pictures. In general, the varistor goes between hot and neutral AFTER A FUSE. The color code on the switches doesn't appear to be standard. Black is hot and white is neutral so switch contacts should normally have two black wires or one black and one red in lieu of a black and white. The white wire on a switch leg is actually a hot side connection rather than a neutral.

Bob  AA4PB
Garrisonville, VA

Posts: 6252

« Reply #2 on: August 17, 2013, 05:16:38 AM »

Are those all the parts?  How about posting a picture of the entire unit?  'PB is right, it is kind of hard to figure things out just by looking at the pictures you've supplied.  It's also possible that there is another point that two ground wires would connect to--to ground the case.  Have you looked in the case itself to see if there is a couple of tabs that the two free ground wires would connect to?

Posts: 2

« Reply #3 on: August 30, 2013, 08:46:43 PM »

Quote: "In general, the varistor goes between hot and neutral AFTER A FUSE."  The failure mode of a varistor is a short...that's when the magic smoke comes out.  Ask me how I know.

Posts: 2005


« Reply #4 on: August 31, 2013, 04:37:30 AM » will be paired with a vintage computer (Kaypro) I am using for CW operation--yes, I collect vintage computers.

KayproII or Kaypro10? I have had them both! I remember asking the guy at the store about the MS-DOS thing that was being put out by some guy in Washington and he said, "Don't worry about that Bill Gates guy. CP/M is the way of the future!"

Posts: 619

« Reply #5 on: September 07, 2013, 06:30:27 AM »

My 2 cents

The operation functional mode of a varistor is to shunt transients. Failure mode can be either open or short.  If it exceeds capacity, they will explode....If not they degrade slowly with each transient operation.

Just throwing it out there, I work with them a bit.

Anyone interested in learning the big picture on MOV's (Varistors) check out this info:

Explains how they work, interesting stuff. Theres also recommended connection schematics at the end for 110, 220, various phasing.

I had the pleasure of talking to one of the old school guys from GE who where one of the developers. Interesting history.. GE, Harris and now littefuse.

Just follow the schematic after the fuse and you'll be fine.

73, Dave KD8GEH

Posts: 828

« Reply #6 on: October 19, 2013, 12:07:12 PM »

Varistors also have a limited lifespan, they don't last forever. I built a similar surge protector from a Byte Magazine article back before you could run down to the store to buy one. I don't recall for certain but believe they used one varistor across each pair of lines (neutral-hot, hot-ground, neutral-ground) since it is always possible for someone to physically short out any pair of lines "down the block" and that way everything was protected.

But with varistors from the 1980's, I'd replace them all with new if I planned to use the device. Cheap insurance.
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