Regarding your last question, an MOV normally will go into "avalanche mode" when the applied voltage exceeds its threshold value. It becomes a low shunt resistance, which should blow the input fuse in the device it is protecting. Once the voltage is removed and the MOV cools off, it usually recovers. However, "usually" is not always the case, so it may be prudent to replace all three MOVs after a surge event.
As others have noted, an MOV rated at 130 VAC is suitable for a 120 VAC appliance only if its tolerance is tight, say +/- 2%. You won't find these at Radio Shack. It may be a good idea to install MOVs rated at 140 VAC with a 10% tolerance. Always protect the device with a fuse sized as recommended by the manufacturer, and don't substitute a fast-blow fuse for a time-delay fuse, or vice-versa. 73, Eric Lemmon WB6FLY -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:repeater-buil...@yahoogroups.com] On Behalf Of kq7dx Sent: Monday, June 07, 2010 5:39 PM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] MOVs for power supply primary Hello to group, Is putting an MOV from hot to ground, neutral to ground, on the primary of the transformer of the power supply a good idea? I have a ICE surge suppressor on in front as well but thought I would put more inside the supply for back up. Also, are the MOVs that radio shack sell any good? Rated at 130VAC. Any body used them? Last question: when MOVs fail or take a surge do they fail in a shorted condition taking out the fuse till the MOV can be replaced, or do they blow or fail open leaving the supply working? Thanks for the help.. 73s
