Bit of a story, bit of a question - Page 1
Forum section: electronics / beginners

author: Revoku
author_rank: Newbie
timestamp: August 04, 2014, 12:26:09 pm
content: Hi EEVBlog peopleI'm new here, couldn't find the information I needed to check something in the story that follows, then remembered EEVBlog has a forum loaded with people who love electronicsSo my story goes I built a computer for a friend of mine 3 or 4 years ago, and purposely had him buy a good quality power supply for it instead of the cheapo junk you can getlong story short the computer died a week ago, and it turned out to be a dead PSU, we have replaced it and all is wellI got to looking at the PSU and thought its such a waste of a high quality PSU, and how did it blow, took it apart, found a Varsistor between the two main capacitors blown to bits, one of those bits had the part no. on it, TVR07241it was a TVR07241 (240v), not being able to find a replacement for it in NZ, I found a 320v one inside an old dead server PSU(also designed to run on 230v mains) I had laying around, put that into it, and replaced the fuseIt worked!My questions about this are, is the 320v MOV replacement going to cause it to be non-fire/electrically safe in anyway?(and if so in what ways), and the strange part is, there is another TVR07241 closer to the AC mains input that isn't blown or damaged at all...the one that is damaged is attached to where the 230v/110v switch wire leads in, which makes me think the original damage might be because someone flicked the switch to 110v on a 230v mains supply, am I correct in this theory?I have a grasp of basic resistors, capacitors and smoothing caps, but no real grasp on MOV's, what I have read up on it tells me they work well as voltage surge protection, and what I can gather from the old varsistor is it protected the power supply from circuit death?any and all insight into this would be much appreciated

author: kingofkya
author_rank: Regular Contributor
timestamp: August 04, 2014, 09:52:25 pm
content: basically its a surge supresser, when the voltage is below the rated voltage the resistance is very high between the legs think gigohms. And when voltage goes above the resistant quickly drops shorting the line and blowing the fuse.There meant to do exactly what they did in this case, it saved the rest of the supply.They sometimes fail for no reason but most likely there was a powersurge. There meant for gross over load so your likely fine with the bigger one, however you cant be sure the psu was designed for the higher voltage.Here is a cheap souse for a closer one.http://www.taydaelectronics.com/zinc-oxide-varistor-250vac-60j-2500a-10mm.html

author: Revoku
author_rank: Newbie
timestamp: August 05, 2014, 06:38:47 pm
content: Awesome, Great info, Much appreciatedand thanks for the source, that site will also be very useful for a project I'm working on, its difficult to find cheap parts that ship to NZ cheaply with such a large range.I think I will set it aside to run in a scrap parts computer of the never left running un-attended typemost power supplys I try to repair explode (well the fuse does anyway)mainly because they have been overloaded already, I replace the visual broken parts, but the problem remains and the fuse blows again instantly, I expected the same from this one, was a pleasant surprise when it didn't

author: jmoreland79
author_rank: Regular Contributor
timestamp: August 05, 2014, 11:13:31 pm
content: Quote from: Revoku on August 05, 2014, 06:38:47 pmTypically when a fuse blows, it's because a component (say, a transistor) further down the line in the circuit fails in a shorted state. This results in a rise in current which a fuse is designed to protect against. But just replacing the fuse recreates the same situation, so it blows again.A MOV is an over-voltage device so it generally comes into play under different circumstances. Provided the device is hooked to the correct mains, the over-voltage that hits the MOV originates from an external, temporary source. So if the MOV acts quickly enough, there's a good chance the other components (including the fuse) will be ok. There are mant occasions with very high transient voltages where the MOV can't arrest it quickly enough and that results in damage to other components. In those cases, the destruction of the MOV usually trips the breaker as well.