Need to figure out if these PTC will work to disconnect a MOV on failure? - Page 1
Forum section: electronics / projects

author: rwgast_lowlevellogicdesin
author_rank: Frequent Contributor
timestamp: February 10, 2018, 10:05:21 pm
content: Im working on building a digital Linear power supply and I want to add some mains input protection to it. I know that that a surge isn't going to hurt a 150VA transformer, but I dont want any high voltages being passed from the primary to secondary side. For primary side protection Im just running a Delta mains filter module in to a 7.5amp fuse. After that I want to use add three movs one from Hot to G, Neutral to G, and Hot to Neutral. I will be usingPanasonic ZNR 10621U. It is rated at 620V@1ma with a clamping voltage of 1025. I will be using these MOVs on a 120Vac line, I know the specs seem kind of high but the data sheet recommends them for 100-240 mains, I figure with the filtering and transformer these MOVs should work just fine and they will take less abuse than a 170V MOV.The problem is I am having trouble understanding just how to choose the right Fuse or PTC to disconnect the MOV's on break down. Im also trying to build this thing using parts that I have on hand only. If I understand MOVs right at 620V the MOV will let 1ma flow so that means at ~1240V ~2ma will flow? So If each MOV were to have it's own series fuse for failure protection and 240v 2ma fuse will disconnect the MOV after a destructive surge?I happen to have 3 PTC's laying around stamped PTC 886, Im guessing thisdata sheetwill do? Im not sure if I understand it correctly but from what I gather these PTC's will work from 1-500VADC and will hit full resistance at 24ma? If the MOV shorts the mains these should go to full resistance right. Can I use the PTC's? If not im just going to pick up some fuses locally and cheaply, but I want to understand how to match the right failure protection to a MOV.In case anyone was going to suggest it I plan on blast shielding the MOVS with some thin metal walls to isolate them from everything else in the enclose in case they catch fire. Also just an idea but wouldn't a micro controller with ns pin speeds be fast enough to digitally monitor things and switch them out of circuit, fuses, movs. ptc's, and TVS diodes all seem much slower, but I guess switching a relay is also pretty slow too no matter how fast the micro can process and flip its pin on some type of electrical fault?

author: floobydust
author_rank: Super Contributor
timestamp: February 10, 2018, 10:28:22 pm
content: OP you have good intentions but will never actually test your PTC/MOV solution, so it could still burn down the house. It might look good in theory, but easier another way.Protecting (PTC) the protection element (MOV) gets to be very difficult.Adding the PTC series-resistance to a MOV ruins it's clamping ability, and transients will make it to the power transformer primary.I'd use off-the-shelf, UL1449 safety tested approved parts:Epcos/TDK,Littelfuse offer thermally-protected MOV's, using TCO's instead.Littelfuse App note EC635has some smoke.P.S. Remember my rule "the fuse comes first" so the EMI filter module would beafterthe fuse.