Project Post: Zeus Filter – AC Line Protection for Rural Grid Mayhem - Page 1
Forum section: electronics / projects

author: vinthewrench
author_rank: Newbie
timestamp: August 26, 2025, 01:39:30 pm
content: Hey folks,After getting sick of blowing up power supplies every time the grid in rural Arkansas throws a tantrum (reclosing surges, nearby lightning, and general utility weirdness), I built my own line-side protection filter — I call it the Zeus Filter.It’s designed to sit between the utility mains and something like a Mean Well DRC-40 supply. The goal was to knock down:Transients from recloser eventsNearby lightning strikesUtility capacitor bank switchingGeneral line noise and spikesFeatures:MOV-based clamping (upgraded to V275LA20AP)Common-mode chokeX and Y safety capsSlow blow fuse + NTC thermistor for inrushGround reference bleed resistors1M bleeders to discharge filter capsMini-Fit Jr. right-angle output connector (with ground wire brought to chassis)I also did a KiCad 2-layer layout with GND on bottom and isolated signal paths above. Tried to maintain good creepage around component leads poking through the plane — but I’d love critique there especially.Here’s the article with background, photos, test data, and a few sarcastic remarks:🔗https://www.vinthewrench.com/p/kicking-zeuss-assWhat I’d like feedback on:Layout: anything dumb or dangerous I missed?MOV aging/fuse pairing — anything smarter?Are the ground bleed resistors truly needed between N and GND?Any better way to connectorize the output side?Would you trust this in your rack?Let me know what you'd change before I spin the next board. Thanks for reading ——Vinnie

author: vinthewrench
author_rank: Newbie
timestamp: August 26, 2025, 06:33:35 pm
content: Note I added the schematic and PCB beacuse somefolks dont follow links --The first thing we do is to go through a 4A Slow-Blow Fuse for short-circuit and overload protection. I even put it in a nice fuse holder.We follow that up with an SL105R004A thermistor. This device limits inrush current during power-on. As it warms up, its resistance decreases, reducing power loss in normal operation.Next up, I added a Bourns 2036-09-BLF 3-electrode gas discharge tube, between the Hot, Neutral line, and Ground. The tube triggers at ~200V to dump high-energy surges directly to earth ground. This is for the lightning-level events and major faults.I would also suggest an MOV to clamp fast voltage spikes and absorb the medium-energy transients from reclosers or grid switching. For my first attempt, I used a B72220S0271K101 (275VAC MOV), but in retrospect, I think I will use a Littelfuse V275LA20AP. It has higher surge energy capacity (220J vs ~120J). This will give it a little more lifespan and durability. Since it has the same physical footprint on the PCB, it's a no-brainer.Further into the circuit, I added a Wurth 744823210 10mH common-mode choke. This will block the EMI noise that appears identically on both Line & Neutral. This is the kind of chatter you get from utility reclosers. The two windings on the ferrite core cancel out unwanted high-frequency noise (common-mode), but it lets the 60 Hz AC pass unaffected.Before we get to the output, I also added a series of class X and Y “safety capacitors” to filter any leftover high-frequency crap to ground or cancel it across the line. They are called safety capacitors because they are designed to fail in a way that protects people and equipment.To be safe, I added a few 1MΩ, 0.25W resistors across the CX1/CY1-2 capacitors to discharge them after AC is removed. This will make storing and working safer once the power is removed. Otherwise, there is a high chance of getting zapped on the output line.We end up with a discrete AC line conditioning circuit combining overvoltage protection, inrush limiting, and EMI/RFI filtering into a compact single-layer PCB. It should give my upstream circuits clean and stable AC input under real-world conditions, including the grid switching, lightning surges, and automatic recloser events we get out here

author: floobydust
author_rank: Super Contributor
timestamp: August 26, 2025, 06:46:27 pm
content: For safety - I assume it's in a metal box. Looking at thefuse curves,a 4A slow blow fuse can carry 7A forever. This is a problem because say a 5A load/fault, the CM choke is rated 2A @70°C which is kinda hot and theNTCwould slowly get hot and trip, just sit there rated 181°C at 4A and maybe melt the fuseholder.My point is, there is an overload where the fuse will never blow and things just cook which can be a fire hazard. You have to always think of the weakest link.I think the GDT voltage rating is too low. Bourns2036-09glow is 90VDC, for telecom and such which would always be on with normal mains voltages. Compare with LittelfuseAC120as an example it's glow 285VDC typ. which is on par with a 2036-30. So I would up the GDT voltage.Also note a GDT is slow, around 4usec to light up. So a MOV in parallel is much faster and will clamp first and do all the work if it's clamping voltage is near or below the GDT.Eaton Combo MOV+GDTyou can see how they paired them up.Littelfuse App. Note - Combining GDTs and MOVs for Surge Protection of AC Power LinesThe 1MEG discharge resistors are so the 0.1uF caps don't bite if you unplug this and touch the power cord.They should be better than 1/4W for their voltage rating which should end up more than your transient voltage limits they could see i.e. you don't want the resistors arcing across their body and acting as a GDT lol.Lightning, substation switching transients can appear on line, neutral, or ground or any combo of these.You need to know how your building is grounded - the service neutral bus and earth ground bond, as well as what you've got for a ground rod, if it's any good.There's so much more to discuss but outta time.

author: little.tesla
author_rank: Regular Contributor
timestamp: August 26, 2025, 07:56:08 pm
content: I’ve built something along the same lines and thought I’d share in case it’s useful or sparks ideas. All the details for my surge protector PCB are here:https://github.com/little-tesla/Surge-ProtectorIn my setup the PCB is integrated as the second stage of a two-stage surge protection system inside an insulating plastic enclosure. Even the internal mounting uses plastic screws for safety. The power path is as follows:- IEC 60320 C14 panel-mount inlet for power entry- Fuses on both L and N lines (T-10A)- Commercial SPD (LSP05GI240SX3333) as the first surge stage- Heavy-duty EMI/AC filter based on this design:https://q82.uk/mainsfilter- My surge protector PCB as the final stage before the output- IEC 60320 C13 outlet for the loadBoth the commercial SPD and my PCB have health LEDs; if an LED goes dark, the MOV has fired and that module needs replacing. The LEDs are visible externally via light pipes.I’ll attach some pictures showing the internal layout and the enclosure.

author: tggzzz
author_rank: Super Contributor
timestamp: August 26, 2025, 08:28:44 pm
content: See TAoE X-Chapters section 9x.25 for example schematics with explanations.

author: floobydust
author_rank: Super Contributor
timestamp: August 26, 2025, 08:32:38 pm
content: Uh but that Yageo GDT2RP600L-8is 600VDC glow and to 1,300V max. sparkover which seems too high for my liking once you include the seriesMOVTP20V300Tmax clamp is 775V.Nothing happening until over 1.3kV - seems up there. It's a 240VAC design?Big problem with DIY AC power protection is it's difficult and never really tested, so only a theoretical design is possible and that's what you'll see out there.You can copy some of the big company designs to get in the ballpark.

author: vinthewrench
author_rank: Newbie
timestamp: August 26, 2025, 09:27:53 pm
content: floobydust - Fantastic feedback -- just what I wanted.I'll rework the design. I was concerned that I need a 3-lead GDT, though?

author: coppercone2
author_rank: Super Contributor
timestamp: August 26, 2025, 09:28:11 pm
content: I had a old plug in surge protector that looked just like this. It was a white box, kind of looked like a router or something

author: vinthewrench
author_rank: Newbie
timestamp: August 26, 2025, 10:53:07 pm
content: OK did some more research. - Here is my latest BOM ---- SURGE PROTECTION --GMOV-14D151K Bourns MOV+GDT Combo, 150 VAC, 420 V clamping, 6 kA peakB32021A3102M TDK Y2 Safety Cap, 1 nF, 300 VAC, 20% toleranceB32922C3104M003 TDK X2 Safety Cap, 0.1 μF, 305 VAC, 15 mm LS-- INRUSH CURRENT LIMITING --B57236S0500M000 TDK NTC Thermistor, S236/50M, 5 Ω-- EMI FILTERING --744824433 Wurth Common Mode Choke, 3.3 mH, 4 A, 35 mΩ-- CONNECTORS --39-30-3036 Molex Mini-Fit Jr. R/A Header, 3-pos, PCB mount1935174 Phoenix Terminal Block, 3-pos, 5.0 mm pitch-- FUSE & HOLDER --696105003002 Wurth WR-FSH Fuse Holder Block (THT)696122003001 Wurth Fuse Clip Cover0218004.HXP Littelfuse 4 A 250 V Slow Blow Fuse (5×20 mm cartridge)-- BLEED RESISTORS --FMF1WSFTE52-1M Yageo 1 MΩ, 1 W, 400 V, Metal Film, Flame-Proof

author: floobydust
author_rank: Super Contributor
timestamp: August 27, 2025, 02:51:09 am
content: Some things to consider maybe.First, know your spacings (creepage, clearance) target for the voltage class and what transients you are designing to. Is this good for 150V Cat. II 2.5kV or Cat. III 4kV etc.Example table, 1.5mm is a minimum on the PCB in a dry, no dust environment 150VAC Cat. III or 150-300VAC Cat. IIA problem (Phoenix Contact I have complained to) is a terminal block's voltage rating is not really achievable on a PC board, compared to their "tests" and specs.The input terminal block has 5mm pin spacings- the PCB pads subtract from this and there is no fuse on the input so an arc there would be a smoke and burn mess. Pad OD max. I think 1.7mm which is not much for an annular ring with 1mm pins/1.3mm holes.The Molex output connector with 4.2mm pin spacing, transients will have been dealt with upstream of it. But 1.8mm PCB holes kinda huge. Check your footprints.I dislike a ground pour unless you can get some decent clearances but it looks OK on the bottom side is lots of room.The Y2 cap at 1nF doesn't do much, more common is a 2.2nF disc cap.I'd use a three-terminal GDT in case the transient is coming in on both line and neutral, as common-mode. Like the 2036-30 but now you only have one MOV+GDT across L-N?I worked in the utility industry and cap banks are a manner of last resort for an old, overloaded grid. They make some nasty transients with vaccuum-breakers. Then there is the old, tired voltage regulator (tap changer) that also make spikes when switching. How far you are away from the distribution transformer, the quality of it's neautral/ground as well as the run to the substation determine how bad your power will be. Reclosing is more of a squirrel or bird zap problem lol. I'm not sure how the Meanwell is getting nuked.Old famous 2002 recall of 15 million units:https://www.cpsc.gov/Recalls/2013/schneider-electric-recalls-apc-surge-protectors"The firm has received 700 reports of the surge protectors overheating and melting and 55 claims of property damage from smoke and fire, including $916,000 in fire damage to a home and $750,000 in fire damage to a medical facility. There are 13 reports of injuries, including smoke inhalation and contact burns from touching the overheated surge protectors."https://www.cpsc.gov/Recalls/2021/Schneider-Electric-Recalls-Surgeloc-Surge-Protection-Devices-Due-to-Fire-HazardSo even the pro manufacturers screw this up.

author: vinthewrench
author_rank: Newbie
timestamp: August 27, 2025, 06:26:44 pm
content: Fantastic Advice. I will consider it thank you

author: Marco
author_rank: Super Contributor
timestamp: August 27, 2025, 11:22:03 pm
content: Quote from: vinthewrench on August 26, 2025, 01:39:30 pmSCASA? It uses a coupled inductor which has negligible impedance at 50 Hz, but will go high impedance during a short surge with the help of a supercapacitor.PS. On second thought, I'm not sure what the hell that supercapacitor is supposed to do. I don't see how the SCASA circuit could work better than simply using an inductor and a MOV.