AC surge survivability of modern electronic power supplies and surge protection. - Page 1
Forum section: electronics / general

author: Fflint
author_rank: Frequent Contributor
timestamp: May 25, 2025, 09:05:07 am
content: Does anyone know or can point me towards some resources from testing and discussion about what kind of overvoltages cause damage for modern electronics?Specifically I mean things like wall-wart style power supplies. USB power adapters. PSUs built into TVs , PCs, modern appliances such as induction cook tops, inverter driven fridges, washing mashines and ACs and so on.I guess all of these use a switch mode psu. I have not seen a transformer based wall wart come with a new product in decades.Please note I'm in a 240V place so I'll be using that as utility voltage in this post.So all of them will have pretty much a very similar AC input side. There will be a fuse first, then a MOV that's usually rated around 475V (from my disassembly endeavours) then there is an emi filter which consists of a common mode choke and some Y and X capacitors. I read these capacitors are tested for peak voltages of at least 5kV.The we have a bridge rectifier and some electrolytic caps. Diodes used (again only based on my disassembly, correct me if I'm wrong) typically will survive 800V to 1000V. Regarding electrolytyc caps I saw PSUs that used 450V caps and 630V caps. I'm not sure what is the average. Let's say they could survive 500v in a very brief surge, but no more than 600V. Then we have the switching mosfets hopefully also rated up to 750v.The MOV will properly start conducting around 700V. So that gap between the electrolytes and MOV is not looking good to me. Other than that it should be fine for brief transients (let's say lightning caused up to 220uS) of 1~2kV. But if the MOV is burned to the crisp by earlier overvoltages that drops to just an extra couple hundred volts.Now let's say you have proper single point grounding in your house, the main panel surge protection of type 1+2 (as is required by the local electrical code here). That kind of protection considers up to 2.5kV surges acceptable.You read that and you add type 3 just in case . They are supposed to limit the surge to 1.5kV.However it seems that may still be way too high for modern equipment. Also if a lightning strikes the house and goes down on the lightning rod that is maybe a couple of meters away from electric cabling there will be thousands of volts induced in the cabling after the spd. That will propagate in both directions so perhaps these pc surge protection bars are not such a bad idea?Or some sort of add on protection right in front of the actual device. But then, if we treat lightly ing surges as RF (as we should) the ability to "clamp" it will depend on the earthing impedance where the equipment is located. Which is likely very high where the actual device is.Also, this induced voltage is probably induced on all the wires, phase, neutral and ground. If that is so than we should worry more about what happens when it gets dissipated from the cable. It shouldn't cause damage if there is no differential first, but then when it is gone the entire pc is like a small capacitor charged to thousands of volts. Then differentials may arise.I'm not sure if we need any protection for this specific situation or not. But I'd like some input, am I wrong here? Am I right? (with regards to voltages being acceptable).And do we need surge protection where the equipment is plugged into the wall. If we do, how does it work if the path to ground is high impedance ?If you're going to answer "you can't protect from a direct strike" or "all surge protection is a crap shoot" then tell me why we have a lot of stuff that gets hit on a regular basis (like cell phone towers) and survives with no problem. But I'd rather discuss specific methods rather than argue if it's possible.P.S A bonus question, can you measure your grounding impedance with a vna?

author: Seekonk
author_rank: Super Contributor
timestamp: May 25, 2025, 08:10:07 pm
content: AC outlets normally can see spikes over 2KV and most electronics can survive short bursts. A little protection isn't hard to add.You might find this interesting

author: JimboJack
author_rank: Regular Contributor
timestamp: May 26, 2025, 01:15:17 am
content: F,you might want to have a look at Constant Voltage Transformers (CVTs) (ferroresonant transformers) use a resonant circuit on the transformer, they make some noise audio noise , but good for surges, input voltage swings, brown-outs. they simple design been around since, AC mains existed, work everywhere, and used alot in countries that have poor mains supply.link to some info on lightning strikes at bottom, pulse shapehttps://www.aelgroup.co.uk/faq/faq001.phpJ.

author: 5U4GB
author_rank: Super Contributor
timestamp: May 26, 2025, 08:07:14 am
content: "Some noise" is an understatement, they buzz like a pad-mounted transformer. They're also quite inefficient since the core is always in saturation, so a fair bit of power is turned into heat in the core.Apart from that they're pretty impressive in terms of dealing with crap on the mains. I've currently got an OEM Sola 1500 for sale if anyone's after one.

author: jonpaul
author_rank: Super Contributor
timestamp: May 26, 2025, 08:39:10 am
content: Find, read the excellent texts and standards on transient suseptibility and protection.j

author: Circlotron
author_rank: Super Contributor
timestamp: May 26, 2025, 12:29:56 pm
content: Quote from: 5U4GB on May 26, 2025, 08:07:14 amYep. I used to work for Sola Basic back in the late 80s.From memory a 2KVA ferro resonant transformer consumed about 200 watts just sitting there at no load. But they are dead reliable, Short circuit proof too.