From: "Phil Allison"
Subject: Re: Strange failure mode in switchmode power supplies: Followup
X-Newsreader: Microsoft Outlook Express 5.50.4522.1200
Date: Fri, 20 Sep 2002 16:08:58 +1000
NNTP-Posting-Date: Fri, 20 Sep 2002 15:58:02 EST
Organization: Telstra BigPond Internet Services (http://www.bigpond.com)
"Peter Gutmann" wrote in message
> A few months back I posted a message about strange failures in assorted
> switchmode power supplies in computers, printers, routers, etc etc.
> people offered suggestions, and I've been trying those out (along with
> anything else I can think of) to try and find the cause of the problem.
> to recap (from the original post, page down to get to the new stuff):
> The immediately obvious symptoms are that any SMPS plugged into the
> my house starts producing large amounts of high-frequency (ultrasonic)
> switching noise, both audibly and in its DC output. At 50W it's mostly
> inaudible (but enough to cause the usual unpleasant effects of
> if you sit next to it for awhile), at 100W you can sense it easily
> not necessarily directly hear it), and at 200W it's so bad you can't
> being in the room for more than a few minutes without feeling ill.
> What's bad for humans is even worse for any elecronics sitting
> so far this problem has caused about $2K of damage in assorted
> routers, etc etc (the damage is pretty spectacular, one device which
> was an online UPS which I'd been using to protect the PC downstream,
> high-speed switching component in it had been fried).
> The things I'd tried originally:
> - Check neighbour's houses (nothing, only this one).
> - Check times (it's present 24/7)
> - Check for wiring problems (eg ground & neutral reversed, nothing)
> - Check the power quality with a line analyser (nothing unusual apart from
> expected irregular fluctuations, spikes, etc)
> - Check the (input) power with a scope (nothing, with the exception of
> p-p ripple at maybe 1kHz on the sinewave peaks which appears for about
> vanishes for 30-60s, it's not continuous so I doubt it's the problem)
> - Get the power company to check the power into the house. "OK, it's
> 240V, everything looks fine, connections look OK, sorry, that's all we
> do" (well, it was worth a try).
> - Assorted line filters, power conditioners, and anything else I could get
> hands on (no effect).
> - Run a direct line from the power board where the power enters the house
> the affected gear (no effect).
> - Physically disconnect the power to everything else in the house except
> single line going straight to a sacrificial PC (no effect).
> - Get the same power company (after several months of arguing and a
> complaint) to switch the house over to a different phase in case there's
> something like a large reactive load on the same phase causing problems
> - Get a new earth fitted in case the existing one (~25 years old) had
> through about 10cm below the surface (no effect).
> New things:
> - Connect a large linear load in parallel with the SMPS (no effect).
> - Took readings at various locations around the house and in neighbouring
> houses with a scope (much newer than the clunker I used earlier, and a
> portable LCD one you can hold in one hand... drool): A generally clean
> sinewave with some (very slight) odd-order harmonic distortion in the
> 150-300Hz range, but nothing serious, and not much different from either
> neighbouring houses or the power in the house of the person I borrowed
> scope from (which is miles from here). In other words, nothing
> different from locations where there was no problem.
> - (I wanted to see what I could find on the line with a spectrum analyser,
> the only one I could get my hands on starts at 50kHz and doesn't have a
> probe, and the owner wasn't at all enthusiastic about letting me put
> together myself).
> - After much searching tracked down a ferroresonant transformer which is
> probably about the most you can do to filter (or regenerate the signal
> a power line (I know there's some folklore out there about these
> some SMPSes, but I never had any problems with several sacrificial
> I tried) (no effect).
> This means that either it's something incredibly insidious in the power
> doesn't appear on a scope and can walk through a ferroresonant circuit, or
> isn't anything related to the power at all. Unfortunately I can't even
> whatever's causing this on power problems any more now, and there's
> left... at best I can handwave some sort of EMI issue, but (a) I'm nowhere
> near any source of this and (b) I'd have thought an earthed metal box
> case) inside another earthed metal box (PC/router cases) would offer at
> some amount of shielding (modulo the power cord and other impromptu
> provide a means of access, obviously). I really don't want to sell the
> to fix this... does anyone have any other ideas? Anything I missed?
> Witch doctor?
** You do bknow what happened to the man who failed to pay his exorcist?
Well, he got re-possesed.