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'[EE] Dodgy power supply design?'
2006\01\05@121141 by Howard Winter

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I recently bought a "Safecom" external hard drive housing, with USB & Firewire interfaces:
http://www.safecom.cn/code/sub/category.asp?prdid=113&subcatid=12

I added a drive, connected it all up, and started formatting it.  After about three minutes there was a "Pop!"
and it shut down.  I found the plugtop fuse had blown, and replaced it with another - no joy, the PSU was
dead.

I took the whole thing back and had it swapped for another, and when I was about to plug things together, I
had the PSU plugged in and switched on, but the drive-case power switch was off, and as I held the USB cable
from the PC in one hand and the drive case in the other, I got a nasty tingle.  Not enough to hurt, but
certainly uncomfortable.  I got out my trusty DMM and measured between the case and the USB cable:  95V AC!  I
took the disk drive out of the case for safety and measured between the body of the case and mains Earth, and
again there was between 90 and 104VAC.  I haven't measured the current-supplying capability of this "fault",
but I may do that when I've sent this.

I assume there's a design fault with this PSU (which has all the approval markings, CE, UL, FCC and others)
and I imagine it was enough to "blow" the first one, and as I didn't complete the connections, it hasn't had a
chance to do so to this one.  I'm a little surprised that the RCD supplying the mains didn't trip, but I
suppose if the short to Earth is on the output side, is isn't seen as a Phase/Neutral imbalance, and the fault
mechanism could have been failure of a component in the PSU which then caused an overcurrent that blew the
fuse and perhaps itself.

This is a "universal" supply, which accepts 100-240VAC, 50-60Hz, obviously an SMPS and is a small sealed block
with an IEC 320 (Kettle Lead) socket for the input.  It puts out 5V and 12V at 3A each, via a mini-DIN 6-pin
plug, with 2 pins for each of 0V, 5V, 12V.  The shell of this plug is isolated.  I tested the mains cable by
plugging it into a mains circuit tester, and the connections are all present and correct (Earth Loop Impedance
showed 0.7 ohm - not great but then it is through an extension lead).

So what do people think?  Any ideas what might have caused this apparent high-impedance connection to the live
mains from the output 0V line?  Is it likely to be a design or manufacturing fault, given the 2:0 score so
far? :-)

I suppose I'm left with taking it back for a refund, and trying to find another make that has a decent PSU
with it...

Cheers,


Howard Winter
St.Albans, England


2006\01\05@130020 by Harold Hallikainen

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I'm guessing this power supply only has a two wire line cord, right?
Switchers have X capacitors across the line and Y capacitors line to
chassis to attenutate conducted EMI. If the chassis is not grounded, the Y
capacitors form a nice voltage divider, putting half the line voltage on
the chassis. This is considered acceptable if the "leakage current" is low
enough (in the neighborhood of 1mA, less for patient contact medical
equipment). Your DVM, of course, drew considerably less than 1mA, so you
were able to measure the unloaded voltage out of the voltage divider.

Even linear supplies with 50/60 Hz magnetics will have capacity to ground
between the primary and the core. This will place a voltage on the
chassis. Again, leakage current is the concern.

Now for another story...  The first PC compatible computer I bought was a
big 286 machine (still have it!). It had a line cord for the computer and
another for the monitor. Plugged in the computer, then plugged in the
monitor. Loud humming noise and smoke starts coming out of the monitor.
Quickly pulled the plugs! Turns out the monitor line cord had line and
safety ground reversed, so line went to monitor ground, then through the
video cable to the computer, where it found ground. The smoke out of the
monitor was the ground lead from the power inlet to the chassis. Replaced
the line cord and all was well...

Harold


--
FCC Rules Updated Daily at http://www.hallikainen.com

2006\01\05@151155 by Howard Winter

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Harold,

On Thu, 5 Jan 2006 10:00:18 -0800 (PST), Harold Hallikainen wrote:

> I'm guessing this power supply only has a two wire line cord, right?

No, it's a 3-core all the way!

> Switchers have X capacitors across the line and Y capacitors line to
> chassis to attenutate conducted EMI. If the chassis is not grounded, the Y
> capacitors form a nice voltage divider, putting half the line voltage on
> the chassis. This is considered acceptable if the "leakage current" is low
> enough (in the neighborhood of 1mA, less for patient contact medical
> equipment). Your DVM, of course, drew considerably less than 1mA, so you
> were able to measure the unloaded voltage out of the voltage divider.

Right, but there is no chassis as such - the 0V output appears to be isolated from the Earth pin of the input
(on a DC continuity test).  I can't imagine that they would put Y-capacitors from the Phase/Neutral to the 0V
DC out unless it was also earthed.

> Even linear supplies with 50/60 Hz magnetics will have capacity to ground
> between the primary and the core. This will place a voltage on the
> chassis. Again, leakage current is the concern.

I suppose it's possible that the Earth pin is not connected inside, but I can't get inside without destroying
it, and I don't know anyone with an X-ray machine :-)

Cheers,


Howard Winter
St.Albans, England


2006\01\05@164527 by M. Adam Davis

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Double check your power stips and house wiring.

I had a machine connected through a 50ft cable and a powerstrip which
was tingly.  Turns out the ground wire was broken _inside_ the
powerstrip.

Usually this is due to bad ground.  Worse is when someone at a school
cut off the ground wire of the AV cart.  Plug it in backwards, and
it's ok.  Now connect the RF to the school cable system, and suddenly
everyone else sees 120V on the ground of the cable system.  I suspect
a better cable system (well grounded, etc) would have mitigated the
problem...

-Adam

On 1/5/06, Howard Winter <spam_OUTHDRWTakeThisOuTspamh2org.demon.co.uk> wrote:
{Quote hidden}

> -

2006\01\05@170136 by Harold Hallikainen

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> Double check your power stips and house wiring.
>
> I had a machine connected through a 50ft cable and a powerstrip which
> was tingly.  Turns out the ground wire was broken _inside_ the
> powerstrip.
>
> Usually this is due to bad ground.  Worse is when someone at a school
> cut off the ground wire of the AV cart.  Plug it in backwards, and
> it's ok.  Now connect the RF to the school cable system, and suddenly
> everyone else sees 120V on the ground of the cable system.  I suspect
> a better cable system (well grounded, etc) would have mitigated the
> problem...
>
> -Adam


I think I read about a bad ground on the cable television drop to
someone's house caused the neighbor's house to burn down when their TV set
shorted the AC line to the coax shield.

Harold

--
FCC Rules Updated Daily at http://www.hallikainen.com

2006\01\05@171501 by Andre Abelian

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Adam,

Having bad ground it shouldn't effect your machine at all unless
you are talking about nutrall.In your machine ground is connected
to the body only no where else.

Andre Abelian



M. Adam Davis wrote:

{Quote hidden}

>>-

2006\01\08@211058 by Howard Winter

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Adam,

On Thu, 5 Jan 2006 16:45:26 -0500, M. Adam Davis wrote:

> Double check your power stips and house wiring.

Already done - there is excellent Earth continuity from the end of the lead just before it enters the power
block (I used an Earth Fault Loop Impedance meter - the thing professional electricians use - and the
impedance measures 0.7ohm).

I measured the resistance from the Earth pin on the input of the power block to the 0V connection on the DC
side, and they are isolated.  So the Earth pin is present, but not connected inside!  Which probably means
there are capacitors from the Phase and Neutral to the 0V output, forming a high impedance voltage divider,
giving the results I have.  I tried measuring the current that it can supply into as near a short circuit as
my meters can provide, and it's about 0.25mA.  As I said, uncomfortable but not painful...

I suspect this is a unit built down to a price in the Far East, and I don't imagine the compliance markings
are worth much - as it's not double-insulated (and it isn't marked as such) then it must be Earthed to comply
with the various standards, not to mention UK import laws.  I need to take it back and get a refund, methinks!

> Usually this is due to bad ground.  Worse is when someone at a school
> cut off the ground wire of the AV cart.  Plug it in backwards, and
> it's ok.  Now connect the RF to the school cable system, and suddenly
> everyone else sees 120V on the ground of the cable system.

I'm really glad of the design of plug/sockets over here - it would be almost impossible to do that (sockets
have shutters that rely on the Earth pin to open them).

Cheers,


Howard Winter
St.Albans, England


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