[sdiy] Wallwarts and ground?

Barry Klein Barry.L.Klein at wdc.com
Mon Mar 23 21:34:39 CET 2009 

Ahhh, this is now entering the realm of work-related discussion, so I 
no longer feel guilt reading these discussions at work(!)
This "feature" also comes along with an added "feature" - leakage across the
transformer windings cause a ~60VAC potential DC output ground compared to
wall ground.  Perhaps worse internationally?  This leakage is current
limited to what are perceived as safe levels but I have seen some of them
make a pretty visible spark upon ground contact.
Most DC plugs have the center "hot" V+, although I have heard that many
guitar effects have it the other way around - center ground.

Anyway, here is my scenario:
My plug is hot center.  Due to common receptacle design, the center makes
first then the ground.  There is a switch option on the receptacle to make
connection with the ground wiper until the plug is pressed in - but the
switch is not used - it is connected to pcb ground too.  

I am wondering if a person were to hot plug this DC plug to a product which
already has a USB or other interface connection made to a PC or other host,
if then this AC potential can then pass through the product's DC Vin and
cause serious havoc in worstcase scenario to components.  The supply itself
is dependant on normal loading - what happens to its regulation circuitry
here?  If the switch were to be wired to short the center pin to ground, the
product would be protected - but the supply may not be.(?)

Any input on this or how line voltages and/or transients can cause wallwart
failure? Seems most adapters now are very similar in design with a
controller IC and optoisolated secondary feedback, along with SCR
overvoltage protection.  What is curious is that the controller IC usually
does not have overvoltage protection and has a Vdd max. of 20V or so...

Barry


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of George Hearn
Sent: Monday, March 23, 2009 9:20 AM
To: synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Wallwarts and ground?

A little note on this topic of 'Wallwarts'.  Wallwarts, generally speaking
comply with something called SELV (Safe extra low voltage).  The (-) output
from a wallwart is not grounded and can assume a wide range of potentials
depending on what it connects to.  This is a safety feature providing an
additional level of isolation between you and the mains.  
	If you tie the (-) terminal of your wallwart to ground you will no
longer be able to call your equipment SELV compliant but may be able to meet
something called FELV (functional extra low voltage).  For instruments you
will sell its just worth considering this.  George

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of James R. Coplin
Sent: 22 March 2009 17:27
To: music.maker at gte.net; synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Wallwarts and ground?

Thanks all!  I got it now.

James R. Coplin  

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Scott Gravenhorst
Sent: Sunday, March 22, 2009 12:02 PM
To: synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Wallwarts and ground?

"James R. Coplin" <james at ticalun.net> wrote:
>I think I understand where I'm getting confused. A two wire 9v 
>supply with a + and - polarity is actually just delivering +9v 
>and ground, not + and - 9v. Is this correct? If so, why do they 
>call it negative instead of ground? Seems confusing to me... 

Yes! correct!

It's called - because it is more negative than the other wire.

If you look at a dry cell, one side is labeled +, the other side is labeled
-.  Same deal.


>James R. Coplin
>
>-----Original Message-----
>From: Ray Wilson [mailto:raywilson at comcast.net] 
>Sent: Sunday, March 22, 2009 8:39 AM
>To: James R. Coplin; 'SDIY'
>Subject: Re: [sdiy] Wallwarts and ground?
>
>If you already have +V DC and -V DC and ground (three wires coming out of 
>the wallwart) you already have a bipolar power supply and you would not
need 
>an MFOS wall wart board.
>
>
>----- Original Message ----- 
>From: "James R. Coplin" <james at ticalun.net>
>To: "'Ray Wilson'" <raywilson at comcast.net>; "'SDIY'" 
><synth-diy at dropmix.xs4all.nl>
>Sent: Saturday, March 21, 2009 1:56 PM
>Subject: RE: [sdiy] Wallwarts and ground?
>
>
>What if the wallwart is already putting out +/-9v *DC* how would I hook it 
>up?  I think it would be easier to deal with the AC as I understand how to 
>get DC and a relative ground from that, I'm just not sure what to do if I 
>already have a supply with +/-9v DC.
>
>James R. Coplin
>
>-----Original Message-----
>From: Ray Wilson [mailto:raywilson at comcast.net]
>Sent: Saturday, March 21, 2009 2:34 PM
>To: James R. Coplin; 'SDIY'
>Subject: Re: [sdiy] Wallwarts and ground?
>
>Hi James
>
>The MFOS wall wart supply board does supply a ground. The supply is
relative
>to itself and supplies +V, -V and ground (which is the center potential of
>the 2 voltages). If you measure from the board's ground to either supply
>output you get the appropriate voltage. It does not supply a connection to
>"earth ground" since that is one of the benefits of using a wall wart which
>is essentially a transformer with the line side (primary) connected to the
>plug prongs and the secondary connected to the two wires that come out of
>it. The MFOS wall wart supply uses half wave rectification (which is all
you
>can get from a non-center tapper secondary) to take one side of the
>secondary get + and - voltage with two diodes, while the other side of the
>secondary becomes the relative ground of the output.
>
>The page explains the whole idea behind the design too.
>http://www.musicfromouterspace.com/analogsynth/WALLWARTSUPPLY/WALLWARTSUPPL
Y.php
>
>Hope this helps.
>
>Ray Wilson
>
>
>----- Original Message ----- 
>From: "James R. Coplin" <james at ticalun.net>
>To: "'SDIY'" <synth-diy at dropmix.xs4all.nl>
>Sent: Saturday, March 21, 2009 10:24 AM
>Subject: [sdiy] Wallwarts and ground?
>
>
>> So, I’m helping a friend finish up his MFOS Sound Lab project and I
want
>> to run it off of a 9v wall wart instead of batteries.  The power supply
>> delivers +/-9v but no dedicated ground.  I guess I never thought about it
>> as I always just use the power supply for my modular so voltage and
ground
>> paths are always there.  What do I do to provide a ground with the
>> wallwart supply?
>>
>> James R. Coplin
>>
>>
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>> Synth-diy at dropmix.xs4all.nl
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>
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-- ScottG
________________________________________________________________________
-- Scott Gravenhorst
-- FPGA MIDI Synthesizer Information: home1.gte.net/res0658s/FPGA_synth/
-- FatMan: home1.gte.net/res0658s/fatman/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- When the going gets tough, the tough use the command line.

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