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From: Markus Vester <markusvester@aol.com>
Message-Id: <8D14C640B0EA9E4-2EEC-4FCAD@webmail-d258.sysops.aol.com>
Date: Mon, 2 Jun 2014 06:25:00 -0400 (EDT)
Subject: Re: LF: Switch mode power supply nasty
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Hi John,
=C2=A0
Your supply is probably working as designed - don't dump it, ground it!
=C2=A0
The "tingling" this is a fairly common effect=C2=A0of 50 Hz leakage current=
.=C2=A0I find that sliding the dry backside of a finger across metal=C2=A0p=
arts=C2=A0can produce a=C2=A0"rubbery"=C2=A0or vibrating sensation=C2=A0wit=
h any of my laptops and laptop supplies.

=C2=A0
Most=C2=A0small switchmode power supplys=C2=A0are nowadays equipped with tw=
o terminal mains plugs,=C2=A0eliminating cost=C2=A0for country-dependent an=
d=C2=A0clumsy three terminal plugs=C2=A0with a protective earth pin.=C2=A0T=
o reduce RF emissions, they usually contain a shunt capacitor between outpu=
t and one=C2=A0(or sometimes both) of the mains lines. This=C2=A0is meant=
=C2=A0to minimize radiation of switching harmonics from the oputput DC line=
s due to capacitive=C2=A0leakage in=C2=A0the ferrite transformer.=C2=A0As t=
here is no ground to shunt to,=C2=A0going to=C2=A0mains primary is the only=
 option. The capacitor=C2=A0is usually several 100 pF to a couple of nF, a =
tradeoff between EMC (especially at low frequencies) and 50 Hz leakage curr=
ents to the output, and being a safety-critical component=C2=A0it needs to =
be rated for high surge voltages. Whether the leakage=C2=A0current is prese=
nt or not may depend on the polarity of the plug - try reversing it!=20
=C2=A0
The best solution is to=C2=A0externally ground the output or connected devi=
ces to protective earth, which will eliminate the 50 Hz leakage, and very l=
ikely=C2=A0improve LF interference a lot.=C2=A0
=C2=A0
I haven't seen=C2=A025 Hz periodics from one of my power supplies yet. But=
=C2=A0it may be related to=C2=A0a method of improving light-load conversion=
 efficiency by intermittent switcher operation.=C2=A0


Best 73,
Markus (DF6NM)

=C2=A0
-----Urspr=C3=BCngliche Mitteilung-----=20
Von: John Rabson &lt;john.rabson07@gmail.com&gt;
An: GQRP &lt;GQRP@yahoogroups.com&gt;; rsgb_lf_group &lt;rsgb_lf_group@blac=
ksheep.org&gt;; rsgb_lf_group &lt;rsgb_lf_group@yahoogroups.co.uk&gt;
Verschickt: So, 1 Jun 2014 11:58 am
Betreff: LF: Switch mode power supply nasty

While chasing some QRM on LF, the ordinary wall wart which powered my recei=
ver died. I replaced it with a switch mode power supply which I had previou=
sly used with a small computer. This turned out not to be a good idea for t=
wo reasons:

The new PSU generated even more interference, which seemed to have a fundam=
ental frequency of 25 Hz (half that of the mains),
=C2=A0More importantly, I got a tingling sensation when I touched the metal=
 case of the receiver. Although my AC voltmeter did not indicate that the c=
ase was live, my trusty electrician=E2=80=99s screwdriver with built-in neo=
n bulb lit quite brightly.


This PSU is going to the electronics disposal unit at the local waste manag=
ement site. Even if the stray potential is not a simple electrical hazard, =
it could easily provoke an involuntary movement resulting in damage to some=
 piece of equipment. I seem to recall the late G3VA reporting in the Bullet=
in that he had lost an AVO 8 that way.


John F5VLF


----------MB_8D14C640B325E75_2EEC_10B68E_webmail-d258.sysops.aol.com
Content-Transfer-Encoding: quoted-printable
Content-Type: text/html; charset="utf-8"


<div style=3D"FONT-FAMILY: arial; COLOR: black; FONT-SIZE: 10pt"><FONT colo=
r=3Dblack face=3Darial></FONT><SPAN style=3D"DISPLAY: inline-block" content=
Editable=3Dfalse></SPAN>Hi John,</div>


<div style=3D"FONT-FAMILY: arial; COLOR: black; FONT-SIZE: 10pt">&nbsp;</di=
v>


<div style=3D"FONT-FAMILY: arial; COLOR: black; FONT-SIZE: 10pt">

<div>Your supply is probably working as designed - don't dump it, ground it=
!</div>


<div>&nbsp;</div>


<div>The "tingling" this is a fairly common effect&nbsp;of 50 Hz leakage cu=
rrent.&nbsp;I find that sliding the dry backside of a finger across metal&n=
bsp;parts&nbsp;can produce a&nbsp;"rubbery"&nbsp;or vibrating sensation&nbs=
p;with any of my laptops and laptop supplies.</div>
</div>


<div style=3D"FONT-FAMILY: arial; COLOR: black; FONT-SIZE: 10pt">&nbsp;</di=
v>


<div style=3D"FONT-FAMILY: arial; COLOR: black; FONT-SIZE: 10pt">Most&nbsp;=
small switchmode power supplys&nbsp;are nowadays equipped with two terminal=
 mains plugs,&nbsp;eliminating cost&nbsp;for country-dependent and&nbsp;clu=
msy three terminal plugs&nbsp;with a protective earth pin.&nbsp;To reduce R=
F emissions, they usually contain a shunt capacitor between output and one&=
nbsp;(or sometimes both) of the mains lines. This&nbsp;is meant&nbsp;to min=
imize radiation of switching harmonics from the oputput DC lines due to cap=
acitive&nbsp;leakage in&nbsp;the ferrite transformer.&nbsp;As there is no g=
round to shunt to,&nbsp;going to&nbsp;mains primary is the only option. The=
 capacitor&nbsp;is usually several 100 pF to a couple of nF, a tradeoff bet=
ween EMC (especially at low frequencies) and 50 Hz leakage currents to the =
output, and being a safety-critical component&nbsp;it needs to be rated for=
 high surge voltages. Whether the leakage&nbsp;current is present or not ma=
y depend on the polarity of the plug - try reversing it! </div>


<div style=3D"FONT-FAMILY: arial; COLOR: black; FONT-SIZE: 10pt">&nbsp;</di=
v>


<div style=3D"FONT-FAMILY: arial; COLOR: black; FONT-SIZE: 10pt">The best s=
olution is to&nbsp;externally ground the output or connected devices to pro=
tective earth, which will eliminate the 50 Hz leakage, and very likely&nbsp=
;improve LF interference a lot.&nbsp;</div>


<div style=3D"FONT-FAMILY: arial; COLOR: black; FONT-SIZE: 10pt">&nbsp;</di=
v>


<div style=3D"FONT-FAMILY: arial; COLOR: black; FONT-SIZE: 10pt">I haven't =
seen&nbsp;25 Hz periodics from one of my power supplies yet. But&nbsp;it ma=
y be related to&nbsp;a method of improving light-load conversion efficiency=
 by intermittent switcher operation.&nbsp;</div>


<div><br>
</div>


<div>Best 73,</div>


<div>Markus (DF6NM)</div>


<div><br>
&nbsp;</div>
-----Urspr=C3=BCngliche Mitteilung----- <br>
Von: John Rabson &lt;john.rabson07@gmail.com&gt;<br>
An: GQRP &lt;GQRP@yahoogroups.com&gt;; rsgb_lf_group &lt;rsgb_lf_group@blac=
ksheep.org&gt;; rsgb_lf_group &lt;rsgb_lf_group@yahoogroups.co.uk&gt;<br>
Verschickt: So, 1 Jun 2014 11:58 am<br>
Betreff: LF: Switch mode power supply nasty<br>
<br>


<div id=3DAOLMsgPart_1_9d887a66-f0ea-496d-8985-2dc3a9b78b8d>

<div style=3D"WORD-WRAP: break-word; -webkit-nbsp-mode: space; -webkit-line=
-break: after-white-space" class=3DaolReplacedBody>While chasing some QRM o=
n LF, the ordinary wall wart which powered my receiver died. I replaced it =
with a switch mode power supply which I had previously used with a small co=
mputer. This turned out not to be a good idea for two reasons:

<div><br>
</div>


<div>
<UL class=3Daolmail_MailOutline>
<LI>The new PSU generated even more interference, which seemed to have a fu=
ndamental frequency of 25 Hz (half that of the mains),</LI>
<LI>&nbsp;More importantly, I got a tingling sensation when I touched the m=
etal case of the receiver. Although my AC voltmeter did not indicate that t=
he case was live, my trusty electrician=E2=80=99s screwdriver with built-in=
 neon bulb lit quite brightly.</LI></UL>

<div><br>
</div>
</div>


<div>This PSU is going to the electronics disposal unit at the local waste =
management site. Even if the stray potential is not a simple electrical haz=
ard, it could easily provoke an involuntary movement resulting in damage to=
 some piece of equipment. I seem to recall the late G3VA reporting in the B=
ulletin that he had lost an AVO 8 that way.</div>


<div><br>
</div>


<div>John F5VLF</div>


<div><br>
</div>
</div>
</div>

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