double insulated external power supplies - dangerous?

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I hope this is not too off topic here, I tried my best to find a group
that would be appropriate to post my question... I'm a guitar player,
and I'm using an effects processor which is powered by an external 9v
transformer connected to mains. Like most external power supplies I
see, this is a "double insulated" transformer, and has no ground. Now,
knowing how dangerous it is to connect the guitar to an amplifier
which is not grounded, I'm kind of worried whether it's possible that
these doubly insulated ungrounded external power supplies may put a
dangerous voltage on the guitar strings. Anyone?

 

[Tom Biasi]

I hope this is not too off topic here, I tried my best to find a group
that would be appropriate to post my question... I'm a guitar player,
and I'm using an effects processor which is powered by an external 9v
transformer connected to mains. Like most external power supplies I
see, this is a "double insulated" transformer, and has no ground. Now,
knowing how dangerous it is to connect the guitar to an amplifier
which is not grounded, I'm kind of worried whether it's possible that
these doubly insulated ungrounded external power supplies may put a
dangerous voltage on the guitar strings. Anyone?

The supplies are intended for such use.
Relax and play.

Tom

 

[Tim Wescott]

I hope this is not too off topic here, I tried my best to find a group
that would be appropriate to post my question... I'm a guitar player,
and I'm using an effects processor which is powered by an external 9v
transformer connected to mains. Like most external power supplies I see,
this is a "double insulated" transformer, and has no ground. Now,
knowing how dangerous it is to connect the guitar to an amplifier which
is not grounded, I'm kind of worried whether it's possible that these
doubly insulated ungrounded external power supplies may put a dangerous
voltage on the guitar strings. Anyone?

Safety design on electrical appliances is done by answering the question
"how many faults do there need to be before someone gets zapped?".

In the case of your guitar amplifier, there are multiple opportunities
for you to come into contact with metal that is connected to the internal
circuitry. Without a ground connection, a single power supply fault
could put dangerous voltages on your guitar (or at least on the sound
leads). With a ground connection, that power supply fault would blow a
fuse or maybe start a small fire, but it wouldn't kill you right off. So
to have a problem requires two faults: one, the ground connection has to
break, and two, your power supply needs to have a problem.

In the case of your 9V wall wart, the 9V itself won't kill you -- you're
in more danger of someone twirling the thing at 90 miles and hour and
whacking you on the head with it than you are of getting a shock from
9V. So the thing is designed to put not one, but two layers of
insulation between you and the line voltage (and no, I don't know how
it's done in wall warts -- I should find out). Here again, it takes two
faults before a dangerous condition exists.

Make sense?

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

[Phil Allison]

"Tim Wescott"

In the case of your 9V wall wart, the 9V itself won't kill you -- you're
in more danger of someone twirling the thing at 90 miles and hour and
whacking you on the head with it than you are of getting a shock from
9V.

** Which is not the issue.

The possibility of the un-grounded output wires delivering supply voltage
is.

So the thing is designed to put not one, but two layers of
insulation between you and the line voltage (and no, I don't know how
it's done in wall warts -- I should find out).

** Simple having two layers of insulation is no nearly god enough to
qualify as "double insulate" or more correctly class 2 insulation.

With transformer "wall warts" the construction of the transformer is
crucial, so that secondary and primary can never come into contact - even if
the windings burn up.

Normally, a thermal fuse is fitted inside the windings to eliminate that
risk .

Here again, it takes two
faults before a dangerous condition exists.

** Unfortunately not true of many SMPS external supplies that claim to be
Class 2.

Single component failure or water ingress can render them very dangerous.



....... Phil

 

[Eeyore]

I hope this is not too off topic here, I tried my best to find a group
that would be appropriate to post my question... I'm a guitar player,
and I'm using an effects processor which is powered by an external 9v
transformer connected to mains. Like most external power supplies I
see, this is a "double insulated" transformer, and has no ground. Now,
knowing how dangerous it is to connect the guitar to an amplifier
which is not grounded, I'm kind of worried whether it's possible that
these doubly insulated ungrounded external power supplies may put a
dangerous voltage on the guitar strings. Anyone?

Totally safe. That's the whole idea of double insulation.

The guitar amplifier however probably SHOULD be grounded. Isn't it ?

Graham

 

[Daniel Mandic]

The guitar amplifier however probably SHOULD be grounded. Isn't it ?

Graham

Hi Graham!


Isn't the ground amplified through the guitarist, changing the
potential by moving back and forth to the amplifier. ?? ;-)



Best regards,

Daniel Mandic

 

[Paul E. Schoen]

Hi Graham!


Isn't the ground amplified through the guitarist, changing the
potential by moving back and forth to the amplifier. ?? ;-)

That's only for feedback, as described by the Beatles in their song "Help".
They sang, "Help me get my feedback on the ground!"

Paul

 

[Tim Wescott]

"Tim Wescott"




** Which is not the issue.

The possibility of the un-grounded output wires delivering supply
voltage
is.




** Simple having two layers of insulation is no nearly god enough to
qualify as "double insulate" or more correctly class 2 insulation.

With transformer "wall warts" the construction of the transformer is
crucial, so that secondary and primary can never come into contact -
even if the windings burn up.

Yea, I knew I had no clue of the actual details -- just that there was
some magic done to make it happen (or at least claimed magic).

Do you know of any web sites that show how this is done?

Normally, a thermal fuse is fitted inside the windings to eliminate that
risk .




** Unfortunately not true of many SMPS external supplies that claim to
be Class 2.

Single component failure or water ingress can render them very
dangerous.



...... Phil

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

[Phil Allison]

"Tim Wescott"

Yea, I knew I had no clue of the actual details -- just that there was
some magic done to make it happen (or at least claimed magic).

Do you know of any web sites that show how this is done?

** Err - did you miss this ?

" Normally, a thermal fuse is fitted inside the windings to eliminate that
risk "

I know of no sites that go into detail of how to design a class 2 mains
transformer - but just have a look at what is inside the next broken wall
wart you see.

May not look much different to a standard small transformer - but it IS !!

There will be things like fireproof insulation between the windings, a lump
on the primary where the thermal fuse is fitted and the primary and
secondary would on separate halves of a bobbin - ie not over-wound.

The other thing is that all wall warts and external PSUs are REQUIRED to
pass stiff lab test procedures before going on sale - including
deliberately introducing faults on the secondary side. ( Not true of class 2
appliances in general, however.)




....... Phil