double insulated external power supplies - dangerous?

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

Tom

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

"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.

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

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

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

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

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?

--
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

** 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