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Old 25th May 2008, 12:43 AM   #1
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Default 2-Wire AC Question

I have a 2 wire AC supply (no green ground) in one part of an old house. When I plug my amp into the AC receptacle and turn the amp on, the amp's external case rises to 50VAC. There is evidently very little current associated with this voltage .... ~ 45 uA AC tested with my DMM through a 10K resistor. When the DMM is connected as such, amp case voltage reduces to 0.4VAC.

Can anyone tell me what's happening here? Is the setup dangerous?
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Old 25th May 2008, 01:06 AM   #2
TheMG is offline TheMG  Canada
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Is this an amp that you built?

If it is a commercial unit, often they will use a capacitor or a high value resistor and connect the chassis through that to the neutral wire. If the plug is flipped, you get a small current on the chassis as it's now coupled to the live (hot) wire. Newer units use polarized plugs so this is not possible unless something is wired incorrectly (in the house wiring).

If it's something you built and you didn't connect chassis to anything, it's probably just the result of a small amount of capacitive/resistive coupling between the power transformer and/or output stage and the chassis.
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Old 25th May 2008, 01:20 AM   #3
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It's a commercial amp. The voltage also appears on the case of a balanced AC transformer I built whose (green) grounds on the primary and secondary sides are tied together to the metal case.
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Old 25th May 2008, 04:47 AM   #4
TheMG is offline TheMG  Canada
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Most transformers unless they have an earth screen between the primary and secondary windings and that it is connected to ground, will have some leakage from primary to secondary. Toroids especially, since the windings are one over top the other. EI transformers not as much, and depending on how the windings are layed out. This is due to capacitance between the windings. All insulators have some amount of capacitance.

Normally this is not a cause for concern as long as class 2 mains power design practices were strictly adhered to. Otherwise the chassis should be connected to earth ground.
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