Isolation Transformer low voltage question

Chuck Pairsi's picture

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Joined: Jan 23, 2021

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Isolation Transformer low voltage question
January 23, 2021 - 5:03am

I modified  an isolation transformer, but still have some low voltage AC between secondary lines and outlet ground, even though grounds are not connected.   See attached.  When I measure AC voltage, with a DMM, between outlet ground and left L1 or right L2 outlet inputs I read ~6 volts AC on left and ~12 volts AC on right.  Outlets are plastic and not grounded to case, so I'm not sure how I am even completing a circuit.  I measured resistance between outlet ground and left, then right inputs and this reads infinite resistance (open) which makes sense.  So I am unclear how I can be measuring some low level AC voltage between grounds that are NOT connected and the isolated L1 and L2 lines.  Hope to hear back from someone smarter than me!   THANK YOU. 

 

Chuck Pairsi's picture

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Joined: Jan 23, 2021

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thanks for your assistance

Sourav Gupta's picture

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Joined: Feb 12, 2018

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This is called ghost voltage and this is happening due to the DMM impedance.
Maybe a very small amount of leakage current even while the resistance is in giga ohms range making this wrong voltage interpretation in the DMM.

What is the AC voltage between L1 and L2?

Chuck Pairsi's picture

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Joined: Jan 23, 2021

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Thanks Sourav.  AC voltate between L1 & L2 is 120V's.  I assumed voltage reading may be related to the Fluke 77.

 

Sourav Gupta's picture

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I am quite sure it is ghost voltage and this is due to the impedance of the multimeter and the ADC wrong reading. You done the right thing, I guess.