isolation transformer use

[peter g]

hi all, i'am planning on taking waveform measurements in a plasma tv. I will be using both hot and cold grounds. the isolation trans that I have were made for tv servicing. my question is, do I use two iso trans, one for the tv and one for the scope? as I recall the scope shouldn't be isolated, only the tv. is this correct?

 

[ted]

The general rule is that TV may have a dangerous voltage on its chassis. Therefore the isolation transformer is used.

However, another general rule is that the oscilloscope should be safety grounded. And then no isolation transformer is used for it.

 

[D.A.(Tony)Stewart]

I challenge anyone to show me a PLasma TV schematic that does not have a transformer-isolated primary side switching regulator.

They will have 2 pin AC plugs and two different internal ground points. Only one is safe.

One AC gnd is for primary side rectifier return ( not to be connected to scope gnd) and the other one is for secondary DC out which connects to all audio video peripherals safely.
There will be a small valued 3kV+ rated coupling cap and > 1M Resistor bridging the two grounds for noise reduction.