Clarity over using oscilloscope needed

[Cykar]

I know this might be obvious, but I just want to check I have things right.

I am working on a small portable tv/radio. The power cord has only live and neutral, one of those figure of 8 plugs, therefore there is no connection to earth ground. But my oscilloscope does have an earth ground connection.

I have read much about safety using oscilloscopes and how using an isolation transformer is best and safest, but I don't have one. Something that some do is removing the ground pin from the oscilloscope to isolate from earth ground - I am not planning on doing this. But since the tv itself has no connection to earth ground, is that effectively the same as removing the ground from the oscilloscope, but the other way around?

So, is it safe to use the oscilloscope on the Tv, (signal testing nothing to do with the high voltage), as there is no common ground between the TV and the oscilloscope?

[themadhippy]

NO,you are still providing a path to ground via the scope so connecting a live wire from any source will not have a happy ending

[Cykar]

Good to know. Thanks.

[madires]

In most cases neutral is connected to earth ground in the mains distribution box and also at the power substation or pole transformer. That means that mains is referenced to earth/PE. So the TV and the scope have a common ground. To work around that you can use differential probes.

[bob91343]

Always good to use an isolation transformer.  With that you can usually forget about ground problems.

[srb1954]

In most cases neutral is connected to earth ground in the mains distribution box and also at the power substation or pole transformer. That means that mains is referenced to earth/PE. So the TV and the scope have a common ground. To work around that you can use differential probes.

The fact that the neutral might share a common ground with the scope ground does not mean that the signal common within the TV set shares that same ground.

If the TV set doesn't have a mains transformer and uses a full wave bridge rectifier directly off the mains then the signal common within the TV set will be bouncing up and down at full mains voltage. Connecting a scope probe ground lead to the common in such a set will result in lots of sparks and probably a damaged scope or scope probe.

[mikerj]

As said it depends entirely on the type of power supply the unit use internally.  If it has conventional transformer based power supply, or a fully isolated switching supply (most modern stuff) then you should be safe to clip your scope ground to the circuitry after the power section.

[Cykar]

As said it depends entirely on the type of power supply the unit use internally.  If it has conventional transformer based power supply, or a fully isolated switching supply (most modern stuff) then you should be safe to clip your scope ground to the circuitry after the power section.

It looks like the the live and neutral go straight into a transformer, 2 wires in, 2 wires out which then go to a power board for rectification.

[CaptDon]

Sometimes a cheap replacement for an isolation transformer is
a transformer with a 120/120 primary and 240/240 secondary
4 fully separate windings generally used as a step-up/step-down
transformer and sometimes called a 'control' transformer. This
device can be used in many different ways including isolation.
It will undoubtedly have a bit more capacitance between windings
like 250pf vs 30pf of a true isolation transformer but at 50 or 60hz
that extra capacitance amounts to only a couple of microamps of
stray leakage. Often these type of control transformers can be had
as junk surplus for very little cost. One rated at around 250va to
500va would be all you would ever need for a home lab setup.
 

[bob91343]

That is true but many of us have the makings of isolation transformers.  Specifically, equipment that offers a choice of mains (line) voltage of either 120 or 240.  This is usually accomplished by a series or parallel connection of the primary windings.  If one changes the setup and uses just one winding as a primary and the other as the secondary with no interconnect, we have an isolation transformer suitable for many uses.

One important step is to disconnect all other windings otherwise there is risk of overloading.  And the  power rating is cut in half, since the windings cannot share current.  There is still the capacitance between windings but in many cases that isn't important.

[THDplusN_bad]

Good Day,

for the benefit of your personal safety and learning about proper grounding of equipment when probing, please can I recommend the following brochure: "ABCs of probes" from Tektronix
https://download.tek.com/document/02_ABCs-of-Probes-Primer.pdf Read and make sure to understand pages 46 and following.

Good luck,

THDplusN_bad