Im trying to scope a TV signal. Where should I ground the probe's ground? Over the common ground on the PCB board? Isn't that a floating ground?
There seems to be 30V ac with reference to the scope ground when I use a VOM to measure voltage. Will it blow or damage the scope? Anyway to prevent sending live current down the scope ground and blowing it up? Thanks.
On Fri, 17 Nov 2006 10:18:57 -0800, rogersk8ter Has Frothed:
Need an isolation supply.
-- Pierre Salinger Memorial Hook, Line & Sinker, June 2004 COOSN-266-06-25794
The first thing you do is obtain an isolation transformer 110/110V or
240/240V that allows TV to float up or down relative to the scope ground. Second you learn about the potential (in both senses) dangers associated with TVs-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on
Can I just ground t to the TV chasis? Where the springs is? Or a isolation transformer is a "must"?
"rogersk8ter" wrote in news: snipped-for-privacy@f16g2000cwb.googlegroups.com:
Don't forget DCV;the chassis can be as much as 170VDC difference from earth ground for a 120VAC line supply.
It's a MUST. Repeat;a MUST.
TV chassis can be electrically hot WRT earth ground. Just look at any switching power suppply schematic. There's no ground,instead "common".
Hook your scope ground to it and you blow the TV and/or your scope,make lots of sparks and smoke.
Or your scope case becomes electrically hot and you touch it and a real ground,you get electrocuted.(that's why you don't use groundbusters.)
-- Jim Yanik jyanik at kua.net
If you have to ask, it's MANDATORY - NO EXCEPTIONS !!!
GG
Could you explain to a Limey what a groundbuster is and I'll add to this "2 nations divided by a common language" file
UK / USA Tool Terminology Translator
"2
I take it from Googling that a ground buster / groundbuster is an adaptor that simply converts a 3 prong AC plug into a 2 prong. In the UK they don't exist as such , the third earth/ground pin is needed to open the safety sliding shrouds covering the powered sections of the socket. "wall warts" for the UK always seem to have plastic earth/ground pins which have to be there as dummies so you can physically plug into UK power sockets.
-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on
"N Cook" wrote in news:ejmif3$cjd$ snipped-for-privacy@inews.gazeta.pl:
Correct.
But one could still cut the ground WIRE either at the plug or inside the scope to disable the ground(to "float the scope"). At least one scope offered a SWITCH to do this,others solder in diodes to "isolate" the ground.Bad idea,as you have no idea if the diodes are always functional and not shorted or open.
Tektronix used to offer a pamphlet about scope measurements and ground safety.I used to include one with every scope I repaired that had the ground pin cut off or otherwise disabled.It explained why various methods of "floating" a scope are unsafe. IIRC,TEK now has something similar on their website.
-- Jim Yanik jyanik at kua.net
Jim Yanik wrote in news:Xns987F8172D2250jyanikkuanet@129.250.170.86:
Yep, here it is:
Quoting from the web page,
"Floating" a ground referenced oscilloscope is the technique of defeating the oscilloscope's protective grounding system - disconnecting "signal common" from earth, either by defeating the grounding system or using an isolation transformer. This allows accessible parts of the instrument such as chassis, cabinet, and connectors to assume the potential of the probe ground lead connection point. This is dangerous, not only from the standpoint of elevated voltages present on the oscilloscope (a shock hazard to the operator), but also due to cumulative stresses on the oscilloscope's power transformer insulation. This stress may not cause immediate failure, but may lead to future dangerous failures (a shock and fire hazard), even after returning the oscilloscope to properly grounded operation!
Not only is floating a ground-referenced oscilloscope dangerous, but the measurements are often inaccurate. This results from the total capacitance of the oscilloscope chassis being directly connected to the circuit under test at the point where the common lead is connected.
WARNING
Never attempt to defeat the protective grounding system of your oscilloscope by using an isolation transformer or disconnecting the ground connector on the power plug. Failure to follow safety warnings can result in serious injury or loss of life.
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