Why is GND of scope channels connected to case?

luka · 14th February 2007, 09:07 AM

I don't know why is that so, but there is a problem if the case is connected to earth in socket and you try to measure floating voltage with probe (like + goes to 12V and GND goes to 5V). This makes short between 5v and earth and coused/destroyed one of the bypass transistors in the supply

The unit under test was push-pull smps for 12v input,supplyed from regulated (now dameged) supply with adjustable voltage/current limit and scope being connected to primary CT of smps with probe GND, + was unconnected.

Geek · 14th February 2007, 09:27 AM

What kind of scope you have?

Only scope I've used with an earth grounded "-" lead was an ancient DuMont.

luka · 14th February 2007, 10:12 AM

Hi

Well one is Tektronix 2235 and other one is LeCroy 9400A and Instek GOS658. LeCroy is digital scope.

Geek · 14th February 2007, 10:20 AM

Hmmm, other than a special adaptor that allows differential measurements, I can't think of any advice

The scope's I've used were all either B+K, HP, Hickok and that one little DuMont.

luka · 14th February 2007, 10:35 AM

Hi

Seems to me like all do have this "problem". The only solution I see is no to ground (earth) the scope, but then I would tuch the case of it

SY · 14th February 2007, 12:12 PM

You hit it on the last one- it's for safety. If the grounds were isolated, some fool would measure the waveform across a 1 ohm resistor in a 500V B+ feed. As he reaches over to change the vertical sensitivity, his hand touches the case of the scope.... ZAP. RIP.

A floating diff amp is pretty easy to cob together. And it won't kill you.

KISS · 18th February 2007, 06:07 AM

Psst. Ever try the A-B mode. Or put the isolation xformer on what your measureing.

luka · 18th February 2007, 07:35 AM

Hi

The A-B mode is not an option since I need both channels at the same time, and why I would use isolation xformer if this could be already done in/with scope.
I still don't see the logic in channels gnd connection to case.Case is suppose to be only for safety if something goes wrong inside of scope, like phase gets loose and tuches the case of scope.

Hi SY

Quote:

some fool would measure the waveform across a 1 ohm resistor in a 500V B+ feed

Then how can you do that??

14th February 2007, 09:07 AM	#1
luka diyAudio Member Join Date: Sep 2005 Location: in Slovenia :)	Why is GND of scope channels connected to case? I don't know why is that so, but there is a problem if the case is connected to earth in socket and you try to measure floating voltage with probe (like + goes to 12V and GND goes to 5V). This makes short between 5v and earth and coused/destroyed one of the bypass transistors in the supply The unit under test was push-pull smps for 12v input,supplyed from regulated (now dameged) supply with adjustable voltage/current limit and scope being connected to primary CT of smps with probe GND, + was unconnected. __________________ home page @ http://www.classdaudio.co.cc @ 24/7 all year long I FEEL SLOVENIA

14th February 2007, 10:12 AM	#3
luka diyAudio Member Join Date: Sep 2005 Location: in Slovenia :)	Hi Well one is Tektronix 2235 and other one is LeCroy 9400A and Instek GOS658. LeCroy is digital scope. __________________ home page @ http://www.classdaudio.co.cc @ 24/7 all year long I FEEL SLOVENIA

14th February 2007, 10:35 AM	#5
luka diyAudio Member Join Date: Sep 2005 Location: in Slovenia :)	Hi Seems to me like all do have this "problem". The only solution I see is no to ground (earth) the scope, but then I would tuch the case of it __________________ home page @ http://www.classdaudio.co.cc @ 24/7 all year long I FEEL SLOVENIA

14th February 2007, 12:12 PM	#6
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	You hit it on the last one- it's for safety. If the grounds were isolated, some fool would measure the waveform across a 1 ohm resistor in a 500V B+ feed. As he reaches over to change the vertical sensitivity, his hand touches the case of the scope.... ZAP. RIP. A floating diff amp is pretty easy to cob together. And it won't kill you. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

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14th February 2007, 09:27 AM	#2
Geek diyAudio Member Join Date: Sep 2004	What kind of scope you have? Only scope I've used with an earth grounded "-" lead was an ancient DuMont.

14th February 2007, 10:20 AM	#4
Geek diyAudio Member Join Date: Sep 2004	Hmmm, other than a special adaptor that allows differential measurements, I can't think of any advice The scope's I've used were all either B+K, HP, Hickok and that one little DuMont.

18th February 2007, 06:07 AM	#7
KISS diyAudio Member Join Date: Jan 2007	Psst. Ever try the A-B mode. Or put the isolation xformer on what your measureing.

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