Measuring floating voltages with oscilloscope - diyAudio
Go Back   Home > Forums > Design & Build > Equipment & Tools

Equipment & Tools From test equipment to hand tools

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 12th May 2012, 07:15 PM   #1
akis is offline akis  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
Default Measuring floating voltages with oscilloscope

In another thread I was asking what is the recommended way to look at and measure a floating voltage using the oscilloscope. The problem is that the oscilloscope probe is earthed and effectively you will earth the component under test, usually with very undesirable results (like meltdown).

There is some good information here:
http://www.newark.com/pdfs/techartic...tronix/FFM.pdf



Solutions that were mentioned in the other thread were:

a) use an isolation transformer on the oscilloscope to "float" its own earth.

This solution means messing with the power side of the scope, not sure how you'd go about earthing it (it has a huge metal chassis) and it still leaves both its probes connected to the same "virtual earth". In any case I do not have a mains isolation transformer and it sounds dangerous.

b) use the two probes together (only the tips) and then "invert" one channel and "add" function on the scope in order to see the curve. This is the "A minus B" technique.

This technique works, sometimes, but it ties down both channels on the scope, and it also requires a reference ground, so cannot be used in a lot of cases where you want to measure a true floating voltage without any references to the ground.


I have therefore decided to experiment with an isolation device that I can put between the component under test and the probe.

I have come up with the simple circuit below, a textbook differential amplifier. I have tested it and it works OK, but more tests to come. If successful I was thinking of building it in the tiniest package possible, so itself looks like a probe, or a probe extension and can be used easily.

Please let me know what you think.
Attached Images
File Type: jpg differential-probes.JPG (68.4 KB, 250 views)
  Reply With Quote
Old 12th May 2012, 07:27 PM   #2
diyAudio Member
 
Join Date: Mar 2002
All you have to do is disconnect the 3rd wire (earth)
on the (scope) mains plug and make sure no other
equipment is attached. This puts the ground of the
scope @ the potential of the second output, but it works.
  Reply With Quote
Old 12th May 2012, 07:31 PM   #3
diyAudio Member
 
Join Date: Sep 2004
Location: UK
Quote:
Originally Posted by hitsware View Post
All you have to do is disconnect the 3rd wire (earth)
on the (scope) mains plug and make sure no other
equipment is attached. This puts the ground of the
scope @ the potential of the second output, but it works.
Agreed! This is what I do, but, WARNING! This is potentially dangerous and cannot be recommended here.

Those experimenters confident enough to do such things do not need advice such as this.
  Reply With Quote
Old 12th May 2012, 07:35 PM   #4
diyAudio Member
 
Join Date: Sep 2004
Location: UK
AKIS, why don't you give real examples of what you are trying to measure?

Your circuit is effectively what a 'scope differential plug-in does.

You will have significant errors in CMRR, bandwidth and DC accuracy unless you spend the equivalent of a decent TEK diff plug-in will cost you!
  Reply With Quote
Old 12th May 2012, 08:01 PM   #5
diyAudio Member
 
luvdunhill's Avatar
 
Join Date: Jul 2006
Does your scope have a battery pack option? I ended up ordering a high voltage differential probe though.
  Reply With Quote
Old 12th May 2012, 09:08 PM   #6
akis is offline akis  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
I am not It trying to measure tube-like voltages, but simply to be able to measure anything on any circuit, without shorting it to the ground.

My first experiment was OK, except the frequency response is weird. It is correct until about 80KHz and then it starts to rise with a peak at about 500KHz and then falls rapidly.
I am not sure if this is an artifact of the way I have mounted the bits on the breadboard, or the breadboard itself (parasitic capacitances).
  Reply With Quote
Old 12th May 2012, 09:23 PM   #7
diyAudio Member
 
Join Date: Sep 2004
Location: UK
"...but simply to be able to measure anything on any circuit .."

Well, good luck with that!
  Reply With Quote
Old 12th May 2012, 09:25 PM   #8
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Probably too much stray capacitance at - op amps inputs. A small cap across the feedback resistors might help.
  Reply With Quote
Old 13th May 2012, 06:29 AM   #9
akis is offline akis  United Kingdom
diyAudio Member
 
Join Date: Jul 2008
I will try to fix it. Seeing as differential probes cost thousands of dollars.
  Reply With Quote
Old 13th May 2012, 06:38 AM   #10
diyAudio Member
 
Loudthud's Avatar
 
Join Date: Nov 2007
Location: Near Dallas Texas USA
Checkout the OPA117 by Burr Brown. A differential amp with a gain of one, 300V common mode range on the input when used with +/- 15V rails. Not totally isolated but fairly high impedance.

Never float the ground on your scope!
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Measuring ultra high voltages with DMM Alexontherocks Equipment & Tools 9 14th April 2012 02:16 PM
measuring voltages and currents simple se pforeman Tubes / Valves 1 10th August 2009 02:38 AM
cheap usb oscilloscope for measuring amplifiers umut1001 Class D 27 5th September 2007 01:33 PM
Floating voltages eeka chu Solid State 12 29th January 2004 03:57 PM
Floating voltages eeka chu Parts 2 27th January 2004 09:10 AM


New To Site? Need Help?

All times are GMT. The time now is 10:24 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2