safe way to use grounded scope with non isolated equipment ? - 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
Closed Thread
 
Thread Tools Search this Thread
Old 9th August 2013, 12:46 PM   #1
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Default safe way to use grounded scope with non isolated equipment ?


I want to start experimenting with SMPS.

The scope will still be directly plugged into the mains outlet socket and connected to Protective Earth via it's own earth wire.

I plan to use an isolating transformer to power up the mains side of the mains equipment.

Does this arrangement remove the risk of killing myself and stop blowing up my equipment?

Is there anything else I should be aware of?
__________________
regards Andrew T.
 
Old 9th August 2013, 01:56 PM   #2
diyAudio Member
 
Join Date: Jan 2007
Location: BC, Canada
Just get the Fluke/Tektronix portable scopemeters with ISOLATED channels. Gives you the freedom of measuring each point of different reference with no risk of blowing up the scope or electrocuting yourself.

Your proposed setup will work but you are bound to make mistake. Above solution is way better and cheaper than buying differential probe adapters.
 
Old 9th August 2013, 01:59 PM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
Quote:
Originally Posted by hitachi_nut View Post
.......Your proposed setup will work but you are bound to make mistake.........
What types of mistakes to blow me or the equipment up?
__________________
regards Andrew T.
 
Old 9th August 2013, 02:02 PM   #4
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
No. The risk you have eliminated - well, reduced some anyway - is that of the direct connection to the mains. And you can now connect the scope ground to some point that would otherwise be hot. But if the SMPS rectifies the mains and makes 340vDC, then you still have 340v, even if it is no longer referenced to earth. a 340v battery in your hand is dangerous.

A tube amplifier uses a power transformer, so all its circuits (other than mains wiring) are isolated. But those 500v circuits can still kill you perfectly dead. Just so the SMPS.

Here in 120v land, one side of the mains is neutral, which is bonded to earth. In my typical SMPS, that is rectified into +170v and -170v with respect to earth. The primary side runs on that. In many if not most of them, the control circuits for the switching run on about 12v. But the "ground" reference for that 12v is the -170v rail. The controller IC sees +12v on its power pin, but from my perspective, that pin is at -158v. I cannot connect my scope ground to that -170v to probe the control circuits without the iso. The iso allows me to do just that. After the iso, my -170v rail is no longer -170v with respect to my earth. But it is still 340v from the other end of the filter caps. I cannot forget this, and should not feel like the SMPS is risk free.

And never lose sight of the fact your scope ground - and with it the frame of your scope - is connected to that point in the circuit. So even with the iso, if you connect the scope ground to that -170v, you cannot also connect some other earthed test equipment ground to some other circuit point. It would short those two points together through the ground leads.


SO yes, you need the iso for your SMPS work, but as to danger, it makes the SMPS no less dangerous than a tube amp.
 
Old 9th August 2013, 02:13 PM   #5
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Andrew, what you propose sounds OK but be aware that HF leakage currents can be more than you might expect. It would be OK once connected but you could still feel quite a jolt from earth to the running but isolated PSU if you touched the two points. It depends what you intend doing. You could also run into trouble if you connect more than one grounded piece of test equipment at a time to different parts of the circuit.

(I used to work all day with SMPS and we had each bench powered from its own isolation tranny. All the test equipment and soldering irons were fed off isolating trannys too. There was no protective safety earth anywhere near, all earth leads in the equipment were isolated, the benches were wooden, the flooring rubber matting. This allowed us to be able to directly connect say a scope probe "ground" lead to the reservoir cap on the primary side of a non isolated PSU with no danger of shock or zapping anything and to be able solder and work "live" using a "mains soldering station on the running PSU)
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
 
Old 10th August 2013, 09:27 AM   #6
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
My reply which includes a further question is missing.
__________________
regards Andrew T.
 
Old 10th August 2013, 09:40 AM   #7
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
Quote:
Originally Posted by AndrewT View Post
My reply which includes a further question is missing.
All your posts have gone through for this thread Andrew. Post#3 was your last.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
 
Old 10th August 2013, 09:46 AM   #8
AndrewT is offline AndrewT  Scotland
diyAudio Member
 
Join Date: Jul 2004
Location: Scottish Borders
No It wasn't.
I proposed a scenario and asked if that helped solve the safety issue.
I also gave feedback to post4.
__________________
regards Andrew T.
 
Old 10th August 2013, 09:58 AM   #9
Mooly is online now Mooly  United Kingdom
diyAudio Moderator
 
Mooly's Avatar
 
Join Date: Sep 2007
There's nothing showing as unapproved or deleted on this thread.
__________________
-------------------------------------------------------
Installing and using LTspice. From beginner to advanced.
 
Old 10th August 2013, 10:38 AM   #10
diyAudio Member
 
Join Date: Dec 2001
Location: South Sweden
You might want to consider some proper true differential HV(as in 600V) hf probes to be able to measure gate drive signals in high side drivers fex. Better CMMR than using the scopes A-B function, and safer for the scope too.

Using a isolation transformer om the supply is wise, as the scope is still earth connected. using it on the scope, while having a direct feed to the supply is lethal, but you knew that already probably.
__________________
listen! Can you hear that ...
 

Closed Thread


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
Test equipment for sale (DMM, scope) USA grenert Swap Meet 6 23rd February 2013 10:38 PM
Isolated vs non-isolated DC converters for car audio amplification Stonner Power Supplies 5 17th July 2012 02:23 AM
USB Scope - Real scope and not too expensive Algar_emi Equipment & Tools 11 16th March 2010 11:40 PM
isolated or non-isolated psu for UcD in a car? zilog Class D 1 2nd February 2009 02:56 AM
Test Equipment (TTi Dual Current-limiting PSU, LFM4 Generator, Hameg 'Scope) thermionic Swap Meet 3 18th September 2008 04:10 AM


New To Site? Need Help?

All times are GMT. The time now is 10:56 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