EEVblog #279 - How NOT To Blow Up Your Oscilloscope! - Page 1
Forum section: electronics / testgear

author: iDeeW
author_rank: Newbie
timestamp: April 04, 2014, 05:13:10 pm
content: Hi eevblogers ,....This is my first post on eevblog. Thanks for all your hard work Dave.I just watched the EEVblog #279 - How NOT To Blow Up Your Oscilloscope!.This exact same thing happened to me a while ago, when I bought my first scope (Tek2232).Test circuit blew up right in front of my face.So I was wondering, what if I use 2 X 12V xformers back to back and isolate AC line while using the ground connection.(Dave mentioned this is not safe in the vid#279)Check the circuit below ....http://www.seekic.com/circuit_diagram/Communication_Circuit/INEXPENSIVE_ISOLATION_TRANSFORMERIMPROMPTU_SETUP.htmlIsolation and ground protection both can be achieved.....Am I wright or wrong?!?Thank youiDeew

author: Shock
author_rank: Super Contributor
timestamp: April 04, 2014, 11:46:34 pm
content: That circuit is ok but you will still need to put it in an enclosure and earth and fuse it properly.

author: David Hess
author_rank: Super Contributor
timestamp: April 05, 2014, 12:12:10 am
content: I have exactly this on my bench for working with off-line power supplies where the neutral needs to be isolated from ground to prevent ground loops. I used a pair of large transformers that I rescued with most of their secondaries wired back to back. Since they have split primaries intended for 120 or 240 volt operation, I wired the output as a pair of isolated outputs and I have another low power pair of 120 volt isolated outputs from the smaller 120 volt secondary windings that they happen to have.

author: Shock
author_rank: Super Contributor
timestamp: April 05, 2014, 05:46:56 am
content: Quote from: David Hess on April 05, 2014, 12:12:10 amWhat did you pull them out of? Just wondering if there is anything common I could do the same with. Less and less stuff these days has dual ~240/120v operation.

author: David Hess
author_rank: Super Contributor
timestamp: April 05, 2014, 07:22:40 am
content: Quote from: Shock on April 05, 2014, 05:46:56 amThey were part of some really big switching power supplies which I suspect were for old Cyber or DEC era computer systems.It is hardly uncommon for power transformers to have dual primaries which can be wired for 120 or 240 volt operation.

author: iDeeW
author_rank: Newbie
timestamp: April 08, 2014, 05:55:39 pm
content: Thanks for the replies...Look at this commercial isolation transformer ... check the circuit diagramwhy is neutral and ground connected together???!!!http://www.carebase.com/Isolation-transformer-line-conditioner-neutral-ground-bond-EI-core-low-impedance-120V-120vac-600VA-1000VA-1kVA-ILc-600B-1000B.htm

author: mcinque
author_rank: Unknown
timestamp: April 08, 2014, 07:04:43 pm
content: I don't know why, but that connection thwarts the purpose for which that device was built. If you buy that device, you should disconnect that connection, like Toddfun did

author: David Hess
author_rank: Super Contributor
timestamp: April 08, 2014, 07:04:53 pm
content: Quote from: iDeeW on April 08, 2014, 05:55:39 pmAt least in the US, that is how the power is wired anyway. Neutral and ground are connected to each other at a single place which is the distribution panel and that is what equipment that uses ground, neutral, and hot expects. The neutral voltage will be different than the ground voltage because of the current flowing through neutral. You can actually measure the neutral resistance this way by applying a known load between hot and neutral and measuring the voltage from neutral to ground.In this case they are duplicating how the existing power connection is wired and preventing the isolated output from drifting away from ground and probably meeting some electrical code requirement. The isolation transformer is isolating the neutral and hot but not the ground.When connecting a test instrument with the probe ground tied to neutral, two connections are made:1. The grounds are tied together forming a loop through the isolation transformer and the test instrument but since no current should be flowing through the grounds, this should not matter. In reality there is all kinds noise between the two grounds even when they originate from the same place so the ground loop results in all kinds of measurement noise.2. The ground to neutral connection in the isolation transformer is to a different point on the neutral than the probe ground to neutral connection. The different in potential between those two neutral connections will generate a current through the ground loop and cause all kinds of problems. In extreme cases, the instrument ground can be damaged.Given the above, I would want to break the ground to neutral connection and do it myself through the probe ground or instrument chassis ground to preserve a single point ground. The safer alternative is to use differential probing so no direct ground connection to the probe is necessary but that is not always feasible.The situation is even worse if you have multiple earth grounds going to different places but that is more likely in an industrial setting. In that case the isolation transformer may be used to trade one earth ground for another (electrical code permitting?) but it amounts to the same thing if the neutral and ground are connected through your test instruments and not the isolation transformer.

author: iDeeW
author_rank: Newbie
timestamp: April 09, 2014, 12:06:36 pm
content: thanks for all the replies...

author: theoldwizard1
author_rank: Regular Contributor
timestamp: December 21, 2014, 11:40:17 pm
content: This is an old topic for me, but Dave's explanation was a good review. While not as good as as an isolation transformer for the DUT, I would like some feed back on this kind of wacky idea.Why not take the ground wire off the wire, cut it, solder in a piece of 26-30 AWG wire and cover with heat shrink tubing. In the automotive world this is called a "fusible link". Yes, it will fail catastrophically, but hopefully the heat shrink will contain any sparks.Not a perfect solution because there could be a "weaker" link somewhere in the system and in case like the Dell power supply in the video, the 1K resistance may not be low enough to melt the wire.Comments ?

author: mcinque
author_rank: Unknown
timestamp: December 22, 2014, 12:27:54 pm
content: because it's dangerous: if you put live on the ground of the probe, you will have live on the ground of the others bnc of the oscilloscope, and of course over any other chassis instrument that is grounded.Moreover, depending your country, putting live on ground, would trip your circuit breaker every time (i>30mA)