Got my new isolated power supply - what?? - Page 1
Forum section: electronics / testgear

author: Tony.T
author_rank: Regular Contributor
timestamp: May 04, 2015, 12:11:24 pm
content: Just bought this one..http://www.peaktech.de/productdetail/kategorie/trenntransformator/produkt/p-2240.htmlWanted to have this when troubleshooting SMPS:s.....But when i plug in this powersupply...the ground is connected through the powersupply directly to the SMPS circuitboard. And if i start to probe...of course it will blow!!..Read somewhere that even when bying this kind of powersupply...it mostly comes with earth connected anyway??!! true?? Have i bought the wrong powersupply???All warning lamps are flashing.....because....regarding the eeeblog video..avoiding blowing up your scope, the earth should not be connected. Scope yearth and SMPS earth. The power supply should float via the isolated transformer (L and N).Soo..to my question. Should i disconnect the earth cable inside the powersuppply that goes to the output outlet (still connected to the chassie though)...to have the possebility to connect the probe earth even on the hot ground?The oscilloscope is still connected to earth. ....Please...help on this.....

author: tom66
author_rank: Super Contributor
timestamp: May 04, 2015, 01:04:30 pm
content: Should be OK, if you are probing only across L-N stuff. As you will simply be connecting one side to earth. The transformer should not internally connect neutral to earth.

author: Kjelt
author_rank: Super Contributor
timestamp: May 04, 2015, 01:07:58 pm
content: No but in some housholds the N and earth are connected in the utility box so never use earth secondary.Plug out the supply and measure with a multimeter in ohms if the N and F primary are not connected to the N and F secundary (this should not be the case) then replace the earthed mains output with a non earthed output.

author: BurtyB
author_rank: Regular Contributor
timestamp: May 04, 2015, 01:12:15 pm
content: From the manual linked on that page.Quote

author: dom0
author_rank: Super Contributor
timestamp: May 04, 2015, 01:15:42 pm
content: This has been discussed extensively in the following two threads:https://www.eevblog.com/forum/testgear/reasonably-priced-differential-probes/https://www.eevblog.com/forum/testgear/best-practice-for-protecting-a-scope-that%27s-probing-mains-attached-devices/or, tl;dr verison:Quote from: Kjelt on May 04, 2015, 01:07:58 pmPrecisely. And read up on safety with isolation transformers. It's important to know when working with them. They have the "only for use by qualified personnel" sticker for a good reason.

author: electr_peter
author_rank: Unknown
timestamp: May 04, 2015, 01:30:32 pm
content: Connecting together isolated and non isolated circuits results in non-isolated extra dangerous circuits. Also, if you use PE/earth ground in context of isolated circuits, you likely have no idea what you are doing and how dangerous it is.Disconnect everything in your setup from the mains and test all items for galvanic isolation. Proceed only if you know exactly what is connected to what.Much better solution is to work on powered off equipment or use isolated differential probes (see above) with RCD/GFCI. In contrast, isolation transformer does not provide additional safety at all in many practical cases.

author: Tony.T
author_rank: Regular Contributor
timestamp: May 04, 2015, 02:16:15 pm
content: https://www.dropbox.com/s/yi8p1m74f9t0fli/SMTP.jpg?dl=0This is how everything is connected.OK?I am aware of the danger....that is why i ask first....and i have also connected the isolateted powersupply as the scope to residual current adapter. Just in case....thanks for the replies...

author: Jeroen3
author_rank: Super Contributor
timestamp: May 04, 2015, 02:30:37 pm
content: Instead of disconnecting earth inside the appliance, it is more obvious to other people if you use an cable from schuko to type c (or an schuko socket with earth ripped out)Theoretically, using such adapter on your scope results in an isolated scope. Especially the old ones with large ring transformers. However, the smps in a newer scope might introduce issues due to capacitors or noise.Remember that the outside of your BNC connectors, USB chassis+gnd and ethernet chassis are now potentially live. Depending on what you are measuring. Isolation should also be applied to all non-probe inputs and outputs. Such as usb isolators to pc's and plastic case usb sticks. Just something to be aware of.

author: Tony.T
author_rank: Regular Contributor
timestamp: May 04, 2015, 03:14:54 pm
content: Just probing the SMPS circuitboard. Aware of that the earth now is missning in all units that are connected to this isolaterd powersupply. But i do not intend to use it for normal usage...But i just was thinking of why they said that this is for safety as it is isolated powersupply...and then the earth was connected....??

author: electr_peter
author_rank: Unknown
timestamp: May 04, 2015, 03:26:08 pm
content: Tony.T, connection scheme looks ok. It may be easier to connect and make less of mess with wiring if you have simple "2 pin plug" -> cable -> "3 pin socket" adapter.Just keep in mind that all scope channels use the same GND. That is, if you use two probes, only attach them to the same spot - otherwise you will have a short.The earth is carried through, it is clearly stated in the description of that supply (or simply 1:1 transformer where earth is connected through). RTFM and be more careful next time.Quote from: Jeroen3 on May 04, 2015, 02:30:37 pmPlease do not disconnect earth from scope, it is the wrong way of doing it. It is dangerous, it puts pressure on scope's power supply and it creates havoc in measurement (massive capacitance is added to circuit, circuit is unequally loaded). See Tektronix app note:Fundamentals of Floating Measurements and Isolated Input OscilloscopesQuote from: Tony.T on May 04, 2015, 03:14:54 pmIt is designed for uses where peopleknowWTH they are doing. You lack knowledge and mess around with dangerous stuff, please stop it or educate yourself.

author: Marco
author_rank: Super Contributor
timestamp: May 04, 2015, 03:30:11 pm
content: Why would you float the scope instead of the DUT? It's functionally equivalent, parasitic capacitances to ground will screw up your circuit either way (use true differential probing if you want to avoid this) but you are generally more likely to expect dangerous voltages in your circuit than on the case of your scope.

author: Tony.T
author_rank: Regular Contributor
timestamp: May 04, 2015, 03:38:17 pm
content: Well....i do not know why one should disconnect the scope from earth....it should remain "safe".I only let the DUT float, while testing the primary side. Scope ground on Hot ground (Main CAP -..) When it is tested. Then secondary side will be the main fokus...and during this testing... normal power supply would be used as usual. Scope ground on secondary cold ground...But everything is going through...residual current adapter...all the time....Thanks electr_peter for your advices....

author: paulie
author_rank: Frequent Contributor
timestamp: May 04, 2015, 03:50:20 pm
content: There seems to be some confusion here about who wants to do what to what. I am curious about this. Is the ground terminal post on those high end supplies connected to earth mains or not?I only have experience with ATX type where it is and cheap wall wart switchers that are all fully isolated. Which way are bench type PSUs wired?

author: DanielS
author_rank: Frequent Contributor
timestamp: May 04, 2015, 04:55:25 pm
content: Quote from: Kjelt on May 04, 2015, 01:07:58 pmSome? In most electrical codes I have skimmed, building ground and utility neutralMUSTbe bonded at the main power distribution panel andONLYat the main distribution panel. Outdoors, neutral will also be bonded to ground at nearly every utility pole and underground bunker.Neutral and ground are technically interchangeable except for electrical codes restricting ground's use to fault currents for safety reasons.Quote from: paulie on May 04, 2015, 03:50:20 pmIf the ground terminal is present, it should be connected to the equipment's ground. The PSU's outputs on the other hand may be floating relative to ground and it is up to the user to tie ground to isolated outputs to un-float them.

author: Marco
author_rank: Super Contributor
timestamp: May 04, 2015, 05:13:49 pm
content: Quote from: Tony.T on May 04, 2015, 03:38:17 pmIf you think so you obviously shouldn't be doing it ... if you float the scope and put the ground clip on something at a high voltage relative to earth then congratulations, your scope case is now live (or if it's plastic, your touch screen may be live or your plastic buttons might put your fingers only a couple of mm of arcing away from something live, etc etc).

author: dom0
author_rank: Super Contributor
timestamp: May 04, 2015, 05:30:29 pm
content: Quote from: paulie on May 04, 2015, 03:50:20 pmThe great majority of bench power supplies has a floating output, only some specialty devices (mainly high powered ones not intended for EE but other kinds of lab) have an earthed output.You will find information about what common mode voltage the output can withstand in the manual or sometimes directly on the front panel.

author: paulie
author_rank: Frequent Contributor
timestamp: May 04, 2015, 09:45:43 pm
content: I was hoping that was the case. Since price was dropping on these I thought maybe about time to pick one up. Not if it was mains grounded like ATX though so this is good news. Strange that the "isolated" AC supply discussed is not fully independent of the mains.

author: dom0
author_rank: Super Contributor
timestamp: May 04, 2015, 09:46:29 pm
content: There are different kinds of isolation transformers for different purposes.

author: akis
author_rank: Frequent Contributor
timestamp: May 05, 2015, 09:14:41 am
content: Quote from: Tony.T on May 04, 2015, 02:16:15 pmWhy do you not just get a battery powered scope that works just as well with its mains cable pulled out ? That way the scope becomes similar to a hand held multimeter then you can plug it anywhere with impunity (just be careful the second probe, its earth clip is still connected to the first probe's earth clip, so you could still kill something this way).

author: macboy
author_rank: Super Contributor
timestamp: May 05, 2015, 01:55:43 pm
content: Quote from: Tony.T on May 04, 2015, 03:38:17 pmWhen you say "Residual current adapter" I believe you are referring to a GFCI, "Ground Fault Circuit Interrupter". This detects any imbalance in the current between the mains live and neutral, in other words, it detects when some current has found some other path to flow into ground rather than back into neutral. Hopefully this will save your life if that 'other path' was through your body.

author: Wim_L
author_rank: Regular Contributor
timestamp: May 05, 2015, 02:30:32 pm
content: Quote from: akis on May 05, 2015, 09:14:41 amBe careful with this. Some battery powered scopes (e.g. Fluke scopemeters) are good for this. Other battery-powered oscilloscopes (e.g. some of the Tek TDS 3000 series) can be battery powered, but must still be grounded separately for safety.

author: akis
author_rank: Frequent Contributor
timestamp: May 05, 2015, 04:05:34 pm
content: You do not ground your handheld multimeter so what's so special about the scope? Genuinly curious.

author: electr_peter
author_rank: Unknown
timestamp: May 05, 2015, 04:23:06 pm
content: Handheld scopes/DMMs are battery operated and fully isolated, that is, there is no metal part you can touch (those metal parts can be connected to mains).In case of conventional battery operated scopes, they leave exposed metal BNC shell, so it should be grounded if used anywhere near mains.Real life situations may be different, excessive grounding is not always safe/required/needed.

author: V8Snail
author_rank: Newbie
timestamp: May 06, 2015, 03:25:07 am
content: Your situation is no different from servicing the old CRT TV's with a 'live' chassis. The scope should remain earthed, the input of your isolation transformer should remain earthed, but there should NOT be an earth connection between your Device Under Test and the output of the isolation transformer.This ensures that although your DUT may be floating above earth potential, it remains safe and prevents any possibility of creating a fault circuit with your scope, or you, through earth.This only applies if only ONE device is plugged in to the isolation transformer at a time, ie. Don't EVER plug your scope and DUT together through a double adapter or power board to the output of your isolation transformer (or even two DUT's) or you'll create a hazard that can either blow your scope, device, or electrocute you without tripping any RCD protection you may have installed.

author: Tony.T
author_rank: Regular Contributor
timestamp: May 07, 2015, 12:26:27 pm
content: Quote from: macboy on May 05, 2015, 01:55:43 pmYes....correctly...could not find the right word for it :-)... only in Swedish :-). Ground Fault Circuit Interrupter it is.....Thanks...

author: Tony.T
author_rank: Regular Contributor
timestamp: May 07, 2015, 12:27:33 pm
content: Quote from: V8Snail on May 06, 2015, 03:25:07 amThanks....love when it is easy explained and answers my questions directly...

author: ebastler
author_rank: Super Contributor
timestamp: May 07, 2015, 02:57:38 pm
content: Quote from: V8Snail on May 06, 2015, 03:25:07 amI understand the hazard when DUT plus scope are connected to the same floating supply. But could you elaborate on the risk of connecting two DUTs to the floating supply at the same time, while keeping the scope grounded? What is the fault scenario and hazard here; why can't I consider the situation as equivalent to just one (more complex) "compound" DUT?(I am not questioning your advice -- the instruction manual of the isolation transformer mentioned above does include the same warning to connect only a single device if the output earth remains unconnected. But I can't envision the fault scenario right now, and would like to fully understand. Thanks!)

author: V8Snail
author_rank: Newbie
timestamp: May 08, 2015, 01:50:01 pm
content: Quote from: ebastler on May 07, 2015, 02:57:38 pmSure. It's an admirable quality to question. You can have just enough knowledge to be dangerous, but full knowledge of the dangers will ultimately keep you safe from the bitey bits.There are a few scenarios where having two or more DUT's connected to an isolated supply simultaneously could potentially be dangerous.One may be where you have two live chassis devices connected. One may have the chassis connected to what would normally be the active side of your plug. The other may have its chassis connected to the neutral side of your plug (either accidentally or intentionally). You touch one chassis with one hand, and the other chassis with your other hand. You have just created a short circuit through yourself which an RCD won't detect as an earth fault as you haven't created an imbalance across the poles.Same scenario exists if you have an active short to the casing on one device and a neutral short to the casing on the other. Unlikely but not worth the risk. This can also happen if there's a short or connection from one pole to the case on one device and both devices have earth connections to the plugs. There may not be an earth connection in the isolation transformer but both device earths will be tied together through your power board. This means just having your hand resting on the chassis of either device and accidentally touching something inside connected to the opposite pole will create a fault circuit through your hand.Even if no fault exists, just by touching something live on one device with your finger and accidentally touching something connected to the opposite pole on the other device with your elbow will play havoc with your reflexes. If you think about it, this probably isn't as far fetched as it sounds, especially when you become engrossed in what your doing.This is exactly why power distribution systems, including your house wiring, ended up having an earthed system of some sort. Originally, power distribution systems had no connection to earth. Many faults with appliances could exist and not shock the user. However, imagine neighbouring houses, one house has a toaster with an active fault to the frame, the other house has a toaster with a neutral fault to the frame. Both neighbours touch their toasters with one hand and their kitchen sinks with the other. They have just created a fault path through ground (the water piping) and each other. Fallen power lines were also a problem then as with no earth connection to create a fault path and blow a pole top fuse, the power lines remained live and within reach of anyone bridging themselves across two of the conductors.Of course, even with an isolated supply (transformer or otherwise) you can still kill yourself by touching both poles at the same time. The safety only comes from not having any current flow to earth should you touch only one pole at any one time.Cheers. . .

author: Monkeh
author_rank: Super Contributor
timestamp: May 08, 2015, 02:39:41 pm
content: Quote from: DanielS on May 04, 2015, 04:55:25 pmWhich electrical codes are those? You'll likely find the rest of the world has a few different ideas.

author: ebastler
author_rank: Super Contributor
timestamp: May 08, 2015, 03:14:30 pm
content: Quote from: V8Snail on May 08, 2015, 01:50:01 pmThank you very much for the detailed and helpful reply, V8Snail! The scenarios you describe make perfect sense. Indeed, two separate devices (especially when connected with opposite polarity) do offer more possibilities than a single DUT to get confused, or to unintentionally touch the two wrong points at once... I will keep this in mind!