YOUR TEST EQUIPMENT'S **OSCILLISCOPE** VERY BEST FRIEND.... - Page 1
Forum section: electronics / projects

author: Hank69
author_rank: Contributor
timestamp: February 19, 2023, 07:14:08 pm
content: Some years ago when I had an early Hantek DSO scope, I have LONG understood that the scope's signal ground and mains ground shallnevermeet. To that end I bought the pictured isolation transformer for about 20 bucks and started my build. In my "early" days the famous grey BUD BOX was the project box of choice and as this project involved AC mains power a plastic box would be suitable. This sturdy plastic Glad food storage container worked nicely. The transformer pictured is rated for 56 VA, about 60 watts, and since my brand spank'in new Siglent SDS1202X-E is stated to draw only 25 watts max, the "loading" would be at a "safe" 50%. Even with the cover on the transformer measured ~95'F which is acceptable. The picture shows a good build with all the niceties of using grommets and the AC cable, (a cut in half extension cord), secured with cable clamps. Although my NASA soldering "certificate" has long since expired, any soldering work is like riding a bike and it's really easy peasy to get a goodjoint.... My AC mains measures a nominal 121 volts @ 60 Hz and without a load the transformer outputs ~ 141 VAC and under load measures ~ 137VAC. As this is just a "straight" - one to one transformer and is not regulated, the output is in the scopes range of 100 to 230 VAC @ 60 Hz. Although it is not very elegant it does the job of avoiding scope replacement IF/WHEN I make a dumb mistake.... P.S. @ 72 YO they are prone to happen a bit more frequently....

author: alm
author_rank: Super Contributor
timestamp: February 19, 2023, 07:23:30 pm
content: Quote from: Hank69 on February 19, 2023, 07:14:08 pmThat's wrong. What made you think that? On common, not isolated mains-powered scopes, all measurements are relative to mains ground. If you need something else, use isolated scopes or differential probes.Quote from: Hank69 on February 19, 2023, 07:14:08 pmSomeone might have to replace you, but at least the scope will still fine. A scope as life insurance

author: wasedadoc
author_rank: Super Contributor
timestamp: February 19, 2023, 07:24:21 pm
content: Quote from: Hank69 on February 19, 2023, 07:14:08 pmNo!The isolation transformer should be used to isolate the equipment being tested. The scope should remain grounded.

author: tggzzz
author_rank: Super Contributor
timestamp: February 19, 2023, 07:40:46 pm
content: Quote from: Hank69 on February 19, 2023, 07:14:08 pmTo any newbie that happens to come across this thread....That statement is completely incorrect. On most scopesthe scope's shield ground is connected to the mains groundin the scope itself. Anybody reading this thread should pause and think how many of the OPs other statements are correct and incorrect.For those wanting to why the OP is wrong and the possible consequences should read the references athttps://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/In particular, look at the reference in the paragraph about "floating the scope", pause and think. Key phrase: "This guy was WELL AWARE of the danger and took a lot of precautions to prevent shock . . . Bottom Line: He died anyway."

author: Hank69
author_rank: Contributor
timestamp: February 19, 2023, 07:45:17 pm
content: Thanks for some good points noted.... BUT...Say you are TS a problem with a devices power supply circuit, which would be a common area to diagnose with a scope for a voltage output issue. Now, what IF - you are probing across a component in lets say the regulator section and you probe the output line and you connected the scopes ground clip across the device under test. No sweat IF the device's common output line is NOT tied to any common input ground. If it is there will be grounding back to the device's input ground if not isolated. WAY back in time, when the "Sam's Photofax" schematic service was available showing the schematic for most anything electrically powered, you could easily navigate your TS efforts with a schematic to work from. In summation - as long as a circuit is isolated either the device itself or the scope, there is zero chance of making sparks, or worse....

author: bdunham7
author_rank: Super Contributor
timestamp: February 19, 2023, 07:56:05 pm
content: So if I understand correctly, you have cut the ground wires off in the extension cord and there is not any connection between the ground pin of the plug that goes in the wall and the ground socket of the end where the scope plugs in? This is an insanely bad idea and even if you think you have thought it through, you haven't, I assure you.If you isolate your scope like this and attach a probe ground clip to anything that is 'live' relative to ground, there may not be any sparks at first but the BNC shells and every external part of your scopeand any other devices connected to itwill be live, as in touch them and get fried. So, for example, if you set up your isolated scope and connected the ground clip to an old live-chassis TV set or tube radio and then plugged in a metal-shelled USB drive to your scope, you and the USB drive would get a full 120V electrocution. I'm betting you hadn't thought of that.

author: tggzzz
author_rank: Super Contributor
timestamp: February 19, 2023, 08:19:44 pm
content: Quote from: Hank69 on February 19, 2023, 07:45:17 pmTo make sure you don't display the Dunning Kruger Syndrome,read the effin referencebefore you kill yourself (I don't care) or encourage other people tounnecessarilyput themselves in danger (I do care about that).Quote from: tggzzz on February 19, 2023, 07:40:46 pmThereareseveral safe ways to proceed in your use-case. Your way isdefinitelynotone of them!Read the references.Learn from other people's experiences.Makenewmistakes; don't repeat old mistakes.If, after reading and understanding all that, you still want to float a scop,startby doing this:

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 19, 2023, 08:28:28 pm
content: Hi Hank69,As other members already said above, isolating you scope or your DUT with an isolation transformer is not a good practice and you should avoid using it.Because of the isolation transformer, the RCD/GFCI of your mains installation won't trigger if current goes through your body.The most important thing is keeping the operator/repair guy alive.The second most important thing to take care of, depends on the value of the TE vs the DUT.To be on the safe side when mesuring on DUT with mains/live exposed parts (like boards within situSMPS), use a multimeter completed withIEC1010 compliant accessories.You may also check out thesetest leads with selectable safety category and double-insulated silicon wire. Same stuff but withbanana connexion.Another set fromPomona.Should you need an oscilloscope for probing, the cheapest safe solution is using a differentail probe.The most adequate solution is to use a truly isolated oscilloscope with compliantfully isolated probes. However it's really expensive stuff.If the DUT has a metallic enclosure, you may use a strong alligator clip to make an additional link from chassis to earth.Even if it's not practical, wearingclass 00 or class 0 insulated glovesprovides an additional level of safety.[update #1 : typo corrected]

author: Hank69
author_rank: Contributor
timestamp: February 19, 2023, 08:39:59 pm
content: Thanks for ALL the replies, even the "politely mean" ones.... I will reconsider my safety protocol when using any of my gear and what I was mostly trying to avoid was smoke from my scope and DUT's, ME - not so much.... Thanks

author: tggzzz
author_rank: Super Contributor
timestamp: February 19, 2023, 09:09:31 pm
content: Have fun, safely!

author: khs
author_rank: Regular Contributor
timestamp: February 19, 2023, 10:28:41 pm
content: There are some isolated scopes like the TEK THS 720.https://www.ebay.de/itm/195594255859?hash=item2d8a537df3:g:nTQAAOSwmbxj5713&amdata=enc%3AAQAHAAAAoHxdTL%2BpbSPNFIHJmnR%2Fk3qm2NDRL8e7NBgZeusU4ufQIdHVMchJkt%2BmCpaEgmHS4gwY5Y6tbdQF1GKZ%2BmaztBv1qOaZhDDzqq8%2BWAXNPbUUFap0N6ONMU0gUv4eNirLeXFyJ%2BpPhrFa0ODgbzIkBy5VIr4O%2FsmWqoGEmJPI5Hb9LXOIgfeHRcu47rpyIPW4jdqPstw2E2yqVRZfAeke%2FYc%3D%7Ctkp%3ABk9SR76LmLzNYQ

author: tautech
author_rank: Super Contributor
timestamp: February 20, 2023, 10:19:24 am
content: Quote from: Hank69 on February 19, 2023, 08:39:59 pmDo yourself a big favor and get a differential probe.And keep your scope grounded, always !

author: pcprogrammer
author_rank: Super Contributor
timestamp: February 20, 2023, 11:08:11 am
content: I read the original post yesterday and decided to sit back and waitWhatever way you look at it, mains voltage is a dangerous beast, even through a separation transformer it is still high voltage and potential risk of touching both wires coming from it is a real one.Keep your attention up at all times and think about the possible risks when measuring on it, with whatever measuring device.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: February 20, 2023, 12:33:18 pm
content: Quote from: tautech on February 20, 2023, 10:19:24 ammy (late) dad was a TV technician, from back in the old days of live-chassis valved B&W TV sets. he was born in 1923. dad had several scopes, all valved, typically 5MHz bandwidth, that had 'lifted' earth added either by a switch or similar. likewise, there were several tapon mains plugs with the earth pin removed. thinking back, it makes me shudder a little... still, he survived to the ripe old age of 92.it does occur to me that with the ground on your scope lifted, you are depending on the insulation of your scope probe (in particular the coax cable) being good for 230v (no wimpy 110v here in NZ). i'd not have that sort of faith in the sort of coax used in modern scope probes.what sort of designs are there out there for DIY 20MHz differential probes? i'd not want to buy one - not something i'd use my scope for usually - but would be curious about building a low-bandwidth one perhaps. for fixing switchmode power supplies you'd not need much bandwidth over even 1MHz.cheers,rob :-)

author: tggzzz
author_rank: Super Contributor
timestamp: February 20, 2023, 12:58:30 pm
content: Quote from: robert.rozee on February 20, 2023, 12:33:18 pmI have a "The Boy Electrician" book with a chapter on how you can use your homebrew X-ray machine. It does warn you that if your skin is becoming red, you are may be using it too much.I used to have fun making stink bombs in the kitchen. Hydrogen sulphide is supposed to be dangerous, but I never came to any harm. I still have a lump of sodium, and some saturated Weston standard cells with cadmium and liquid mercury in a thin glass phial.And we used to run into roads without looking. Now adults only walk into roads (without looking at anything other than their phones).I've taken my early teens daughter backbacking round India, and sent her flyng on her own.Some risks areworthtaking, because they bring benefits difficult to achieve otherwise. Floating a scope doesn't fall into that category

author: timeandfrequency
author_rank: Frequent Contributor
timestamp: February 20, 2023, 01:09:16 pm
content: Hi Robert,Quote from: robert.rozee on February 20, 2023, 12:33:18 pmPerhaps you may find thisbasic diff probe designuseful. BW is 600kHz, max voltage is 600V and attenuation factor is 1:100.

author: alm
author_rank: Super Contributor
timestamp: February 20, 2023, 03:08:52 pm
content: Quote from: robert.rozee on February 20, 2023, 12:33:18 pmI would be more concerned that you are depending on yourself not to touch any metal parts on the scope, like BNC or USB connectors, or the metal case. And that those flimsy rotary encoder knobs provide sufficient isolation. Hence the picture that tggzzz posted with fiberglass rods.I do remember reading a story about how some people used to test 10x probes on 110V mains (while being grounded, obviously), but that a new series of HP miniature probes had come out, and the probe burnt out because it could not survive 110V RMS. But that was the attenuator breaking down, not the insulation.

author: Gyro
author_rank: Super Contributor
timestamp: February 20, 2023, 05:35:04 pm
content: Quote from: alm on February 20, 2023, 03:08:52 pmThat sound pretty low for attenuator breakdown. I do remember HP mini-grabbers that were only insulated by the hard anodizing on their Aluminium sleeves.

author: tautech
author_rank: Super Contributor
timestamp: February 20, 2023, 08:20:49 pm
content: Quote from: robert.rozee on February 20, 2023, 12:33:18 pmAs floating scopes is no longer accepted designs don't cater for it but of highest risk is any metal part of the instrument as it is no longer at mains ground potential as ALM points out.Usage strategies normally involve the use of just one hand with the other fixed behind you back to minimize the risk of an across body shock.However now differential probe cost less that half the price of a 4ch DSO float a scope is a foolhardy practice.QuoteSMPS are probably the most common need for Diff probes but there are safe strategies to enable work on SMPS once familiar with their operational principles.Refusal to operate can simply be diagnosed with a DMM, ESR/Cap meter and a PSU.DIY design is a different matter and to properly quantify performance a DSO with Power Analysis and differential and current probes required.

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 21, 2023, 01:14:02 am
content: Today's VFD troubleshooting involved an isolation transformer on DUT supply and THS720 isolated (not floated) scope.I could have got away with one or the other, when you heard test equipment may as well use it...It always scares me how often this same subject springs back up !Safe measurement practices also mean understanding the equipment's limitations such as isolation voltage.

author: Bud
author_rank: Super Contributor
timestamp: February 21, 2023, 03:03:42 am
content: @OP in Internet culture typing in all CAPS mean you are yelling. Do not type in all caps unless you are mad and want to show that to everyone.

author: tggzzz
author_rank: Super Contributor
timestamp: February 21, 2023, 10:55:15 am
content: Quote from: Bud on February 21, 2023, 03:03:42 amSush! Don't give away the tell-tales, since it will enable kooks/etc to avoid them

author: Hank69
author_rank: Contributor
timestamp: February 21, 2023, 07:43:56 pm
content: As Steve Martin would say, "Well, Exxccuussee Meee. It is a "HEADLINE", don't get yourpantiesin a bunch. Scheese....