Probing SMPS with oscilloscope - Page 1
Forum section: electronics / testgear

author: tony359
author_rank: Frequent Contributor
timestamp: July 08, 2020, 03:52:37 pm
content: Hi allSurely a silly question but better safe than sorry.I am trying to fix a SMPS and on the primary I have 420V p/p which is normal. My scope says "400V p/p max" on inputs but I am using a 10:1 probe, rated 500Vp/p.Am I right in thinking that with a 10:1 probe the actual voltage reaching the scope is 42V and hence I am perfectly safe to use it that way?Thanks!

author: tony359
author_rank: Frequent Contributor
timestamp: July 08, 2020, 04:56:08 pm
content: Thanks,I suspected that - even though my scope is indeed 400V p/p input so while I should be safe with a 10:1 probe, it would be dangerous and borderline.

author: PKTKS
author_rank: Super Contributor
timestamp: July 08, 2020, 05:31:42 pm
content: Quote from: blueskull on July 08, 2020, 04:02:15 pminteresting comments but nevertheless - NEVER cross sides.A common mistake - taken sometimes by distraction - is tohook your clip on the secondary "ground".. and go probingthe primary... bad things happenQuote from: blueskull on July 08, 2020, 04:02:15 pmALL MY BENCH IS ISOLATED.DUTs by dedicated trafo and scopes by another set of trafos and so forthUSE GLOVES. - I never check mains powered without them.It is not about getting zapped... the panic of touching a 400Vhot spot can do a lot of bad things... even without killing you,it is a fast reaction hard to controlUSE GLOVES... USE GLOVES... USE GLOVES... USE GLOVES...Paul

author: Per Hansson
author_rank: Unknown
timestamp: July 08, 2020, 05:34:13 pm
content: Quote from: tony359 on July 08, 2020, 04:56:08 pmIt would not be borderline: the second you put the negative of the probe to B- the probe will vaporize in your hand.EEVblog #279 - How NOT To Blow Up Your Oscilloscope:

author: bdunham7
author_rank: Super Contributor
timestamp: July 08, 2020, 05:50:15 pm
content: You can avoid killing yourself for now by getting a 100X HV probe set, like this:https://www.ebay.com/itm/P2301C-High-Voltage-Oscilloscope-Passive-Probe-5kV-100X-300MHz-For-Tektronix-HP/332835001398?hash=item4d7e831436:g:2vcAAOSwEIhZg-ywANDremoving the ground lead and probing where you like with just the tip.If you can get by only measuring points that are ground-referenced, than you're good. Anything with bridge input, for example, will not typically not have DC- as ground referenced.To measure non-ground-referenced voltages--IOW to use a scope like you would a DMM--you need either a HV differential probe, an isolation interface or an isolated scope. If you already have the scope, the cheapest solution that won't shock you is likely the MicSig line of HV differential probes.

author: nctnico
author_rank: Super Contributor
timestamp: July 08, 2020, 05:57:28 pm
content: Get a differential probe. Very useful. Not just for measuring mains but there are cases where being able to do a floating measurement is very handy.

author: james_s
author_rank: Super Contributor
timestamp: July 08, 2020, 06:07:17 pm
content: The only safe way to do this is with a HV differential probe. I picked one up last year and now wonder how I ever got by without it. Poking around with a scope on the primary side of a SMPS is a classic way of destroying your scope and the PSU you're trying to fix.

author: David Hess
author_rank: Super Contributor
timestamp: July 08, 2020, 09:21:00 pm
content: Quote from: tony359 on July 08, 2020, 03:52:37 pmThere are two different limits.The operating range which produces a reasonable display is simply based on the probe attenuation factor and the oscilloscope sensitivity. So a 10x probe at 5 volts per division, which is typical, yields 50 volts per division. 8 vertical divisions then are 400 volts peak-to-peak. When the position control is taken into account, that means that voltages up to about + or - 400 volts can be displayed hich is generally sufficient for line side circuits.The 400 volt peak input rating has nothing to do with the above. Instead, it is related to the input protection, switching, and coupling circuits.Where you can get into trouble is that while a 10x probe attenuates the input signal to one tenth, it only does this for DC when the input coupling is set to DC. When the input coupling is set to AC, then the full DC input voltage is passed to the input of the oscilloscope as the AC coupling capacitor charges, so the maximum input voltage remains 400 volts. The same applies to most 100x probes.For safety, you really should be using a high voltage differential probe but with care, it is usually possible to use 10x probes on line side circuits, but be very careful about where you attach the probe's ground lead, or just remove it for safety. I use an isolation transformer on the device under test whether I use a high voltage differential probe or not.

author: bdunham7
author_rank: Super Contributor
timestamp: July 08, 2020, 09:59:29 pm
content: Quote from: David Hess on July 08, 2020, 09:21:00 pmJust FWIW, I'm sure my legacy scopes all behave this way, but I did just check the input resistance (by DMM) on my Tek TPS2024 (isolated) and Siglent 1104X-E. The Tek does read 1M on DC-coupled and open circuit on the AC and GND settings, but the Siglent has a steady 1M input resistance regardless of the coupling setting. Hmmmmmm. Cheap scopes save the day? I wonder if the very robust Tek has additional input protection against the scenario you outlined.

author: tggzzz
author_rank: Super Contributor
timestamp: July 08, 2020, 10:04:13 pm
content: Quote from: PKTKS on July 08, 2020, 05:31:42 pm1) isolation solves some problems and introduces others. It is neither necessary nor sufficient.2) you had better explains what you mean by trafo, and its relevance; I have no idea.3) mains isn't hard to control; it is impossible to consciously override muscle contractions. I know, from personal experience.Nowadays the only sane way to do this is with an isolated HV probe. End of story.

author: PKTKS
author_rank: Super Contributor
timestamp: July 09, 2020, 02:16:30 pm
content: Quote from: tggzzz on July 08, 2020, 10:04:13 pmTRUE. However one of them most hard to overcome problemis to actually hook your scope chassi to the mains or otherbad reference.Having instruments and DUTs isolated solves this particular badproblem. You will be isolated from the mains which ultimatelyis the primary cause of scope dead shorts.Quote from: tggzzz on July 08, 2020, 10:04:13 pmops.. that one is a more or less ancient term (acronym) for TRAnsFOrmer(TRAF was also found on some ancient schemas)that term may be nostalgia for some HAMsQuote from: tggzzz on July 08, 2020, 10:04:13 pmDespite the fact that those DIFFERENTIAL HV probes are very handy...They are NOT ISOLATED. Their input is mostly a cascade resistorladder with very high impedance... but not isolated.Add that the required power needed by these probes andsometimes you have introduced another factor to considerHaving DUTs and INSTRUMENTS "isolated de facto" (like havingbattery scopes and DMMs) is very handy most of the time.For those critical things where I WOULD NOT RISK EXPENSIVEGIZMOS.. i got myself one of those handy battery operatedhand scope.Combined with 100x probe operated on battery onlythey are a peace of mind.. in case o faulty devices doingsome harm they are cheaper than expensive bench gizmos.You see.. repair and R&D are quite 2 different things.Expensive stuff to repair garbage bricked stuff just do not matchPaul

author: CDaniel
author_rank: Frequent Contributor
timestamp: July 09, 2020, 05:35:19 pm
content: Using an isolation transformer for the supply under test is necesary and there are no problems . However measuring the primary side of the SMPS with the bench scope should be done only when absolutly necesary , not jumping to measure if the mains voltage is coming in , the filter cap has voltage or too much ripple or if the power switches are workingFor troubleshooting in 99% cases the multimeter is enough , I'm not referring at designing a SMPS and so on

author: tony359
author_rank: Frequent Contributor
timestamp: July 09, 2020, 05:45:59 pm
content: Thank you all for the many replies - I really appreciate the time you spent to warn me!Well, before I posted here I did actually probe the primary with the ground connected to chassis ground. Nothing exploded and the scope (Siglent 1104X-E) has survived but I won't do that again!The reason I was poking around is to try and identify where some ripple (more of a spike) was coming from - it's present on all 5 rails and I could see it before and after the switching transformer too - see pics attached.

author: AVGresponding
author_rank: Super Contributor
timestamp: July 09, 2020, 06:13:18 pm
content: You could use HV differential probes, or... you could just use a scopemeter.

author: nctnico
author_rank: Super Contributor
timestamp: July 09, 2020, 06:52:22 pm
content: Quote from: PKTKS on July 09, 2020, 02:16:30 pmFor all intents and purposes these probes are isolated. Their input resistance is extremely high so no dangerous current can flow and, more importantly,they are safety rated.

author: bdunham7
author_rank: Super Contributor
timestamp: July 09, 2020, 07:27:50 pm
content: Quote from: nctnico on July 09, 2020, 06:52:22 pmYou should specify 'extremely high'. It is actually a lot less than most single-ended HV probes. One common type of these is 4M each side to ground. So measuring a small signal at 1400V common mode voltage, you would have a net resistance of 2M to ground, with a resultant leakage current of 700uA. That's not even getting into what the current might be at the highest frequencies it is rated at. In many cases this is not problematic, but it is still there and has to be considered in some situations.

author: David Hess
author_rank: Super Contributor
timestamp: July 10, 2020, 10:05:10 am
content: Quote from: bdunham7 on July 08, 2020, 09:59:29 pmThere is a clever alternative "two path" input buffer design which newer oscilloscopes may use which does not require the input coupling capacitor, but it is not something to count on for safety.

author: MathWizard
author_rank: Super Contributor
timestamp: July 11, 2020, 06:52:59 am
content: Quote from: bdunham7 on July 08, 2020, 05:50:15 pmIsn't the Neg probe of Diff-probes also a few Megs from MainsEG, like the pos. probe?Isn't the whole point of diff-probes is so that you could (if transients aren't a concern) put them anywhere (both on hot/clod side tho)

author: dustooff
author_rank: Contributor
timestamp: July 11, 2020, 03:03:38 pm
content: After looking at a few HV diff probes available to me, some state the resistance to ground (4M), and they would be balanced to maintain the differential input.Having a new scope that can also do multichannel power measurements, I'm looking at HV diff probe adaptor/kit. In the past any measurement that was more than about 60v was either with the Fluke 87 or PM97, both have served well and still provide relevant results.Looking at the various available HV diff kit, in no particular order,https://emona.com.au/products/electronic-test-measure/oscilloscopes/gdp-050.htmlhttps://www.eevblog.com/product/hvp70/http://www.micsig.com/html/41.htmlhttp://hantek.com.cn/products/detail/6163I think I will go with the micsig DP10013.They all seem to have similar CAT ratings, though the micsig prominently displays the MAX rating not +/-700 or +/-600 DC+AC pk.The HVP70 does have a CAT III 1000v rating where the others are CAT II 700v or 1000v. I've been thinking about drawing a compatibility chart of which accessories can be connected with devices to maintain CAT rating. ( for all CAT rating and examples)I do find it interesting that Hantek have added the word Isolated to their description of HV diff probe kit. This seems a bit misleading / unjustified.

author: PKTKS
author_rank: Super Contributor
timestamp: July 11, 2020, 03:28:05 pm
content: Quote from: dustooff on July 11, 2020, 03:03:38 pmLike many others.. there is an assumption that aHIGH IMPEDANCE is equivalent to ISOLATION.Obviously there is no GALVANIC ISOLATION.In the unfortunate event of a cascade failurethe whole surge may enter the diff ampfrying whatever is in the wayGALVANIC ISOLATION is somewhat in limboonce the heavy bulky transformers are hardto deal with and hard to build.Nevertheless I have never seen a primary faultentering secondary or vice-versaNothing compares the isolation core itself.There is real ISOLATION in the strict sensePaul

author: nctnico
author_rank: Super Contributor
timestamp: July 11, 2020, 03:35:36 pm
content: Quote from: PKTKS on July 11, 2020, 03:28:05 pmJust check how galvanic isolation is defined... Several hundreds of micro-amperes is allowed. In they end there is always a possibility for creepage. The fact is that CAT rated differential probes are designed to withstand high voltages. Those probes have a much wider barrier between dangerous voltages and the operator compared to -for example- an opto coupler or windings in a transformer.

author: PKTKS
author_rank: Super Contributor
timestamp: July 11, 2020, 04:03:16 pm
content: Quote from: nctnico on July 11, 2020, 03:35:36 pmWhen the ENAMEL or the isolation layer on the core fails...The magnetic flux stops (or drastically tend to zero)leading the surge to void - no harm in the chain.Opposite results in the resistor ladder failure.The dead short surge propagates until the energy dissipates..Paul

author: nctnico
author_rank: Super Contributor
timestamp: July 11, 2020, 04:11:36 pm
content: Quote from: PKTKS on July 11, 2020, 04:03:16 pmThe problem is not the enamel isolation from a winding but leakage between the windings! Just look at a teardown of a differential probe. There is not 1 resistor but a whole bunch of them in series. IOW you need a catastrophic failure in multiple parts before there is a dangerous situation. Also when a differential probe starts to fail it won't work right so you are aware of problems immediately.

author: PKTKS
author_rank: Super Contributor
timestamp: July 11, 2020, 04:29:16 pm
content: Quote from: nctnico on July 11, 2020, 04:11:36 pmCORRECT.Assuming the sub-millimeter dimensions of today SMTit is easy to high energy surges propagate once thelayout is cracked - arcs won't even need to go viasmd components they can spread or arc ad hoc.Old school HV probes are LONG and sharp to preventthose catastrophic surges to arc ad hoc..those small things are not. tight and too closeto prevent - in the event of a surge failure - random arcsPaul

author: nctnico
author_rank: Super Contributor
timestamp: July 11, 2020, 04:31:58 pm
content: Quote from: PKTKS on July 11, 2020, 04:29:16 pmPlease take a look at how these probes are constructed before making any other comments; the parts they use are huge! There is nothing sub-millimeter about the resistors you find in CAT rated differential probes. CAT rating implies that these designs can handle surges.

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2020, 04:49:46 pm
content: Quote from: nctnico on July 11, 2020, 04:11:36 pmIn any proper isolation transformer there isn't any 'leakage' (in the DC or resistive sense) between windings, just capacitive coupling on the order of 1nF or less. Nowadays they typically are on separate plastic bobbins. They are tested with thousands of volts for DC leakage. I don't think there's much of a case to argue that properly designed diff probes, opto-isolators or isolation xformers are unsafe if properly designed, tested and used. I didn't raise the issue of the diff probe impedance as a safety issue, it's a circuit loading issue IMO. 2M to ground may be too much in some cases, although the nice low ~2pf typical capacitance may be optimal in others.

author: PKTKS
author_rank: Super Contributor
timestamp: July 11, 2020, 07:15:17 pm
content: Quote from: bdunham7 on July 11, 2020, 04:49:46 pmExactly. There is no such thing as leakage 'current' onhealthy transformers - you do have inherent loss butthis is not "leakage" in the sense we are talking.The more recent higher voltage are indeed separatedplastic bobbins - some of higher frequency built withproper high freq. wiring.For the record. I never said HV differential probes areunsafe or unrated. Thing is: their size by using exclusiveSMD parts are indeed small (compared to old school long ones)and they do not allow for proper arc safety when thelayout of the ladder is cracked by a strong surge.I like them very much. Very handy. Although I would neverconsider them isolated and rather prefer to use batteryoperated scopes in critical cases.For half the price of a differential here locally a 1000x old school probehttps://www.aliexpress.com/item/4001233837188.htmlPaul

author: bdunham7
author_rank: Super Contributor
timestamp: July 11, 2020, 08:02:05 pm
content: Quote from: PKTKS on July 11, 2020, 07:15:17 pmThe MicSig appears to be SMD. You can look at the Sapphire versions (details at about 7:00 in video) and see what you think of the 'HV ladder'. Both are purportedly CAT rated at some level.The HV probe you mentioned, besides probably not working properly on anything but DC due to compensation issues, doesn't work for non-ground referenced measurements. One side is grounded. And if you use a floating scope, you need to be prepared for it to float up to whatever you are measuring--and that system is not designed for that since the ground side is not going to be well insulated, if at all.

author: tggzzz
author_rank: Super Contributor
timestamp: July 11, 2020, 08:03:31 pm
content: Quote from: PKTKS on July 11, 2020, 07:15:17 pmI wouldn't touch that except with a (fibreglass) bargepole.The lack of a specification might lead you to think it is 20kV at 10MHz. Even decent Tek probes (e.g. P6015) don't get anywhere near that - and state the frequency derating.

author: tautech
author_rank: Super Contributor
timestamp: July 11, 2020, 09:19:50 pm
content: Quote from: tggzzz on July 11, 2020, 08:03:31 pmIt's a DMM HV probe and that's all !AC rating is there but garbage for anything serious:AC : 1 ~ 28KV (50 / 60 Hz)

author: tggzzz
author_rank: Super Contributor
timestamp: July 11, 2020, 10:08:45 pm
content: Quote from: tautech on July 11, 2020, 09:19:50 pmWell, that's exactly what itlookslike! But it was posted in relation to scope probes and mentions "Bandwidth 10MH z".As I said, get out your bargepole

author: tautech
author_rank: Super Contributor
timestamp: July 11, 2020, 10:18:39 pm
content: Quote from: tggzzz on July 11, 2020, 10:08:45 pmManufacturers webpage clearly lists it's only a 50/60 Hz DMM probe:http://www.pintek.com.tw/product_detail/landersound/index.php?Product_SN=19259&Company_SN=6002&Product_Site_Classify_SN=17071Some of their other products are the the real thing for scopes and quite like their compact HVP-08:http://www.pintek.com.tw/product_detail/landersound/index.php?Product_SN=127265&Company_SN=6002&Product_Site_Classify_SN=17071

author: tggzzz
author_rank: Super Contributor
timestamp: July 11, 2020, 10:49:44 pm
content: Quote from: tautech on July 11, 2020, 10:18:39 pmI have Testec probe similar to that. Remarkably it has an MoD test sticker indicating the next service/cal date was as late as last SeptemberQuoteThe spec says 3pF, 40MHz, <6kVrms => 4Arms through the probe. Should be entertaining - from a suitable distance.I'll stick with my well-specified Tek P6015 (or even my P6013).

author: bdunham7
author_rank: Super Contributor
timestamp: July 12, 2020, 01:05:30 am
content: Quote from: tggzzz on July 11, 2020, 10:08:45 pmAssuming you could find a low-impedance source for 28kV @ 10MHz, your bargepole might not keep you far enough away.

author: tggzzz
author_rank: Super Contributor
timestamp: July 12, 2020, 08:07:45 am
content: Quote from: blueskull on July 12, 2020, 02:46:50 amNormally I'm the one to make that point, but in this case 10MHz can be assumed to be a sinewave. Not that it makes any practical difference, since the spec is clearly rubbish, possibly dangerous rubbish.

author: tggzzz
author_rank: Super Contributor
timestamp: July 12, 2020, 08:09:42 am
content: Quote from: bdunham7 on July 12, 2020, 01:05:30 amWell, bargepole isn't a well-defined standard. Personally I regard it as a punt pole as used in Cambridge or Oxford. That's 4.9m or 16ft, which ought to be enough

author: PKTKS
author_rank: Super Contributor
timestamp: July 12, 2020, 02:57:22 pm
content: try to keep as short as possible.I have dealt with a large number of SMPS for CRTsPCs and mostly kind of consumer gear for repairand for my own projects.I am not sure what kind of SMPS you folks aredealing with.. but 10MHz and other numbersplaced here are really way too far to me.I have a pretty large baggage of SMPS from 70s and 80swere I stopped servicing CRTs (by late 80s) definitelyAll of my Service Manuals are pretty clear in details foralignment and service those SMPS - for those not alreadyfamiliar they were 15KHz or in some rare cases some multiplein 3x order. Tube based SMPS (as in the figures) were always trickya typical figure of flyback input is 1.2KV input 25kV outputMethods to attach the probe are very specific to avoid sparkingin the "AQUADAG" tint and safety net. Also a common source os issueswas polarized dust around the tubes which could detonatesparks rather randomly and very dangerously.Figures of my bench stuff attached with rich schemas anddetailed Service Manuals mostly 70s/80s era. I got spoiledby computers and Internet on the 90s and stop servicingSMPS CRTs since themBUT NEVER FOUND A 10MHz SMPS with 25KV so far...I had one os those HV DMM probes on th 80s which I soldfor a fellow technician by early 90s. This probe allowed myto service a great number of SMPS mostly on that rangeof 30KV up to 70kHzThe form factor of these probes are carefully crafted to servicethose rather tricky spots on furniture like CRTs almost 50Kg eachI miss that probe I even consider getting another one today.They were very expensive at that time.All those comments I just can not fit with those decadesof real life service...Paul

author: Fungus
author_rank: Super Contributor
timestamp: July 12, 2020, 04:55:27 pm
content: Quote from: tony359 on July 08, 2020, 03:52:37 pmYes.But:* I personally would get a fixed x10 probe if I was going to do it on a regular basis.* There's safer ways to do it.

author: tony359
author_rank: Frequent Contributor
timestamp: July 22, 2020, 10:53:55 am
content: Thank you all for the contributions.I was thinking of adding a 100x probe to my toolkit. I appreciate a differential probe would be ideal but it's beyond my budget. I understand that as long as I do not attach the ground clip to anything but actual ground, I should be safe even though I imagine that measurements may be somehow inaccurate.I was thinking of this one:https://uk.farnell.com/multicomp-pro/mp770213/test-probe-100mhz-oscilloscope/dp/3265030?st=oscilloscope%20probeI see the max voltage allowed lowers with frequency but SMPS shouldn't really go up to Megahertz, am I mistaken?Thanks!

author: bdunham7
author_rank: Super Contributor
timestamp: July 22, 2020, 03:07:27 pm
content: Quote from: tony359 on July 22, 2020, 10:53:55 amYour scope should already be grounded and the safe thing to do with the ground clip is to remove it from the probe entirely and put it in a drawer.The probe you posted would be fine, I use this one and I think it is better:https://www.ebay.co.uk/itm/P2301B-P2301C-Oscilloscope-Clip-Probe-300MHz-5000V-High-Voltage-For-Tektronix-HP/163762675918?hash=item26210420ce:g:UP8AAOSwUTddHFJSThe concept of derating voltage by frequency or dV/dt is important and unfortunately, both the probe you posted and the one I have come with datasheets that are a bit suspect and should not be relied upon too literally. However, I can tell you that the one I have works well in reasonable circumstances and I have no desire to test it's limits. Or, I do, but don't have the equipment and a supply of probes!Anyway, it works on a Tek 22XX scope HV supply that is about 2kV RMS @ 20kHz. You aren't likely to find 1MHz on a high-power SMPS, although I'd recommend RTFM before probing where possible and hands-off-only probing where there is any doubt.

author: CDaniel
author_rank: Frequent Contributor
timestamp: July 22, 2020, 04:14:36 pm
content: If you work in the primary side of a SMPS you must use an isolation transformer ... without it you can't measure anything right with the ground clip not usedAnd you can't touch the SMPS primary ground , as the scope ground is tied to earth.If the farnell probe is not made by a crappy chinese company like the one on e-bay , for sure I would chose it .

author: bdunham7
author_rank: Super Contributor
timestamp: July 22, 2020, 05:03:25 pm
content: Quote from: CDaniel on July 22, 2020, 04:14:36 pmYes, you typically need isolation to make any sense out of it, but a better practice is to isolate and then deliberately ground one point, typically starting with the negative side of the bridge, with a separate, secure ground connection and not rely on the scope ground clip for your ground reference path. And still leave the scope ground clip off unless the readings are too noisy.I assure you both probes are Chinese made. The one I have is made by YPioneer, not a crappy company at all.

author: tony359
author_rank: Frequent Contributor
timestamp: July 22, 2020, 05:57:57 pm
content: TESTEC is not exactly a well known brand but I hope Farnell sell stuff of reasonable quality - as a minimum they must meet all EU regulations which I'm not sure about the one on ebay.That said, thank you bdunham7 for the link, appreciated!

author: 2N3055
author_rank: Super Contributor
timestamp: July 22, 2020, 06:53:40 pm
content: Testec is a known OEM wirh decent quality

author: nctnico
author_rank: Super Contributor
timestamp: July 22, 2020, 07:09:17 pm
content: Quote from: CDaniel on July 22, 2020, 04:14:36 pmWrong on both accounts.1) Never ever use an isolation transformer. Only use differential probes. Isolation transformers are a death trap in untrained hands c.q. in an unsuitable measurement setup.2) Without an isolation transformer the primary side of the power supply is ground referenced just like the oscilloscope. You can still make useful measurements but there will be some offset. I have done this myself many times.

author: tony359
author_rank: Frequent Contributor
timestamp: July 22, 2020, 07:25:04 pm
content: Quote from: 2N3055 on July 22, 2020, 06:53:40 pmThat's great to know, thanks! I buy some stuff from China but I didn't feel like buying a 100:1 probe on Ebay. Even if it's of good brand, it's quite easy to get a copycat.I believe I now fully understand what to do and not to do when playing with an SMPS, thanks you all for the great advice!

author: bdunham7
author_rank: Super Contributor
timestamp: July 22, 2020, 08:15:04 pm
content: Quote from: nctnico on July 22, 2020, 07:09:17 pmEveryone has their opinions and this subject goes round and round endlessly. There's nothing wrong with the way I mentioned and it is often a lot more convenient and less noisy. You can even do it with isolated or differential probes if you like. I fail to see how the isolation transformer itself is unsafe--the unsafe situation always involves some mistake or another and you could just as easily make the same--or different--mistake without one. At least a proper isolation transformer will limit current to a few amps and then stop (fuse or breaker) if you do short something.

author: nctnico
author_rank: Super Contributor
timestamp: July 22, 2020, 08:38:26 pm
content: Quote from: bdunham7 on July 22, 2020, 08:15:04 pmIsolation transformers are bad in the hands of novices and people who think they understand the risks (and I hate to say it but you seem to fall in this category). Think about the situation where you clip the ground of your grounded oscilloscope to a random point in the primary side of a PSU. You'll ground the PSU AND bypass the GFI (you should have) making the isolation the isolation transformer provides go down the toilet. On top of that: If things go wrong you can fry your probe (and scope if you are unlucky). It is very easy to clip the ground of the second probe to a different point and creating a short between those points. Working safely should not depend on not making mistakes; people do make mistakes.Besides all that a differential probe costs the same as a good (=with safety rated isolation between primary and secondary) isolation transformer. The whole point of trying to prove an isolation transformer is a good solution is totally moot. There are better ways to limit the current through a circuit as well. For example by putting a lamp in series or use a quick acting low amp breaker. But even then there are much better ways to test SMPS safely. A method I use when the control circuitry is acting weird is to use a low voltage PSU to power the controller chip and provide the DC bus voltage (say 30V to 40V); that way you can usually test everything without needing to apply mains at all.

author: tony359
author_rank: Frequent Contributor
timestamp: July 22, 2020, 08:51:32 pm
content: Quote from: nctnico on July 22, 2020, 08:38:26 pmUhm... I like that! I think I'll give it a go, thanks!

author: tautech
author_rank: Super Contributor
timestamp: July 22, 2020, 08:54:13 pm
content: Quote from: tony359 on July 22, 2020, 08:51:32 pmAlways check SMPS IC datasheets for VCC max beforehand.

author: tggzzz
author_rank: Super Contributor
timestamp: July 22, 2020, 09:36:32 pm
content: Quote from: nctnico on July 22, 2020, 08:38:26 pmPeople need to consider how thingsfail, as well as how theywork. Hence you make sound points.But I don't like the series (incandescent) bulb technique.An SMPS keeps the output power constant. If the input voltage is reduced then it will try to increase the input current, which can damage components.

author: CDaniel
author_rank: Frequent Contributor
timestamp: July 22, 2020, 10:23:38 pm
content: Quote from: nctnico on July 22, 2020, 07:09:17 pmHow you can state such nonsense ...Then how do you measure and not die in the secondary side ( isolated from mains ) of any device ? Isolation transformer make the primary of the SMPS just like a secondary side , isolated from mains . There isn't any difference ( beside the output voltage of course ) between a 230V(120V)-12V transformer and a 230V(120V) - 230V(120V) . The concept is not complicated at all , if you think a moment . If you touch one of the secondary 230V(120V) wires nothing happens ...

author: bdunham7
author_rank: Super Contributor
timestamp: July 22, 2020, 10:51:40 pm
content: Quote from: nctnico on July 22, 2020, 08:38:26 pmNovices and idiots should stay out of SMPS guts no matter what equipment they have. I don't know why you think I fall into this category, I have routinely worked with systems and devices much more dangerous than a garden variety SMPS and I still have 10 fingers and 2 eyeballs after all these years. CRT Color TV's anyone? I'm overhauling an old HP 6515A 1600V power supply right now and I assure you, I'm not getting buzzed.QuoteDUDE! Can you read? Did you bother to read? Did you miss the 'no ground clips' part? Who in the f*ck would do any of those things? And do you think the result would be any different without the isolation transformer? If they did, the big smoking mess and fried scope would be a good lesson--that they should adopt a different hobby or line of work. And why do you keep harping about the isolation transformer not being effective against touch voltage when I've expressly said that I'm not using it that way on an SMPS? Working safely means understanding what you are doing and following the rules you have laid out using that understanding. If I'm working on something like this using a ground-referenced scope, it is all hands-off and no ground clips, or if necessary one ground clip attached to my reference ground, not some 'random' point.QuoteIf you like diff probes better, by all means do it that way. Inventing silly scenarios that might happen due to sheer ignorance by the user isn't much of an argument--and don't forget, your diff probes won't protect you if you lick the filter capacitor terminals.

author: 2N3055
author_rank: Super Contributor
timestamp: July 22, 2020, 11:23:00 pm
content: Isolation transformer solves only isolating from distribution network. First moment you connect ANY part of circuit on isolated side of transformer to a ground clip of scope YOU and your circuit is NOT isolated anymore. You just referenced it to scope ground that is earthed... Also it doesn't solve anything if you want to measure on BOTH primary and secondary side of SMPS at the same time with same scope..Or if you want to measure voltage on inductor AND output voltage at the same time.Diff probe has no grounded side, so you can scope around anything. like you would with multimeter. That alone is worth buying diff probe..Micsig diff probe is very inexpensive. Buying good 100x or 1000x probe and iso transformer is not cheaper. And is more limited in use and dangerous in many ways. Fact that it CAN be done is not proof it SHOULD be done..

author: CDaniel
author_rank: Frequent Contributor
timestamp: July 23, 2020, 06:59:31 am
content: And how and why would you want to measure both primary and secondary side in the same time if your scope's channels are not isolated? This is not about safety , even at 12V separate windings transformer you can destroy your circuit or measure garbage doing that . And of course in the same circuit if you don't know that your channels ground clips are connected ...

author: 2N3055
author_rank: Super Contributor
timestamp: July 23, 2020, 07:06:17 am
content: Quote from: CDaniel on July 23, 2020, 06:59:31 amWith two differential probes....And there are many reasons. For instance, on PFC corrected PSU with isolated secondary you might want to look at output while looking at regulating or PFC circuits that are on the primary side...

author: CDaniel
author_rank: Frequent Contributor
timestamp: July 23, 2020, 07:23:26 am
content: If you want to do that for the living better buy a scope with isolated channels ...Many differential probes are for low voltage and don't have good bandwidth.

author: 2N3055
author_rank: Super Contributor
timestamp: July 23, 2020, 07:32:19 am
content: Quote from: CDaniel on July 23, 2020, 07:23:26 amIsolated channels scope are more of industrial/automation stuff.. Most of them are very basic scopes, with some specialised stuff..SMPS R&D scope would be 12 bit Lecroy with hi voltage diff probes and current probes...And probes are for voltages you buy them for. And most of them are NOT low voltage. Low voltage is 50 -70 V..1300V is not low voltage and neither is 5300V.

author: tggzzz
author_rank: Super Contributor
timestamp: July 23, 2020, 07:35:43 am
content: Quote from: 2N3055 on July 23, 2020, 07:32:19 amAnd frequencies.Never forget to understand the specification of max voltage as a function of frequency!

author: 2N3055
author_rank: Super Contributor
timestamp: July 23, 2020, 07:52:08 am
content: Quote from: tggzzz on July 23, 2020, 07:35:43 amAbsolutely, frequency derating curves are very important... Thank you ...

author: CDaniel
author_rank: Frequent Contributor
timestamp: July 23, 2020, 08:59:55 am
content: And don't forget that the max voltage stated is peak to peak , so for rms is not that much1300Vpp even without derating is not enough for probing safely across the transformer primary , especially a flyback where could be some high voltage spikes

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2020, 09:08:08 am
content: Very unusual to find a SMPS without back EMF control on the primary.Lots of MOSFETS would be toast without it.

author: CDaniel
author_rank: Frequent Contributor
timestamp: July 23, 2020, 09:16:38 am
content: OK , but not allways you deal with a lets say "modern" flyback where a 700V Mosfet can be used

author: tautech
author_rank: Super Contributor
timestamp: July 23, 2020, 09:19:16 am
content: Quote from: CDaniel on July 23, 2020, 09:16:38 amNa, shit RDSON

author: 2N3055
author_rank: Super Contributor
timestamp: July 23, 2020, 09:45:31 am
content: Quote from: CDaniel on July 23, 2020, 08:59:55 amWhen working on SMPSs RMS rating is not important, only P-P... As I said, if you work on something that is 2000 V you buy probe for that...

author: CDaniel
author_rank: Frequent Contributor
timestamp: July 23, 2020, 09:50:16 am
content: Yes you measure peak to peak , but I mentioned clearly that the probe rating is for peak to peak not rms ...

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2020, 06:21:32 pm
content: Quote from: 2N3055 on July 22, 2020, 11:23:00 pmI don't know why the dangers and difficulties of SMPS testing have take on such mythical proportions compared to other similar devices which have been around for many years. There's no special sauce, extreme energy levels beyond compare or complications beyond understanding of anyone with basic knowledge of the subject.First, while isolation may protect you from touch voltage in a basic setup, I'm not using it that way here, so there is still a touch voltage hazard if you don't use the hands-off method. This is no different than using diff probes--if you are probing circuits by hand, they're live to touch. I'm intentionally referencing a point in the circuit to ground, the most common point being the negative terminal of the input bridge. And yes, in many cases you CAN measure both primary and secondary sides simultaneously with a single non-isolate scope with no voltage hazard or damage. I'll provide an example here, measuring "the voltage on an inductor AND output voltage at the same time".I have an old Delta ATX SMPS, unfortunately it is non-PFC, which makes it a lot less interesting, but it will still serve. It's the only handy 'laying around' PSU I have right now. I take the case cover off and add two test points at the + and - of the bridge, where there just happen to be handy empty holes. I then install a jumper to tie the negative of the bridge to ground. Then the PSU gets plugged into the isolation transformer.The ground clips are initially removed from the probes, and here I've connected one probe (1X) to the 3.3V output and the other (100X) to the +BRIDGE test point I installed. Unfortunately the 100X probe hook is obstructed in the picture and I've already put everything away. I turn the power supply on and observe the result on the scope, then set the trigger to capture the noise pulse on the 3.3V output. Note that CH1 is +350V, CH2 is 3.3V or so, one primary, one secondary and nobody died.Now I've said 'hands off' for safety, but in reality I often do some careful poking around. I'll bet the differential probe advocates do so as well and probably with both hands at times. As long as I use one non-grounded probe and keep my hand behind the guard, I'm pretty safe. The main danger is damage due to shorting two close points with the probe, a danger that has nothing to do with isolation or probe type--it can happen with a DMM as well. So I find an interesting point on the input side of the main transformer, attach a test point and hook it up.Well, there's some noise, which is what happens when you don't have ground clips. Clearly we can see what part of the CH1 signal is correlated to the noise (not that it was a big mystery), but if we want a more accurate and proportional signal, we can connect a ground clipto the ground reference point.It's tempting to use a more convenient ground point, but that's where the errors can start to happen. Add the ground clip and the signal looks good.And there you have it. This is, IMO, a completely safe--as safe as anything else with exposed high voltages--method of probing an SMPS. Yes, is requires some knowledge, planning and care--but differential probes are no more an excuse for carelessness than isolation transformers are. Stupid hurts, no matter how much gear you have. Obviously in this case, the ATX-type PSU has a grounded-common output, which may not be the case in other types of devices, so as my favorite math professor used to say, "mutatis mutandis".As for all of the other issues that have been raised, I don't want to bother quoting everyone, but every system has it's advantages and disadvantages. I can probe 4 points simultaneously this way with one scope--that would require 4 differential probes. A cheapo diff probe might be less expensive than my iso-variac, but then you start talking about 12-bit LeCroys and Rogowski coils. I'm not an SMPS researcher and if I was I wouldn't be asking someone like me for advice on EEVBlog, would I? I'm just fixing stuff and this is one way to do that.EDIT: My pictures got out of order somehow, I'll maybe sort them later.

author: nctnico
author_rank: Super Contributor
timestamp: July 23, 2020, 06:41:29 pm
content: Quote from: CDaniel on July 22, 2020, 10:23:38 pmBut if you clip it to ground using an oscilloscope probe (otherwise you can't measure anything) the SMPS will be ground referenced without any protection from the GFI. But this has been written and explained so many times before....

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2020, 07:08:40 pm
content: Quote from: nctnico on July 23, 2020, 06:41:29 pmTrue, of course. But relying on GFI to mitigate touch voltage hazards is just as foolish as anything else, especially in an SMPS where intermediate stages may either be isolated enough not to trip the GFI or contain sufficient energy to finish frying you even if the GFI trips. Of course, if you really want to, you can add a GFIafterisolation easily enough.

author: 2N3055
author_rank: Super Contributor
timestamp: July 23, 2020, 08:01:28 pm
content: Quote from: bdunham7 on July 23, 2020, 06:21:32 pmYou can write whole book of rationalizations to justify your opinion... It's irrelevant.You are wrong.I said it before, fact that so far YOU survived and YOU didn't destroy valuable equipment is just something that didn't happen to you.It will if you keep pushing luck. I was thinking same as you, and then one day ground clip that was on right place snapped off and while it was falling touched stuff.. BOOM....I knew everything right and it still happened. Shit happens.Now imagine someone that is beginner, with limited funds.. Odds are they will destroy little equipment they have and maybe hurt themselves.Because you preach that it is safe, if you know what you're doing.Problem with beginners is that most of them are not aware how little they know...They tend to doing things hastily, and they overestimate their abilities and understanding.Most of them are used to how with multimeter they can poke around ANYWHERE with any of the probes...It is how they think and it is not natural for them to think where is ground reference... That is something you have to learn and have to get used to...For you it was so long time ago you forgot how it is ...It is against the law in most world countries to work on ungrounded equipment. Because it is dangerous. When things are grounded properly, you KNOW where your ground reference is...And with diff probe you don't get crap injected in signal. That is what they do, reject common noise and common mode signals...

author: nctnico
author_rank: Super Contributor
timestamp: July 23, 2020, 08:30:58 pm
content: Quote from: bdunham7 on July 23, 2020, 07:08:40 pmNo, wrong. If you touch anything in the primary side of a PSU which causes a current to flow to ground through you, the GFI trips because there is an unbalance between hot and neutral.QuoteThe latter is the stupidest thing to do because you'll need to ground reference the secundary side of the isolation transformer and thus voiding the purpose of the isolation transformer. I sincerely hope this is a brain fart and you wrote this in error...And ofcourse a GFI isn't 100% fail safe but you can have 2 in series if you like (a regular one as part of the installation of your home and an extra sensitive one for your bench).

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2020, 08:31:57 pm
content: Quote from: 2N3055 on July 23, 2020, 08:01:28 pmWhat does all that have to do with isolation or not? I've burnt up my fair share of stuff before I learned, and got buzzed once or twice. The reason I wrote all that is to show a safe way to do things, not a way to 'get away' with something. This is the result of learning, not luck.QuoteNow that you mention it, I think the most common probing mistakes are actually made with DMMs. Poking around and short two points with the probes and ZAP!--dead chip. Forget to move the cables from the AMP jack and BRZZT, burnt probes. And you want to hand these 'beginners' another toy so they can 'safely' poke around inside a high-energy device before acquiring knowledge about....grounds? Probably using both hands at once? Seriously?QuoteNow you and ntnico are smart guys, I'm sure, but did you READ? The whole point of how I do it is that I select the ground point and ground it. Obviously I know where my ground is, that's the point. And I've no idea what laws you are talking about--does that include battery powered camera flashes? Are you saying isolation transformers are illegal?QuoteYours must be magic because the CMRR of bargain-basement diff probes is the main reason not to have them.

author: bdunham7
author_rank: Super Contributor
timestamp: July 23, 2020, 09:02:35 pm
content: Quote from: nctnico on July 23, 2020, 08:30:58 pmI appear to have put too many concepts in one sentence. Let's simplify and you can agree or disagree with each as you like.First,relyingon GFI for protection against touch voltage hazards is improper. Period. It's nice to have, but should not be your primary plan because 1) you still get a shock and 2) sometimes GFI doesn't trip or trips slowly.Next, depending on the architecture of the SMPS, there may be portions of it that are at high potential relative to ground but do not cause a current imbalance that the GFI can detect, or detect easily; this would be in high-frequency switched sections. The GFI may have trouble sensing currents at high frequencies, or some intermediate stage may have a fault in its isolation. Or, in some cases, the output side of the SMPS may be high voltage and ground referenced. I know that isn't 'primary' but zapping is zapping.Last, the filter capacitors may retain a significant charge and the GFI can't do much about that.QuoteYou're a smart guy, but here's an opportunity to learn something from a knuckle-dragger like me. Please read at least twice:GFI devices, despite having the term 'ground' in their name, typically do not have or need any internal reference or connection to ground.Yes, seriously. Two power lines in, two power lines out. If there is a ground connection on the device, it is there for the electrician's convenience and is not connected internally. Many GFI devices are actually permitted to be installed in older homes that do not have ground wires at all by simply omitting the ground wire and putting on a sticker that says "No Equipment Ground". And I have actually tried GFIs with the isolation transformer to measure their trip current so this is not a theory.QuoteAt least with my home's electrical equipment, that is only possible if using something called a combination Arc-Fault detector in the service panel, which has a much higher GFI trip level (it trips after you are dead, not before). Otherwise, the manufacturer specifically prohibits using the devices in series. I can't remember why.

author: nfmax
author_rank: Super Contributor
timestamp: July 23, 2020, 09:06:19 pm
content: Quote from: nctnico on July 22, 2020, 08:38:26 pmI've used this approach as well. Many 'universal' type power supplies will actually start up on 50V to 60V, at least unloaded. This lets you check them out functionally, for example after you have found and fixed 'the' fault (there's always at least two). If you use a low current limit on the PSU, you also have some protection against dead shorts, MOSFETs being turned hard on, etc.

author: David Hess
author_rank: Super Contributor
timestamp: July 24, 2020, 04:11:48 am
content: Quote from: bdunham7 on July 23, 2020, 08:31:57 pmThat is what I use my isolation transformer for; it allows me to move the safety ground to exactly the same point as my probe ground preventing the ground loop which would otherwise contaminate the measurement. This also applies to the ground for differential probes for the reason given below; differential probes have a third connection whether it is explicit or not.QuoteExactly, common mode rejection falls with frequency and commonly available differential probes have poor common mode rejection to start with. Good common mode rejection is the exception and not the rule.