How to test a switch mode power supply with an analog oscilloscope - Page 1
Forum section: electronics / beginners

author: j57H8I4
author_rank: Contributor
timestamp: March 04, 2017, 08:34:11 pm
content: I would like to test the PWM integrated circuit of a switch mode power supply with my analog oscilloscope, like it is tested from the minute 7:36 in this video:https://youtu.be/g4Jvm5Mh594In that video that guy connects the PWM output and the mosfet gate of the switch mode power supply to a digital oscilloscope, I think working with a battery. When I tried to reproduce the same test using my analog oscilloscope the switch mode power supply fuse blew up, because the probe ground is connected to the mains ground. I would like not to use an isolation transformer because I am unfavorable to isolation transformers.Is there a way to test a switch mode power supply PWM using an analog oscilloscope without an isolation transformer?

author: Ian.M
author_rank: Super Contributor
timestamp: March 04, 2017, 08:48:40 pm
content: Not without spending hundreds of bucks for a high voltage isolated differential probe.

author: j57H8I4
author_rank: Contributor
timestamp: March 04, 2017, 09:19:25 pm
content: So the question now is: is there any grounded testing point in a switch mode power supply that allows to test the power supply using an oscilloscope connected to the mains without an isolation transformer and an high voltage isolated differential probe?

author: Ian.M
author_rank: Super Contributor
timestamp: March 04, 2017, 09:34:21 pm
content: NO

author: mcinque
author_rank: Unknown
timestamp: March 04, 2017, 09:46:57 pm
content: Using both channels with probes on 10X, without GND leads connected and doing MATH A-B as a differential probe (with much less performance) should work.EDIT: nevermind. Didn't read "analog oscilloscope". Do you have 2 channels and "Invert" on your analog oscilloscope? It should work like MATH A-B.

author: Ian.M
author_rank: Super Contributor
timestamp: March 04, 2017, 10:27:26 pm
content: Not a good idea on the primary side of a SMPSU, unless you've got a pair of 1KV rated x100 probes. With modern high frequency PSUs and >300V on the DC bus, its all too easy to exceed the probe's voltage limit derated for frequency. Also, unless the scope's gain calibration is spot-on there will be a lot of 50Hz breakthrough.

author: j57H8I4
author_rank: Contributor
timestamp: March 04, 2017, 10:32:40 pm
content: Yes, my oscilloscope has got two channels and the second channel has got invert function.

author: j57H8I4
author_rank: Contributor
timestamp: March 04, 2017, 10:46:10 pm
content: No, my oscilloscope has got no calibration at all: I just bought it used and I received it yesterday! My oscilloscope is rated for 250V PK MAX, one probe is rated for 200V PK at 1x and 600V PK at 10x, I do not know yet about the voltage rate of my other probe because I have bought it today on eBay, it is the original probe for my oscilloscope but its voltage rate is not indicated in the user's manual. So, could I make an attempt to use it that way or the risks are too high?

author: j57H8I4
author_rank: Contributor
timestamp: March 04, 2017, 10:54:24 pm
content: Do you think that my probe rated for 600V PK at 10x could be used the way mcinque indicated?Please let me know that because I would buy another one of those probes because they are very cheap, it is this one:http://www.banggood.com/1-Set-P6100-DC-100MHz-Oscilloscope-Probe-100MHz-Scope-Clip-Probe-for-Tektronix-p-1025192.html?rmmds=myorder;

author: Ian.M
author_rank: Super Contributor
timestamp: March 04, 2017, 11:34:41 pm
content: Chinese ratings are rarely trustworthy. Using those probes on any circuit with more than 50V on it would be a gamble that could kill your scope, and also you, if you attempt to handle the probes with power on.If you are probing higher voltage circuits, you need brand-name probes, manufactured and tested to real specs (i.e. backed by a 1st word standards org), with an adequate voltage rating, at least 50% more than the max peak voltage you expect to find, and a minimum of a CAT II rating if working on the primary side of PSUs or on circuits fed by HV DC rails with large reservoir caps. If the probe doesn't have a manual (paper or online) with a curve for voltage derating with frequency, its almost certainly not reputable enough for this sort of usage. Beware of fakes - if its far cheaper than brand-name's RRP and sold outside of official distribution channels, odds are it isn't genuine and wont meet its specs.Use an isolating transformerand hard-wire a ground to the negative side of the primary side reservoir capacitor, and your scope and probe will do just fine for probing around the chopper chip, provided youDO NOT attempt to probe the MOSFET drain (or BJT chopper collector), or any part of the primary snubber network.With crappy Chinese probes I wouldn't try to look at ripple on the DC bus either, even though its within their nominal voltage rating.However an isolating transformer removes any RCD/GFCI protection from circuits downstream of it so be very careful when probing the primary side of the PSU. Its not a good idea to try to hold the probe on a pin - if you cant hook it on without shorting anything and work hands-free, switch off, take a 1" piece of tinned solid wire (e.g a cropped component lead), form a small loop in the end and twist to secure, then solder it on as a testpoint you can hook the scope probe to before switching the power back on.

author: j57H8I4
author_rank: Contributor
timestamp: March 05, 2017, 12:21:52 am
content: I have just ordered two of these oscilloscope probes:https://www.aliexpress.com/item/P4250-High-Voltage-Oscilloscope-Probe-2KV-100-1-250MHz-Alligator-Clip-Test-Probe-Electronic-Measuring-Instruments/32433214360.html?spm=2114.13010608.0.0.8hCoj5and also this safety face shield:https://www.aliexpress.com/item/Adjustable-Electrowelding-Dustproof-Face-Mask-Protective-Shield-Full-Safety-Workwear-Eye-Protection-Welding-Helmet/32787027294.html?spm=2114.13010608.0.0.8hCoj5I know, they are Chinese. Now I have to wait a couple of months for them to arrive. Thank you for all your very appreciated indications and advices.

author: Ian.M
author_rank: Super Contributor
timestamp: March 05, 2017, 03:44:30 am
content: More dubious off-brand Chinese stuff. It may be good or it may be totally dangerous shite.Edit: See Tautech's reply below.When you get those HV probes, BEFORE USE, take a BNC socket, connect a 1Meg resistor from the center pin to the shell, plug the lead into it and HiPot test it, between the tip and the BNC shell at 1KV test voltage, and between the shell and all joints in the probe body. Also check ground clip continuity to the shell. 100:1 probes should read an insulation resistance of 100Meg from the tip to the BNC shell. After that, check the attenuation is as specified on a waveform <100V and then (and only then) I'd trust them up to 1KV if you keep them clean and undamaged.N.B. the probe *MUST* be loaded by a 1Meg resistance at the BNC connector, same as a scope input, otherwise if you apply a high voltage to the tip there is a risk of insulation breakdown in the lead. Never disconnect the BNC if the probe is connected to a HV circuit.Get the right PPE for the job - that way you are far more likely to use it every time, all the time.You are very unlikely to need the full face shield when working with electronics unless you are in the habit of fitting large electrolytics backwards or other destructive activities. A pair of safety glasses with side shields (not fully enclosed goggles) would be more appropriate and far more comfortable, and can be ordered with prescription lenses if you need them.

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2017, 04:36:10 am
content: Quote from: j57H8I4 on March 05, 2017, 12:21:52 amThose probes will be fine, they're made by one of the largest Asian probe manufactures that supply probes to several scope OEM's.They are rebranded to Siglent, Rigol etc.

author: innkeeper
author_rank: Unknown
timestamp: March 05, 2017, 05:04:01 am
content: NOTE: doing this is highly dangerous as there are high voltage / current pathways to ground, and the chassis of the scope can be at high potential. I've added this warning good conscious - If you do not understand the dangers of doing this then DON'T EVEN TRY ITI've done it... this goes back now circa 1985 ish.....never liked working on switching power supplies btw.. anywayif you do this though and you hurt yourself or blow up your scope or worse .. don't come crying to me...with that out of the waywhat we used to do is use a plug to isolate the scope from ground so the chassis would be floating (either we tore out the ground pin on the power plug or used one of those 3-2 prong adapters that breaks out the ground.then we would signal ground the scope to the switching power supply ground and go at it.we would put the switching power supply under test on a variac. (not isolated) - this was a shop were we would get supplies someones already had a go at, so we would ramp up the voltage.is it the right way to do it, no. does this assume a lot about how your scope is wired... yea... is it safe ... probably not.

author: tautech
author_rank: Super Contributor
timestamp: March 05, 2017, 05:43:40 am
content: Quote from: innkeeper on March 05, 2017, 05:04:01 amNever is this method safe and should never be suggested on a public forum.Users with limited knowledge of scope use and without the appropriate probes can kill themselves, I will not have that on my conscience. Get the proper equipment or give the repair to somebody that knows what they are doing.

author: David Hess
author_rank: Super Contributor
timestamp: March 05, 2017, 06:53:09 am
content: Quote from: Ian.M on March 04, 2017, 10:27:26 pmOn an analog oscilloscope, the variable controls can be used to trim the combined gain of the probes and oscilloscope to match for good low frequency common mode rejection. Analog oscilloscopes actually work better for this operation than almost all digital oscilloscopes (1) because they can subtract one channel from another without adding quantization noise.If you watch carefully when doing this, you can *see* the noise level fall when the vertical gain adjustment is correct.(1) A weird exception is the old Tektronix 2232 series of DSOs which do the subtraction in the analog domain before the digitizer.

author: tggzzz
author_rank: Super Contributor
timestamp: March 05, 2017, 08:58:22 am
content: Quote from: innkeeper on March 05, 2017, 05:04:01 amThere isONE AND ONLY ONE WAY TOSAFELYFLOAT A SCOPE; see the photo.It is very dangerous to encourage beginners to do dangerous things they don't understand.

author: mcinque
author_rank: Unknown
timestamp: March 05, 2017, 09:13:33 am
content: Quote from: Ian.M on March 04, 2017, 10:27:26 pmI've done it with my DSO with 10X probes but your're right: I made the assumption he has better input specs on his oscilloscope.Quote from: Ian.M on March 04, 2017, 11:34:41 pmThis is true, you cannot trust specs from a not respectable manufacturer.

author: sibeen
author_rank: Frequent Contributor
timestamp: March 05, 2017, 09:43:31 am
content: Quote from: tggzzz on March 05, 2017, 08:58:22 amLove that photo. Back in the day I used to use a bit of electrical tape around the exposed edges of the cro and across the buttons most used. Wouldn't do it nowdays, but I was once young and stupid.

author: dave_k
author_rank: Frequent Contributor
timestamp: March 05, 2017, 11:10:15 am
content: If you just want to test the PWM function of the SMPS controller IC, try powering it using a low-voltage bench supply .. no need to dick about with floating scopes or differential probes. Stay away from high-voltage stuff unless you know what you're doing...

author: guenthert
author_rank: Frequent Contributor
timestamp: March 06, 2017, 07:21:45 pm
content: Quote from: j57H8I4 on March 04, 2017, 08:34:11 pmYou had me confused there with the 'digital' vs. 'analogue'. I watched now the video (sadly I don't speak a word of Italian, love the sound of it though) -- the fellow there is using a (digital)handheldoscilloscope, which is (implicitly) floating off ground. 'digital' vs. 'analogue' is the wrong dichotomy, as most digital oscilloscopes arenot floating off ground. The common (GND) on digital bench oscilloscopes is connected to chassis and mains earth, exactly like that of their analogue counterparts. Ian and others already explained how to work around that, I just thought above was worth clarifying.

author: james_s
author_rank: Super Contributor
timestamp: March 07, 2017, 08:53:17 pm
content: Why on earth would you try so hard to avoid an isolation transformer? They exist specifically for situations like this, and they work very well.You're lucky you didn't seriously damage your scope and the probe, and the power supply you were trying to test.

author: innkeeper
author_rank: Unknown
timestamp: March 07, 2017, 09:26:39 pm
content: Quote from: tautech on March 05, 2017, 05:43:40 amPoint taken, note added to the post

author: Ian.M
author_rank: Super Contributor
timestamp: March 07, 2017, 09:50:33 pm
content: Your note is wrong.The hazard is the live chassis of the floated scope, which makes all its controls a touch hazard, and any exposed metal parts a deathtrap..Also, if you float it by lifting the ground wire, it is far too easy to blow up the the scope as its PSU transformer insulation could be stressed with double the supply voltage between the primary and secondary..Lastly, again with the ground lifted, an unsafe condition could exist even with nothing connected to the scope as it almost certainly wont meet the safety standards for a Class II (double insulated) appliance.

author: j57H8I4
author_rank: Contributor
timestamp: March 08, 2017, 12:05:48 am
content: Quote from: james_s on March 07, 2017, 08:53:17 pmI have not found in any book or user's manual the indication to use an isolation transformer. It is very hard to find isolation transformers on the market, there must be a reason for that.

author: tautech
author_rank: Super Contributor
timestamp: March 08, 2017, 12:17:34 am
content: Quote from: j57H8I4 on March 08, 2017, 12:05:48 amThe safest method for measuring/testing elevated non ground referenced voltages is with a Differential probe.Get one or let someone else do the repair that knows what they're doing.

author: james_s
author_rank: Super Contributor
timestamp: March 08, 2017, 01:06:32 am
content: Isolation transformers are readily available, I would not attempt to service a SMPS or anything with a hot chassis without one. If you can't find one ready made, you can wire two identical transformers back to back.

author: ataradov
author_rank: Super Contributor
timestamp: March 08, 2017, 01:10:34 am
content: Quote from: j57H8I4 on March 08, 2017, 12:05:48 amWhat books have you read on the topic?Quote from: j57H8I4 on March 08, 2017, 12:05:48 amWhat are you talking about? Here is the first one out of a dozen appearing on a first page of amazon search:https://www.amazon.com/Tripp-Lite-Isolation-Transformer-Outlet/dp/B00006HPFH/ref=sr_1_2?ie=UTF8&qid=1488935050&sr=8-2&keywords=isolation+transformer

author: j57H8I4
author_rank: Contributor
timestamp: March 08, 2017, 01:12:17 am
content: Quote from: james_s on March 08, 2017, 01:06:32 amThat is what all people say on forums and videos, but, as I already said, I have not found that indication in any book, nor I have ever seen an isolation transformer manufactured by a primary electronic equipment producer.

author: ataradov
author_rank: Super Contributor
timestamp: March 08, 2017, 01:13:37 am
content: Quote from: j57H8I4 on March 08, 2017, 01:12:17 amWhat is "primary electronic equipment producer"? Tripp Lite is a fairly well known brand for this kind of stuff.You are talking about specialty tools. You won't find one make by Sony or Samsung.

author: j57H8I4
author_rank: Contributor
timestamp: March 08, 2017, 01:15:01 am
content: Quote from: ataradov on March 08, 2017, 01:10:34 amTell me a book or an oscilloscope user's manual where it is indicated to use an isolation transformer.

author: ataradov
author_rank: Super Contributor
timestamp: March 08, 2017, 01:17:00 am
content: Quote from: j57H8I4 on March 08, 2017, 01:15:01 amIt is called common sense. This is not specifically oscilloscope-related. It is about general safety rules when working with high energy circuits.Show me a book that tells that it is a bad idea to shove fingers into the wall outlet. The same book will be applicable here.Also, if you want to kill your scope, or yourself - go right ahead. There is no real point arguing here.

author: j57H8I4
author_rank: Contributor
timestamp: March 08, 2017, 01:18:25 am
content: Quote from: ataradov on March 08, 2017, 01:13:37 amHave you ever seen an isolation transformer produced by Tektronix, or Rigol, or Keysight, etcetera?

author: ataradov
author_rank: Super Contributor
timestamp: March 08, 2017, 01:19:11 am
content: Quote from: j57H8I4 on March 08, 2017, 01:18:25 amWhy would they? You do not isolate the scope. You isolate the device under test.Also, isolated inputs is a feature of high-end scopes with a huge price tag.

author: ataradov
author_rank: Super Contributor
timestamp: March 08, 2017, 01:23:09 am
content: I'm sure it was posted in one of your threads already. But I'll repeat. Watch this:

author: j57H8I4
author_rank: Contributor
timestamp: March 08, 2017, 01:24:46 am
content: Quote from: ataradov on March 08, 2017, 01:19:11 amIf the use of an isolation transformer was so important you should find as many isolation transformer on the market as many oscilloscopes, because for each oscilloscope sold also an isolation transformer should be sold. Instead you can very hardly find isolation transformers on the market and you find them on eBay, not on primary electronic dealers shops.

author: ataradov
author_rank: Super Contributor
timestamp: March 08, 2017, 01:27:34 am
content: Quote from: j57H8I4 on March 08, 2017, 01:24:46 amThis is not true. Not everyone works with non-insulated mains-powered devices. Furthermore, people purposefully design things to be isolated, if possible. This is a consumer safety requirement as well. But sometimes it is unavoidable.For example, if you work with digital circuits and power them from your Korad power supply, you are fine without any isolation transformer. Your power supply acts as one.But if you want to look at the guts of that power supply, then you need one.

author: james_s
author_rank: Super Contributor
timestamp: March 08, 2017, 01:31:06 am
content: Most people are not using scopes to poke around in line powered switchmode power supplies, and those that are have an isolation transformer. The transformer isolates the DUT, *not* the scope. The scope should *always* be earth grounded.Walk into any TV repair shop around, if such a place even exists anymore and I guarantee there will be an isolation transformer on the bench. Any lab where switching power supplies are designed will have one, we certainly had one in the lab at the last place I worked that did hardware. It is a mandatory safety item, it protects you as well as your test equipment.

author: james_s
author_rank: Super Contributor
timestamp: March 08, 2017, 01:33:38 am
content: Quote from: ataradov on March 08, 2017, 01:27:34 amHeh that makes me visualize a light socket with a warning label that says "Do not insert tongue"I shouldn't even joke about it, people expect to be warned and protected from everything these days, common sense is not so common.

author: djnz
author_rank: Regular Contributor
timestamp: March 08, 2017, 04:20:53 am
content: Rather than risking you life, why don't you ask all the questions you have about isolation transformers here on the forum? I am sure people will be willing to help!Lots of nasty outcomes are possible here - from loss of life to causing a fire to causing damage to your oscilloscope or the power supply. Why not take the time and learn to do this safely? This is also about habits and developing some respect for electricity. Perhaps you will be lucky a few times and nothing so bad will happen. But if you do not develop the attitude of safety, some day your judgment and your luck might both fail and cause real damage.There is a third option apart from an isolation transformer and differential probes, which is using an isolated channel oscilloscope. All isolated channel scopes I know of are battery powered, but not all battery powered scopes have isolated channels, so be careful. I bought a used Tek THS720A recently, was around US$200. Even with that, all the standard precautions like probe de-rating with frequency apply.Good luck.

author: james_s
author_rank: Super Contributor
timestamp: March 08, 2017, 05:30:38 am
content: Even if I had a fully isolated scope with a differential probe I'd still use an isolation transformer. One slip of the finger and you could end up with 340V with lots of current behind it going through your body. Working on non-isolated mains powered gear without an isolation transformer is a deathwish.

author: Ian.M
author_rank: Super Contributor
timestamp: March 08, 2017, 06:40:55 am
content: Unfortunately the safety provided by an isolation transformer vanishes the moment you connect a non-isolated scope ground clip to the primary side of the SMPSU under test, and worse, it prevents any upstream RCD/GFCI tripping. To retain the safety advantage, either use a fully differential HV probe, or an isolated input scope or a CAT II or better battery powered scopemeter, and beware of sneak grounds introduced by other testgear, etc.Otherwise its live working rules:Start with a clear and tidy workbench and make sure there is nothing grounded where you may accidentally come into contact with it..Remove all rings, bracelets, metal watches, phone or music player headsets etc..Keep one hand in your pocket to avoid shocks across your chest..Test before touching!.Don't hold the probe in place with your hand..When ever possible switch off before moving the probe..Use a large heavy insulating mat, checked for damage, to isolate yourself from ground..Do not work alone..Do not work if tired, distracted or intoxicated..Do not allow pets or young children in the room..Have a clearly marked isolating switch to cut power to the bench..In case of electrocution, make sure that all personnel are trained to cut the power before touching the casualty.That's how techs in the TV repair trade worked without killing themselves before GFCIs became common.

author: bitwelder
author_rank: Super Contributor
timestamp: March 08, 2017, 07:11:36 am
content: From the old Sam Goldwasser's Sci.Electronics.Repair FAQ:https://www.repairfaq.org/sam/smpsfaq.htm#smpsint(the whole chapter is worth reading, but below is the quote about isolation transformers)Quote

author: dav
author_rank: Regular Contributor
timestamp: March 08, 2017, 02:17:25 pm
content: Isolation transformers are harder to find in EU rather than in US.If you want a 220V->220V fully assembled one, you can take a look to this:https://www.reichelt.de/Labornetzgeraete/PEAKTECH-2240/3/index.html?ACTION=3&GROUPID=4952&ARTICLE=101937

author: vk6zgo
author_rank: Super Contributor
timestamp: March 09, 2017, 02:22:12 am
content: Quote from: j57H8I4 on March 08, 2017, 01:24:46 amWhen I worked at a TV Studio,we had dozens of Oscilloscopes,& about 3 isolation transformers.Most of the time the 'scopes were used on equipment which did not have power supply faults.For all other circuitry, the Switchmode Power Supply provided isolation, as it is designed to do.As I fixed a lot of Picture Monitors, where SMPS faults were not uncommon, I had one semi-permanently on my bench.Isolation transformers have no moving parts & last for years, so they are pretty much "niche market " items, just as desoldering stations were for many years.For this reason, they are rarely found in Electronics stores like Jaycar, or Altronics in Australia, (& equivalents in other countries).As for your previous comment, it is fairly unusual for such firms as Tektronix, Sony, etc, to manufacture devices like this, even for internal use in the factory, instead, they buy them from companies who specialise in such equipment.They use forklifts, too, but with the possible exception of Hitachi, don't have "own brand" ones.

author: vk6zgo
author_rank: Super Contributor
timestamp: March 09, 2017, 03:02:43 am
content: Quote from: Ian.M on March 08, 2017, 06:40:55 amIn fairness, the hazard of doing this is hardly different to testing the high voltage secondary circuit of a tube type device using a conventional transformer type supply.Many thousands of such circuits were probed with normal Tektronix or other probes over tens of years, with vanishingly small numbers of injuries.One thing,though, is the possibilty of the SMPS having a "common" side which takes up a larger part of the board.It would obviously be wise to place the earth clip on that side,as the "hot bits" would be smaller & harder to touch. ( Of course, this when using an isolation transformer---just in case some " newbies" missed that bit)QuoteDifferential probes were, until comparatively recently, too expensive, so Techs had to rely upon a "functioning brain".Even now, such probes are far from cheap.QuoteReally good idea,& hobbyists should embrace it,but realistically, this was pretty much "honoured in the breach" much of the time in workshops.QuoteThis becomes more important in the current day,where people are wont to festoon themselves with all sorts of crap.QuoteAll good!QuoteI've seen this done a few times, but quite rarely----it sounds more like what you would do if testing heavy industrial switchboards.QuoteVery few TVs would have ever been fixed.This is something we would have adhered to if getting into the power supply side of TV or Broadcast Transmitters, but small stuff like Monitors,TVs, etc were regarded as significantly less hazardous.QuoteIntoxicated is certainly a "no-no",but most Techs are tired most of the time---it's the nature of the job!QuoteYes!,Yes!,& Yes!Quote

author: Ian.M
author_rank: Super Contributor
timestamp: March 09, 2017, 03:22:37 am
content: The rubber mat thing is less important if you are on lino over a wooden floor, but if you are on concrete its strongly recommended. Carpet normally insulates reasonably well but that can change if its dirty and the humidity is high + its an absolute PITA getting solder splashes out of carpet so you want a large mat down anyway. With the mat, if you follow the one hand rule, the worst you will get is a shock across your hand or to your forearm. Nasty, with a risk of burns or even deep tissue damage, depending on how much energy you get bit with, but with a minimal risk of killing you. Not working alone was possible for all except 1 man repair shops and some hobbyists. Its a rule I've broken a few times myself, but if you know there's no backup if something happens, and you have the intelligence to properly assess the risks, you tend to be a lot more careful and methodical. e.g. Alone, I will unplug and put the plug on my lap before moving the probe., but at the shop bench with colleagues nearby, I would frequently move the probe hook onto the next testpoint loop I'd pre-attached without shutting down.Quote from: vk6zgo on March 09, 2017, 02:22:12 amIsolation transformers - at least as bare transformers used to be readily available over the counter in high street electronics stores. IIRC for about 50 bucks for a 250VA one. However that was 25 years ago, when people in general still repaired stuff or at least were willing to pay to have it repaired. If you aren't repairing or designing SMPSUs or working on lighting ballasts or AC power control circuits its an extremely niche market, and I rarely use one nowadays, though I still have the one with a 250VA toroidial I mounted up with a yellow painted double socket in an old wooden instrument case with a hinged lid, as an easily portable one for callout TV repair.

author: stevelup
author_rank: Regular Contributor
timestamp: March 10, 2017, 12:15:39 pm
content: Farnell (Element 14) list about 100 different appropriate types as do RS and therefore, I would expect, so would pretty much any component wholesaler.It would be less than an hours work to put one in a nice box with a power inlet and output.Most of them have split secondaries as well, so it would be easy to add a 240/120V switch too.

author: David Hess
author_rank: Super Contributor
timestamp: March 10, 2017, 03:04:54 pm
content: Here is how I usually work with off-line circuits including off-line switching power supplies.Neutral and ground are connected together at the breaker box which means any current drawn by the power supply raises the potential of neutral away from ground so ground of the oscilloscope cannot be connected to neutral, or any other point, of the off-line side safely or with good or even safe results. Potentially neutral and hot could be reversed which makes things even worse.One solution is to use a high voltage differential probe however this adds complications and may be expensive. Further the differential probe performance could be improved if we could use a ground connection to limit common mode noise.So what I normally do whether I use a differential probe or not, is to use an isolation transformer on the line side of the device under test. Now ground, real earth ground, may be connected to any low impedance point like neutral or hot, with isolation they become the same thing, or even one of the rectifier outputs which is usually the common point for the line side circuits and the probe ground can be connected there as well without creating a massive low impedance ground loop.

author: casinada
author_rank: Frequent Contributor
timestamp: March 10, 2017, 04:38:40 pm
content: On top of Dave's explanation there are other good explanations out there. As explained by others, know what you're doing, understand what you are doing.