Classic don't-blow-up-my-scope question - Page 1
Forum section: electronics / beginners

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: February 19, 2019, 11:19:40 pm
content: I have a 12V AC wallwart. Since it has a transformer inside, would it be safe to scope it? Would it be effectively isolate from ground?Second question... if not, and/or for other devices, is there a cheap way to buy or make an isolation transformer? Amazon has a 250W isolation transformer for 200 bucks! A bit pricey.

author: Psi
author_rank: Super Contributor
timestamp: February 19, 2019, 11:49:54 pm
content: Yep, it's safe to scope the low voltage side. Not on the high voltage mains side.If you keep looking your will probably find a cheap isolating transformer on ebay eventually.You can make an isolating transformer from two mains transformer of the same type.But you will need to put it in a box and make it safe etc..It's usually easier to find two random mains transformer than one isolating transformer.If you found two 110V ->50V transformers you could connect them in a step down then step up combo.input 110V -> 50 output then feed that into 50V of second transformer.The output of the second one would be 110V again.Ya just have to be careful about the wattage, and obviously make sure you get them around the right way. 110V into 50V winding will end badly.Also don't use anything under like 12V on the low voltage side, ideally you want as high as possible.The amps gets really high if the voltage is too low.

author: sambonator
author_rank: Regular Contributor
timestamp: February 20, 2019, 12:07:17 am
content: Quote from: joeyjoejoe on February 19, 2019, 11:19:40 pmDigikey has one for $87.https://www.digikey.com/product-detail/en/hammond-manufacturing/171A/HM2103-ND

author: rstofer
author_rank: Super Contributor
timestamp: February 20, 2019, 12:07:23 am
content: Safe? Maybe not... Watch Dave's video on "How Not To Blow Up Your Scope"If one side of the secondary is grounded then that is the lead that you need to hook to probe ground. Maybe one way to check is to measure, with a DMM, the voltage on each lead with reference to earth ground. On a grounded supply, one lead will read very close to 0V (grounded) and the other won't. I would expect the transformer to be ungrounded so both leads will measure 'something' relative to earth ground but the value will be nonsensical and perhaps erratic.I wouldn't expect one side of the secondary to be grounded but it is possible.It isn't the probe that is the problem, it is that darn ground lead. By definition, it is earth grounded through the BNC connector and through the power cord. You have to be very careful where you place it.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: February 20, 2019, 01:07:03 am
content: Checking resistance between the tip/barrel and both prongs of the wall wart show open circuit, so probably good.However, thanks for linking that transformer. That's a reasonable price for something that will prevent me blowing up my scope. Arrow has it a few bucks cheaper, and has a 200VA model for only a few bucks more. Trying to think of if the lab warrants one and how much I would use it.

author: Psi
author_rank: Super Contributor
timestamp: February 20, 2019, 02:48:54 am
content: Be aware, don't put the isolating transformer on the scope unless youfully understandwhat risks that creates.Put it on the device under test instead.If you put it on the scope you may make the entire outer case of the scope live!

author: vk6zgo
author_rank: Super Contributor
timestamp: February 20, 2019, 12:19:15 pm
content: To find out if your "wall wart" has one side of the output connected to ground, just use your 'scope probe with the "ground clip" & its lead removed from the probe.If one side of the transformer output is connected to the power socket wiring ground ("protective earth" or PE for short) as the 'scope chassis is, you will see zero*volts when you probe that side.If you probe the other side & see 12v ac, you have proven that the side with zero*volts is grounded.If both sides show zero*volts neither side is grounded.This is exactly the same test you could carry out with a DMM.In my opinion, it's quicker to use the Oscilloscope, but that's just me!*If you wind the 'scope sensitivity up enough, youwillsee something, but it is "random pickup" & will be very much less than 12v.

author: exe
author_rank: Unknown
timestamp: February 20, 2019, 12:41:58 pm
content: Common suggestion is to use high voltage isolation probes. I also see people suggest not to use isolation transformers as they prevent gfci from triggering. Isolation transformer also won't help if you touch both wires simultaneously (actually, no method helps in this case). So, some people suggest do measurements using only one hand to reduce risk of current flowing through the heart.No matter what protections you choose, no method is foolproof.

author: capt bullshot
author_rank: Super Contributor
timestamp: February 20, 2019, 02:33:26 pm
content: Quote from: joeyjoejoe on February 19, 2019, 11:19:40 pmThe answer is: Yes, yes. (to be applied in the order of question marks).QuoteAn isolation transformer is overrated / not necessary for most of the things you usually do with a scope. As long as you don't mess around with circuits that are directly connected to mains (e.g. primary side of a power supply), you won't need an isolation transformer, won't need a HV probe and won't need a differential probe.

author: DDunfield
author_rank: Regular Contributor
timestamp: February 20, 2019, 04:17:39 pm
content: Quote from: Psi on February 20, 2019, 02:48:54 amI concur completely with the "understand the risks" - no matter WHAT METHOD you use.But.. I'm not quite as convinced that the traditional "don't float the scope, float the DUT" is as applicable today as it used to be.Sure... when scopes were big metal boxes, you would absolutely NOT want to float it. But modern scopes are not like that.They are well insulated cases and knobs, if you cap off the BNC's and calibration output, and don't go putting your fingers into the network or USB connectors, a lot of modern scopes will be safer to float then the DUT. (Still not something I recommend doing!)The problem is that "floating" the DUT does not make it safe! but does tend to give people a dangerous false sense of security because "I did it the safe way".As soon as you clip the ground lead from your grounded scope to some high voltage point on the DUT, it is NO LONGER FLOATING! - in fact, it is now grounded at a place where the manufacturer never intended it to be grounded. That means lots of spots which were left exposed because "they're grounded" are now sitting at a large offset from ground. Things like that grounded heatsink you've gotten in the habit of touching to gauge the power dissipation level... or the grounded device case you sometimes rest your fingers on while holding the probe in a difficult spot...There are much "safer" ways to look at HV components in a mains connected system, such as:- Use the math functions of your scope to see the differential between two probes (no extra cost option)..- Use a HV differential probe- Use a properly insulated battery powered scopemeterThe reason "safer" is in quotes above is because just because you have the right tools doesn't make you safe. Mains isn't something to screw around with.IMHO the only good advice you can give a newbie who wants to scope something directly connected to mains is "DON'T - wait till you're not a newbie and REALLY know what you are doing".Having said all that - if you just want to see a power grid sine wave, looking at the output of a low voltage power transformer (no connection to mains other than induction) is a good way to do it.Dave

author: IanMacdonald
author_rank: Frequent Contributor
timestamp: February 20, 2019, 05:12:43 pm
content: If working on the primary side of a SMPS you absolutely do need an isolating transformer in its feed. This is because they use a bridge rectifier, so no part of the circuitry is at 0v potential, no matter which way round the supply is connected. The same applies for some vintage TV sets which use a non-isolated bridge rectifier.Vintage radios that are 'live chassis' (no mains transformer) likewise, although some people are happy to work on them after checking that the chassis is connected to the neutral, rather than the live wire. The problem here is that if the neutral were to come off the chassis would become live. Thus, not advisable. The isolating transformer arrangement is safer for theseOn high voltage equipment such as valve guitar amps, which usually have a mains transformer anyway, there is no real advantage in using isolation, and I would just check that the chassis is grounded before working live on it.The important point in all such cases is not to place your resting hand (or any body part) on grounded metal whilst doing high voltage tests. The temptation to do this on a guitar amp chassis, vintage scope case or the like is ever-present, and should be resisted. Any mistake you make in touching a live part whilst one hand is earthed will have far more serious consequences.I cringed at seeing Mr Carlson holding the metal case of a signal probe when doing live tests on valve gear. Much as he is a brilliant vintage equipment restorer, I would disagree on this point. Put some heatshrink on it. Or if nothing else is available wrap some tape round it. Just don't use it bare. Please!Should you isolate your scope? Good question, and the problem of the case becoming live if the probe outer does, is a very good point. It could also be unwise on vintage CRT scopes with some whacking voltages inside, since it could allow such voltages to find their way onto the case. On modern scopes this is less of a concern though, and putting an earthed test lead onto a point which you thought was at 0v but isn't, is a good way to damage the DUT, let alone the scope.The classic trap for young players here is audio amps with bridge speaker drive. Especially car audio. Need to scope the output, so put the probe croc clip on the 'negative' speaker terminal, and 'BZZZZT-POP!' as you fry the putput stage. Reason; in this configuration both sides of the speaker have power on them. Of course isolating the scope won't help if you put two probe grounds on different places. In that case you just create a short between the probe outers.Roll on the day when scope inputs are like multimeters.

author: exe
author_rank: Unknown
timestamp: February 20, 2019, 05:16:16 pm
content: Concerning modern scopes, the bnc connector is grounded. It's not that difficult to touch it.

author: DDunfield
author_rank: Regular Contributor
timestamp: February 20, 2019, 05:45:28 pm
content: Quote from: exe on February 20, 2019, 05:16:16 pmThere are little caps you can get specifically for the purpose of insulating unused BNCs, and there are probes readily available which have the scope end insulated as well (or you can tape it).My point being that unlike the days of large metal boxes, exposed grounds on modern scopes can often be more easily insulated than whatever it is you are testing.Neither (floating scope or DUT) is a good solution. I personally keep an old Fluke97 which is my go to when I need to scope something hot. Battery powered, well insulated even the BNCs and probes are insulated to reduce the chance of unintended contact.

author: exe
author_rank: Unknown
timestamp: February 20, 2019, 06:14:42 pm
content: Quote from: DDunfield on February 20, 2019, 05:45:28 pmI'm not arguing. I'm the opposite, just adding to what you said.I know these topics were discussed quite a bit, but I like we have a condensed version of all typical techniques on a single page.Question: is it worth wearing gloves or something? Is it worth putting on rubber slippers?

author: Ian.M
author_rank: Super Contributor
timestamp: February 20, 2019, 06:30:52 pm
content: A reasonably thick rubber or plastic mat on the floor at the bench is a useful addition for safety if you work with mains or other high voltages and your workshop has a concrete floor or if there's any chance of damp flooring. Rubber soled shoes on their own are less useful because the combination of a cracked sole, moisture and dirt, or simply treading on a drawing pin can cause them to become conductive with very little warning. They are still far better than leather or fabric soles or bare feet. However insulation under your feet or chair is useless if you come in contact with any other ground, so exposed and possibly grounded metalwork in your work area must be minimised.Floating a scope, even a modern one with minimal visible exposed metalwork is still a *LETHALLY* *DUMB* idea when working with any voltage above 50V, as you don't know the insulation rating of the front panel controls. All it takes is a scope floating at HV with one metal grubscrew in a small plastic knob to electrocute you. This is not hypothetical - people have died floating scopes. Also modern scopes with SMPSUs are likely to have excessive noise levels if not grounded as per manufacturer's recommendations.

author: DDunfield
author_rank: Regular Contributor
timestamp: February 20, 2019, 07:55:11 pm
content: Quote from: Ian.M on February 20, 2019, 06:30:52 pmVery good point - and I actually agree with you. If you read my posts, I'm not advocating floating a scope.But I remain unconvinced that floating the DUT is actually any safer than floating a modern scope. In the past it was obviously so because one of them was a huge metal box attached to whatever it thinks ground is ... but IMO they've come to similar levels (neither of which I would consider particularly safe).To put forth your arguments: You don't know the insulation rating between the internal parts the DUT manufacturer thought were at ground potential and equally easily touchable parts on the outside. And the DUT may also have characteristics which make it more noisy or otherwise operate out of spec. when not properly grounded.To reiterate my first comment, telling people to float the DUT because doing it that way is "safe", allows people a false sense of security that what they are doing is safe - it is not. What most people seem to miss is that as soon as you connect a grounded test lead to a part of the DUT that is not normally at ground potential, it is no longer "floating safely" and is in fact "ground reversed" - at least one part normally hot is at ground, and all the parts normally at ground are hot. Not something I like having on my workbench.There ARE better ways!Dave

author: metrologist
author_rank: Super Contributor
timestamp: February 20, 2019, 08:17:01 pm
content: My grandfather would grab house mains all the time. He thought it was silly how afraid people are of mains.I saw on the front panel says 300V RMS CAT I.Measurement Category I:This category is for measurements of voltages from specially protected secondary circuits. Such voltage measurements include signal levels, special equipment, limited-energy parts of equipment, circuits powered by regulated low-voltage sources, and electronics.

author: David Hess
author_rank: Super Contributor
timestamp: February 21, 2019, 04:55:07 am
content: If it is a two power pin transformer, then it is completely safe to hook to the oscilloscope and this is an excellent way to monitor the AC power line. If you are worried, then double check with an ohmmeter from primary to secondary to verify isolation.