Isolation Transformer - a 2nd one for the Oscilloscope ? - Page 1
Forum section: electronics / testgear

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 08:43:27 am
content: When trouble shooting (testing) a Consumer Product - like TV, or its SMPS - we all know it is essential to hook it first up via an Isolation Transformer, before even trying to probe it with a Ground Referenced Test Equipment (like an Oscilloscope).What I was wandering about is why some [info on the Net] recommends to use at the same time a 2nd Isolation Transformer for your Oscilloscope as well ?

author: kripton2035
author_rank: Super Contributor
timestamp: August 11, 2016, 09:05:54 am
content: isn't it easier to use a differential probe for that ?

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 09:15:22 am
content: Depending on whether you them probes, number of available Scope channels, and how many points you have to measure.Still, why use a 2nd one remains interesting.

author: pascal_sweden
author_rank: Super Contributor
timestamp: August 11, 2016, 09:23:07 am
content: What I really don't get it that some people on this forum here buy a commercial isolation transformer from a well respected company, which is designed for medical industry, but that they then still have to do their own modifications to the isolation transformer to further "improve" it.So the people who make these modifications believe that the company who designs the isolation transformer is stupid then?Or in other words, why aren't there any commercial isolation transformers on the market which are good enough, and which require no further modifications?The moment you do a modification to these devices, you are entering risk zone in my opinion, and most likely you don't even have a clue of what you are doing. You are just playing around, without realizing the risks you are building into the device, with your so called "improvement".

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 09:31:48 am
content: The issue of modifying [or not] your Isolation Transformer (whether Commercial/Industrial/Medical grade) - is not related to this thread.Note:[The only modification that is somehow related, is that some vendors tend to hook up the secondary winding to Ground (for better Noise attenuation issues). For testing & safety purposes with a Ground referenced Test Equipment, it is essential to disconnect that Ground wire from the secondary winding.]

author: Falkra
author_rank: Regular Contributor
timestamp: August 11, 2016, 09:33:37 am
content: I think this is the topic about the modification :https://www.eevblog.com/forum/testgear/teardown-and-modification-of-noratel-imed-e-150-isolation-transformer/

author: Ijaz
author_rank: Contributor
timestamp: August 11, 2016, 09:35:02 am
content: Quote from: roli_bark on August 11, 2016, 08:43:27 amIt would be interesting to probe it further. Do you have any link ?

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 09:45:09 am
content: Quote from: Ijaz on August 11, 2016, 09:35:02 amhttp://electronics.stackexchange.com/questions/73991/why-do-we-need-an-isolation-transformer-to-connect-an-oscilloscope

author: pascal_sweden
author_rank: Super Contributor
timestamp: August 11, 2016, 10:08:10 am
content: Quote from: roli_bark on August 11, 2016, 09:31:48 amIsn't the whole point of an isolation transformer to have a secondary winding which is not connected to ground? I honestly don't get it. Why is it then called isolation transformer in the first place, and why do you need primary and secondary winding? Then it is better to just get an extension cable directly connected to the mains outlet

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 10:33:11 am
content: Quote from: pascal_sweden on August 11, 2016, 10:08:10 amThe point is this:For some [especially Hi-Fi Audio] Isolation Transformers are intended to be used for attenuating the Electrical Mains Supply noise, and NOT for safety purposes. So for these types of applications the secondary is hooked up to Earth-Ground.So people who use them for safety purposes, should be aware each time they get hands on an Isolation Transformers, and verify their secondary winding is DISCONNECTED from Earth-Ground.

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: August 11, 2016, 10:42:34 am
content: Quote from: pascal_sweden on August 11, 2016, 10:08:10 am Isolating the equipment, not the safety of the end user - there's other sections of the equipment for that and home RCDs should still work with medical isolation transformers.

author: nctnico
author_rank: Super Contributor
timestamp: August 11, 2016, 10:44:04 am
content: Quote from: roli_bark on August 11, 2016, 08:43:27 amNever ever do that! Use a differential high voltage probe instead. An isolation transformer is dangerous in many ways when used to make floating measurements.

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 10:50:07 am
content: Quote from: nctnico on August 11, 2016, 10:44:04 amYou mean -Never ever - for feeding the Test Equipment thru Iso-Xformer [I guess for safety reasons], butYes - for using an Iso-Xformer for feeding power to the product under test ?

author: Andy Watson
author_rank: Super Contributor
timestamp: August 11, 2016, 10:54:02 am
content: Quote from: RGB255_0_0 on August 11, 2016, 10:42:34 amPaths to ground exist via many routes and not necessarily via the RCDs - for this reason RCDs detect an imbalance in current between live and neutral (the clues is in the name - Residual Current Detector). Putting a transformer in the way (medical or otherwise) will balance the currents and null the effectiveness of the RCD.

author: RGB255_0_0
author_rank: Frequent Contributor
timestamp: August 11, 2016, 10:56:38 am
content: Yeah. I guess depends how sensitive the RCD is and where you were physically using it (probably not wise to run a long cable to your neighbour's house).

author: nctnico
author_rank: Super Contributor
timestamp: August 11, 2016, 11:07:11 am
content: Quote from: roli_bark on August 11, 2016, 10:50:07 amNo for both! Clipping the DUT to ground makes it dangerous to touch in an unknown way while defeating the RCD. With a differential probe you don't have this problem.

author: madires
author_rank: Super Contributor
timestamp: August 11, 2016, 11:15:03 am
content: Quote from: roli_bark on August 11, 2016, 10:50:07 amYup, isolation transformer only for the DUT, not for the scope. If feasable, use the poor man's differential probe (ch1 + ch2 inverted, ground clips removed). For regularly use get a proper differential probe. Those are expensive, but much cheaper than a funeral.

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 11, 2016, 11:19:02 am
content: Wouldn't a battery-powered oscilloscope be the best option? Probably cheaper to do that modification than to buy differential probe. I bet it's cheaper even to get a pure sine inverter and a 12V battery!

author: nctnico
author_rank: Super Contributor
timestamp: August 11, 2016, 11:23:07 am
content: Quote from: TheAmmoniacal on August 11, 2016, 11:19:02 amBut then every metal part of that battery powered scope would become a potential deathtrapA differential probe is the only sane way to go and they aren't that expensive compared to an isolation transformer anway.

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 11, 2016, 11:24:38 am
content: Quote from: nctnico on August 11, 2016, 11:23:07 amHow do the handheld/portable scopes deal with that?

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 11:28:30 am
content: Quote from: roli_bark on August 11, 2016, 10:50:07 amQuote from: nctnico on August 11, 2016, 11:07:11 amQuote from: madires on August 11, 2016, 11:15:03 amSo, one says Nay and the other Yea - what gives ?The Nay answer makes me a little nervous...

author: madires
author_rank: Super Contributor
timestamp: August 11, 2016, 11:49:11 am
content: The isolation transformer renders the RCD useless and is more reliable than a RCD, but it won't help when you touch both output potentials at the same time.

author: tautech
author_rank: Super Contributor
timestamp: August 11, 2016, 11:53:24 am
content: In fifty plus years scopes have been in common use only 2 recognized safe methods of scope isolation have evolved: isolated channel scopes and differential probes.There are several methods that are often used in order to achieve the types of measurements that these safe methods provide, none of which are safe and shouldn't ever be discussed on a public forum for fear of the risk such methods wiil expose the less experienced user to.So don't do it OP, take experienced users advice and put your hand in you pocket and get the correct tools for the job.

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 11:58:34 am
content: Quote from: tautech on August 11, 2016, 11:53:24 amOK, let's not talk about the Do's"...But "Don't do it OP" - what ?Still it is OK to iso the DUT and NOT the Scope ?Also what still makes me nervous is these responses:Quote from: nctnico on August 11, 2016, 10:44:04 am

author: pascal_sweden
author_rank: Super Contributor
timestamp: August 11, 2016, 12:03:06 pm
content: Get a differential probe from Pintek. They are very good quality, and very affordable!This one for example: DP-30HShttp://www.pintek.com.tw/product_detail/landersound/index.php?Product_SN=97781&Company_SN=6002&Product_Site_Classify_SN=17072Note that Pintek has recently launched the DP-60HS which goes up to 60 MHz.They haven't added this model yet on their website.

author: Omicron
author_rank: Regular Contributor
timestamp: August 11, 2016, 12:12:36 pm
content: Quote from: roli_bark on August 11, 2016, 08:43:27 amSay a hypothetical technician has his TV under test connected via an isolation transformer. Our man (or girl) feels very safe now because he's been told he can't get electrocuted as long as he keeps one hand in his pocket. He now clips the ground lead of his bench scope to point A inside the TV. He then absent mindedly prods and pokes around with his finger until he touches point B which is at 230VAC with respect to point A. Unfortunately for our poor fellow some part of his body (maybe just his feet) is also (indirectly) making contact with mains earth. What happens? Because point A is also connected to earth via the scopes ground lead a closed circuit now exists: from B through the technician to earth and via the ground lead back to A. Let's hope the poor man survives. To make matters even worse in this case, the ground fault interrupter that might have saved our technician is completely defeated by the isolation transformer.The recommendation to use a second isolation transformer for the scope is a misguided attempt at preventing this scenario. The problem with it is that as soon as you connect anything else (say you plug your scope into your PC via USB) the dangerous situation re-apears!Moral of the story: isolation transformers give you a false sense of security and are therebyvery dangerous to use! It's far too easy to create a situation that is even more dangerous than no isolation transformer at all. The only correct way to measure on mains connected circuits is to use an isolated instrument with an appropriate CAT rating. Either a scope built for the job (e.g. fluke scope meter) or a high voltage differential probe.

author: TheAmmoniacal
author_rank: Unknown
timestamp: August 11, 2016, 12:13:58 pm
content: Quote from: pascal_sweden on August 11, 2016, 12:03:06 pmBut 65 V p-p max? That's of little use.

author: kripton2035
author_rank: Super Contributor
timestamp: August 11, 2016, 12:18:40 pm
content: yes the DP25 or even DP50 will be better for mains voltages

author: tautech
author_rank: Super Contributor
timestamp: August 11, 2016, 12:25:23 pm
content: Quote from: TheAmmoniacal on August 11, 2016, 12:13:58 pmActually that would do a lot of signal path work but not lots in power electronics.More important is the level of isolation from mains ground it provides, one would hope 1kV or more.

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 12:25:36 pm
content: Quote from: kripton2035 on August 11, 2016, 12:18:40 pmFor SMPS testing, you'll need to measure deferentially a high voltage of 325v min (Europe), and about 270v min (USA)

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 12:28:05 pm
content: Quote from: tautech on August 11, 2016, 12:25:23 pmThe Pintek picture says - 600 CAT (to Earth Ground) - see here

author: rsjsouza
author_rank: Super Contributor
timestamp: August 11, 2016, 12:33:45 pm
content: (while I was writing this 10 other replies were posted; hopefully I am not repeating what others already wrote)IME a differential probe is the method with no side effects to the oscilloscope and the DUT. Both equipments will have paths to ground and you can get shocked if you inadvertently touch the DUT in a single point. However, differential probes are somewhat expensive.In this case, nothing prevents you from using an isolation transformer on the DUT and your brains. Isolating the DUT will not have a path to ground when left alone or connected to ungrounded equipment - useful when probing its innards with portable DMMs, for example. This minimizes the risk of electrical shock as it requires touching two points in the DUT.The moment you probe the DUT with a grounded equipment you create a path to ground, which increases the risk of electrical shock as touching one point on the DUT is enough to cause the current to flow through DUT-->oscilloscope-->ground-->your body-->DUT. In this case the equipment itself is the one "anchoring" the potential to ground - thus touching its alligator clips or its chassis will not cause you any harm (provided you have a reliable ground connection, of course).Isolating the oscilloscope increases the chances of you getting a shock as the path to ground is through the DUT and any alligator clips or the chassis of the oscilloscope will be elevated to the DUT potential. Also, touching a single point in the DUT is enough to cause the current to flow through DUT-->ground-->oscilloscope-->your body-->DUT.Isolating both will help with your safety as there will be no path to ground at any point; this minimizes the risk of electrical shock as it requires touching two points in the DUT. Also, the DUT and the "grounded" test equipment will be at the same potential at all times, which will be a similar scenario as the portable DMM. However, one of the pitfalls of this method is falling prey to its advantages: it is easy to forget or assume the environment is safe and relax on procedure.I suspect I didn't forget any details, but I am sure others will point out any flaws...

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 12:35:38 pm
content: Quote from: TheAmmoniacal on August 11, 2016, 12:13:58 pmThat is if NO attenuation is selected.At 200 attenuation you can measure 1300v=======From datasheet:Attenuation : x 20, x 50, x 200 ( Into 1M? scope ) ; x 40, x 10 , x 400 ( Into 50? load )Maximum operation Voltage ( DC + peak AC ) :? ± 140V at x 20? ± 350V at x 50? ± 1300V at x 200Maximum input Differential Voltage : 1300V ( DC + peak AC )Maximum input Voltage to Ground : 600V ( DC + peak AC )

author: roli_bark
author_rank: Regular Contributor
timestamp: August 11, 2016, 12:38:30 pm
content: Same as 'Pintek' - in eBay for $230 (shipping included):http://www.ebay.com/itm/Professional-Differential-probes-DC-25Mhz-ADP25-Max-Voltage-1300V-3-ATTENUATOR-/380411931457?hash=item5892516b41:g:QHIAAOxye3BRrVln

author: helius
author_rank: Super Contributor
timestamp: August 11, 2016, 02:00:52 pm
content: If you're interested in a project you could develop a device for safer isolation. You would need a contactor, and a sensitive bridge (maybe based on magnetics). As soon as a couple mA begins to flow through the safety ground of the scope, it opens the contactor to cut power to the DUT. That would protect the scope from being burned and also provide some protection to the user.

author: madires
author_rank: Super Contributor
timestamp: August 11, 2016, 02:05:26 pm
content: Quote from: Omicron on August 11, 2016, 12:12:36 pmIMHO, the best approach is to use differential probes and an isolation transformer for the DUT. The differential probes provide the isolation for the scope and the isolation transformer the same for the DUT. The important point is not to rely on a single safety device. A RCD is not as reliable as an isolation transformer and poses more risk when fiddling with a DUT. But it's great for your test and measurement devices. And even with all the safety, one should keep in mind that HV is dangerous and act accordingly.BTW, if you combine an isolation transformer with a variac, meters for current and voltage, and a breaker, you'll get a very useful tool.

author: Jeroen3
author_rank: Super Contributor
timestamp: August 11, 2016, 03:14:08 pm
content: Quote from: TheAmmoniacal on August 11, 2016, 11:24:38 amThey have no touchable metal parts, eg optical data output.Or they have isolated input channels.

author: nctnico
author_rank: Super Contributor
timestamp: August 11, 2016, 06:04:54 pm
content: Quote from: TheAmmoniacal on August 11, 2016, 11:24:38 amThebetterones have isolated inputs. IOW: when selecting a handheld scope be sure to specifically check if the specific model of interest has isolated inputs. Many of the handheld scopes do not have isolated inputs and manufacturers may print 'isolated inputs' on the brochure but if you read the fine print you'll notice that only specific models have isolated inputs.

author: Fungus
author_rank: Super Contributor
timestamp: August 11, 2016, 07:29:55 pm
content: Quote from: roli_bark on August 11, 2016, 08:43:27 amMy response is: Don't believe everything you see on the internet.Floating a 'scope can be very dangerous, eg. If you connect the ground clip of a probe to something live then the other BNC connectors on it become live - shock hazard!If the device under test is also isolated then you're OK, yes, but what if you forget your scope is isolated and go to test something else that isn't isolated? What if somebody else uses the 'scope and doesn't know it's isolated?

author: Assafl
author_rank: Frequent Contributor
timestamp: August 11, 2016, 07:38:22 pm
content: Theoretically, using isolation transformers is a good idea. However, I think the reasons the isolation transformers are out-of-safety-fashion for measurements is that when incorrectly set up can easily become severely more dangerous than without them.As an example - take the two transformer scenario. An isolation on both the oscilloscope and DUT. Depending on the direction of the windings I can have it so that both hits are at 220v, equipotential. But if I flip one of the transformers, I can have 220v feed to DUT and -220v to the oscilloscope hot.I really bad cases one can get ground to float at 220v, touch anything and both the DUT, the scope and perhaps the operator, are toast.It is superbly easy to get wrong. So very discouraged. Especially that isolated scopes and HV differential probes exist.With my Scopemeter I can carelessly prod mains equipment (not that I do). If I were to use a transformer I wouldn't be safe touching the probe...BTW - Systems that float (or are grounded) at a high voltage can - and are - brought down to earth with an isolation transformer. But it is done as part of system design so the technician cannot get it wired incorrectly.

author: timb
author_rank: Super Contributor
timestamp: August 11, 2016, 09:04:25 pm
content: Quote from: helius on August 11, 2016, 02:00:52 pmIt exists! Check out the Tektronix A6901.

author: roli_bark
author_rank: Regular Contributor
timestamp: August 12, 2016, 05:56:44 am
content: Quote from: Fungus on August 11, 2016, 07:29:55 pmYou're absolutely right.If I would believe it[whatever is written in Internet], I wouldn't post this thread ...

author: Someone
author_rank: Super Contributor
timestamp: August 12, 2016, 08:24:44 am
content: Quote from: TheAmmoniacal on August 11, 2016, 11:24:38 amThe very nice Fluke/Phillips handhelds have special BNC inputs that are shrouded with the appropriate clearance/creepage distances, and the matching probes that fit are fully shrouded with no exposed metal. Full isolation for the user as per IEC/EN/UL/BS/AS 61010

author: tatus1969
author_rank: Super Contributor
timestamp: August 12, 2016, 09:06:29 am
content: I would not recommend using isolation transformers for this purpose. They have large coupling capacitances to mains primary. If you then connect your probe's ground clip to a signal in your circuit which you want to measure against, then this signal will have to drive that capacitance.The result can be anything between bad measurement and broken hardware. Use a differential probe, or two probes and math function to subtract them.