Viewing mains waveform - simple isolated adapter? - Page 1
Forum section: electronics / projects

author: David_AVD
author_rank: Super Contributor
timestamp: December 10, 2014, 06:38:12 am
content: I'd like to view the mains power waveform on my 'scope to look for transients while switching a large transformer on and off.So I hooked up a small mains transformer with a voltage divider on the secondary and fed it into my 'scope.The transformer seems to be distorting the waveform at the peaks. It looks the same on my analog or digital 'scope.Is there a way to get a cleaner result?

author: qwaarjet
author_rank: Regular Contributor
timestamp: December 10, 2014, 06:58:21 am
content: Is the output of the transformer earth ground via the probe or is it floating?

author: calexanian
author_rank: Super Contributor
timestamp: December 10, 2014, 07:14:33 am
content: You have core saturation going on. Add some series impedance with the primary of the transformer. Saw 470 ohm or so to get the saturation current down. A low impedance (larger resistor wattages required) would be better with less high pass effects from the transformer. Scale the output voltage divider accordingly. Something to keep in mind is that the low frequency transients will not be passed by the transformer as well. Their amplitude will be much lower due to the low frequency roll off of the transformer itself. To do this properly you would really want to use a truly isolated circuit or float the scope ground. Floating the scope is not really that safe. Its a genuine electrocution hazard. I might also consider using two opto isolators. It would not be linear but may be good enough for just catching errant transients. Otherwise a really good quality audio type transformer could be used.Just my two cents.

author: IanB
author_rank: Super Contributor
timestamp: December 10, 2014, 07:32:10 am
content: Been there, done that:https://www.eevblog.com/forum/chat/when-is-a-sine-wave-not-a-sine-wave/My conclusion: small mains transformers make horrible signal transducers.To get a better result without expensive or more elaborate instruments, you could perhaps use a pair of resistors to divide the voltage down and use two scope probes in poor man's differential mode while leaving the ground clips unconnected.

author: tautech
author_rank: Super Contributor
timestamp: December 10, 2014, 08:39:19 am
content: Quote from: David_AVD on December 10, 2014, 06:38:12 amAs this thread seems to be connected to your other:https://www.eevblog.com/forum/projects/breaker-tripping-when-load-is-switched-off/msg518442/#msg518442Your isolation using a transformer will likely substantially dampen the relay switching spike you are looking for. Inductors are good at that.But again looking at the scope image you have posted, you have the input attenuation set at 100x.Does that imply you also have a 100:1 probe?Could you not use that?Reference lead on neutral of course.

author: David_AVD
author_rank: Super Contributor
timestamp: December 10, 2014, 10:42:42 am
content: Thanks very much for your thoughts so far guys.The output of the transformer goes to a simple resistor divider (approx 5K series and 1K shunt) to give me 2.4V output with 240V input. This is why I have the 'scope set as 100x.Yes, one side of the transformer output (lower side of shunt resistor) is grounded via the 'scope probe.Is there an off-the-shelf solution that will give me an accurate output and as low input loading as possible?If money needs to be spent then that's fine. I've already bought a Fluke i200s 20A/200A current clamp and the DSO-1054Z.

author: tautech
author_rank: Super Contributor
timestamp: December 10, 2014, 10:56:14 am
content: As the snubber is managing the back EMF most of the time, would your test be with it disconnected?I would leave it there for a quick test for piece of mind and use a 100:1 probe.You can switch the relay manually can you not?BTW I was rumaging through my scrap box and came across a !0 uF 230 V mains suppression cap and thought of you.......It is part of the incoming mains suppression network on a F&P washing machine PCB if you can find one,Quite a big beast 40H 20W 50L

author: David_AVD
author_rank: Super Contributor
timestamp: December 10, 2014, 12:16:10 pm
content: I have some 450Vac 5uF caps on their way to me for a test.It would be nice to be able to see the difference in back EMF with various fixes applied.Maybe what I really want is a differential probe for the DSO ?

author: mikerj
author_rank: Super Contributor
timestamp: December 10, 2014, 12:17:00 pm
content: Quote from: David_AVD on December 10, 2014, 10:42:42 amYes, a differential scope probe with suitable voltage rating for mains work. They aren't cheap though.

author: David_AVD
author_rank: Super Contributor
timestamp: December 10, 2014, 12:28:06 pm
content: ThisGW unit from Emonaseems to have reasonable specs.There is a cheaper, lower spec unit but I think the extra money would be well spent to get the better voltage rating.

author: The Electrician
author_rank: Frequent Contributor
timestamp: December 10, 2014, 06:23:17 pm
content: Quote from: calexanian on December 10, 2014, 07:14:33 amCore saturation is not responsible for the flattening of the peaks of the grid waveform. This is a phenomenon seen all over the world these days. The multitude of capacitor input power supplies in all the various consumer electronic devices have all drawn currents at the peak of the waveform, flattening the peaks.Furthermore, if saturation does occur in a transformer, it doesn't occur at the peak of the applied grid "sine" wave.The flux in a magnetic core is proportional to the integral of the applied voltage. This means that the peak flux, and onset of saturation, occurs at the zero crossings of the applied voltage.Here's a scope capture of the grid voltage applied to the primary of a small transformer (yellow), the voltage at the secondary (green) and the primary current (red). The primary current can be seen to exhibit substantial peaks at the zero crossings of the applied voltage. That's when the core flux reaches its maximum, and is somewhat into saturation:We also see that the voltage at the secondary of the transformer (green) is a faithful version of the grid voltage applied to the primary (yellow). Of course, the frequency response of the transformer is limited, but the grid frequency and its first 10 or so harmonics are transmitted to the secondary with a fairly flat frequency response.

author: SeanB
author_rank: Super Contributor
timestamp: December 10, 2014, 06:31:35 pm
content: If you are worried about saturation simply buy 2 of the same small transformer, wire the primaries in series and the secondaries load equally with about 10% of the rated current, about 1k will be good. Then measure across the one load and you will have less distortion.Otherwise a 10n class Y capacitor and a series 1k resistor on live and neutral, with the scope terminating in 1M and you simply use the one with high voltage as the probe signal ( live normally but in the EU it can be swapped) and the other to the scope ground lead will give a good high frequency response and show all the harmonics. Makes a high pass filter so have the scope on a high input voltage range at first then switch down if 50V/div is too small.

author: The Electrician
author_rank: Frequent Contributor
timestamp: December 10, 2014, 06:33:14 pm
content: Quote from: David_AVD on December 10, 2014, 12:28:06 pmYokogawa also has a range of differential probes:http://tmi.yokogawa.com/products/oscilloscopes/voltage-probes/

author: Yansi
author_rank: Super Contributor
timestamp: December 10, 2014, 07:23:21 pm
content: Quote from: David_AVD on December 10, 2014, 06:38:12 amOther possibility is that you mesaure mains waveform in an area, where there is a crapload of switchmode supplies and the mains is heavy-loaded with capacitance after rectifiers in these supplies. But usually there is not that much insane distortion.If you want to probe the waveform through a xformer, without waveform being distorted, you MUST NOT use a mains transformer as a coupling device. You need a linear one, which has the lowest possible primary resistance and the smallest possible magnetising current (=biggest possible primary inductance). One way to achieve this is to use regular transformer, but with lowered primary voltage, to let the xformer work at minimum flux density and maximum linearity. You can use simple resistance divider on the primary side. But be aware, that the more the source impedance (which includes the winding series resistance also!) the more the distortion by the magnetising current. So you just cannot use a divider of kohm resistor in a primary side of a basic mains transformer. It simply will not work either.Interesting solution might be to use an audio grade galvanic isolation transformer. These are usually VERY linear with low flux density. The only two things you need to solve then is the source impedance on the primary side to be the lowest possible, at a maximum of around 100 ohms for proper operation. So the divider might consist of an 100ohm resistor down and 39K 2W resistor on top. That will give around 0,6Vrms output to the primary at a source impedance of little less than 100 ohms. The second thing is SAFETY. You must be absolutely sure, that the signal transformer has an insulation proper for mains operation.This is actually an interesting idea to try, I've never tried using that, but it sounds it might work. Maybe I'll try that, I am also a bit curious how this measurement through signal xformer actually could work.Most things about where the distortion in a transformer comes from is easily seen from a equivalent circuit of transformer. (magnetizing current passing through primary series resistance).Also you can use active parts on the mains side. You can go also without a transformer, just by using an IL300 or so analog voltage coupler.//Note: Small highly saturated transformers with large primary series resistance can distort the peaks a lot, but the signal looks a little different than yours, the core saturation isn't actually that sharp.EDIT: Or this just came to my mind.. only a fast sketch, not a complete circuit. The hardest part is the properly isolated signal transformer.

author: Yansi
author_rank: Super Contributor
timestamp: December 10, 2014, 07:40:33 pm
content: Quote from: calexanian on December 10, 2014, 07:14:33 amDumb idea. That will even increase distortion.

author: ivan747
author_rank: Super Contributor
timestamp: December 10, 2014, 07:57:37 pm
content: What about differential probes?You can build one yourself, it's not a particularily complex project:http://thetooth.name/dev/UK2010090601.pdfThe article is available at Elektor, costs less than a fancy cup of coffee:http://www.elektor-magazine.com/en/magazine-contents/previous-issues.html?tx_elektorarticle_listissues=19455&tx_elektorarticle_listissues=showI've bought the article myself, but didn't build it. Still, it was a good read.There are patterns on PDF of the copper of the board, but no gerbers. The PCB patterns are somewhere in the second link. (or you can ask me for the PDF if you can't find them).The thing I like the most of that article is that it's so well written and the device is so well characterized that it gives me absolutely no doubts it will work and makes me confident that it will meet its spec. The guy does sound like he knows what he's talking about.I believe the first link I posted is for the full article, I hadn't noticed (there is a PDF preview for free and the article is paid and generated with a footer indicating who bought it, seems like the guy in the first link edited that footer out).You should buy it, it's the moral way.