Whats up with my transofmer's AC output waveform? - Page 1
Forum section: electronics / beginners

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: June 26, 2017, 01:39:52 pm
content: HelloI am doing a little project and am using a little mains to ~6.3V transformer to power it. In the past when I have used it I of course got a nice neat sine wave from the output. However at the moment its looking rather odd.For illustration, look at the yellow trace in the attached picture. Thats right across the terminals of the secondary coil. When I put a pure sine wave in from my function generator, its an exact sine wave.What could be causing this?CheersSpanners

author: bktemp
author_rank: Super Contributor
timestamp: June 26, 2017, 01:42:29 pm
content: The waveform is clipped because all the rectifiers + capacitors are drawing current only during the peak of sinewave.

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: June 26, 2017, 01:50:50 pm
content: Quote from: bktemp on June 26, 2017, 01:42:29 pmHiTheres nothing else in the circuit at all, just a 266JB6 transformerhttp://eu.mouser.com/ProductDetail/Hammond-Manufacturing/266JB6/?qs=HMhDvBYWqvPX%2FCwaL6nHJA%3D%3Dhttp://www.mouser.com/ds/2/177/5c0016-17-71099.pdfWhen I have used it before its always given a perfect sine wave

author: MosherIV
author_rank: Super Contributor
timestamp: June 26, 2017, 01:54:31 pm
content: HiI do not know for definite but my guess would be the iron core saturates and therefore give you this distorted looking wavform.I get this when I build linear PSUs. It does not worry me.

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: June 26, 2017, 02:13:17 pm
content: Quote from: MosherIV on June 26, 2017, 01:54:31 pmHm interesting, just read a tiny a bit about core saturation, which naturally has given rise to a daft question:The (insulated) cable carrying the mains voltage was at first running over my magnetic screw holder, which has a pair of quite fierce magnets in it. Is it possible that I have magnetised the core? If not, what else could be leading to it? Its pretty damn hot in the conservatory where I am running it, any chance of that being a cause?I've not put the mains side through the scope, I'm not even sure if my DS1054Z would take it

author: Benta
author_rank: Super Contributor
timestamp: June 26, 2017, 02:22:34 pm
content: You probably have other equipment running with large storage caps. This distorts the mains voltage. Try turning off other equipment, then you'll find the rascal.

author: MosherIV
author_rank: Super Contributor
timestamp: June 26, 2017, 02:29:31 pm
content: I am no expert when it comes to transformersQuoteI am pretty sure magnets near cables do not affect the electricity running through them (unless you are moving the cable past the magnet).QuoteI hope so, that is how they work!The AC current flowing through the primary magnetizes and demagnetizes the core. When magnetizing, there is a finite amount you can push the iron to - hence it saturates.I have been told that modern small transformers tend to be under rated power wise and will magnetically saturate long before the maximum stated power is achieved - ie pick transformers with bigger power rating than you needDo not know much more than that, it was just my guess as to why your waveform looks funny.

author: Ian.M
author_rank: Super Contributor
timestamp: June 26, 2017, 02:35:09 pm
content: Its probably flat-topping of the mains supply to the transformer caused by PSUs that don't have power factor correction. The diodes in a bridge rectifier directly feeding a large reservoir capacitor only draw current close to the voltage waveform peak. The problem may be local e.g. due to a cheap or old PC PSU or a high current linear PSU on the same extension lead or it may be elsewhere in your house or even be the culminative effect of all loads of this type in the neighbourhood.To check the mains waveform before the transformer reasonably safely, without an expensive Cat II or better rated differential probe, connect a string of 10 identical 100K resistors in series between Line and Neutral from a fused plug-in mains lead. *EVERY* time you use it, check the voltage to ground using a CAT II or better rated meter at the point between the two resistors nearest to the Neutral connection. Assuming its within your scope's ratings you can now probe that point with a x10 probe. DO NOT connect the probe ground clip and DO NOT directly probe the Neutral line (as there may be a dangerous voltage on Neutral if there is a fault). Make all test connections *BEFORE* plugging in the mains lead to energise the resistor chain and unplug it before disconnecting anything else. Do not leave the probe connected when not making measurements. This procedure relies on your scope being properly grounded to the mains supply ground.

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: June 26, 2017, 02:35:44 pm
content: Quote from: MosherIV on June 26, 2017, 02:29:31 pmWell I think you nailed it, a quick additional search threw up all sorts of info round it and one or two waveforms that look extremely similar. Not a scoob why it might have happened, but its apparently an issue with cheap ones. Its not powering anything though, thats literally right across the open terminals.Just checked the household AC and its 243VAC (UK) which is no higher than I have used in the past with it, so I am just putting it down to being a cheapy

author: bktemp
author_rank: Super Contributor
timestamp: June 26, 2017, 02:45:02 pm
content: Basically every mains powered device uses a SMPS today. For larger loads a PFC is mandatory, but all small ones don't have anything exept a little bit of emc filtering, doing not much at 50Hz.The THD of the mains waveform here is around 3%.Here is an example of how the mains voltage and current waveform from a 4W LED lamp looks like. It uses a rectifier + SMPS current source instead of the cheaper capactive dropper type. This gives a flicker free light output, but has a poor power factor. You can clearly see the clipping of the sinewave starting at the same time as the current spike happens.

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: June 26, 2017, 03:04:54 pm
content: Quote from: bktemp on June 26, 2017, 02:45:02 pmHiThanks for all the infoI just went round and turned everything else off in the house, no difference at all, and of course theres nothing else in the circuit with the transformer, so I am not sure how any of those things could affect it. Maybe someone next door has a bunch of electronic gear running, I don't know. I am more tending to believe the saturation theory TBH especially given this thread:http://4hv.org/e107_plugins/forum/forum_viewtopic.php?156237As long as it not a huge deal I am not too worried.

author: bktemp
author_rank: Super Contributor
timestamp: June 26, 2017, 03:22:50 pm
content: Even without any large SMPS running in your house the waveform will be clipped, because you can't turn of all other SMPS elsewhere unless you have your own power generator.I don't say the transformer does not affect the waveform, but your waveform does not look like that.Here is good page explaining transformer saturation:https://www.ibiblio.org/kuphaldt/electricCircuits/AC/AC_9.htmlIf you look at the shown waveform, saturation happens slightly before zero crossing not at the peak of the waveform, because at that time the transformer has seen one polarity for the longest time and therefore the core is at its peak magnetic flux.

author: SpannersToSparks
author_rank: Regular Contributor
timestamp: June 26, 2017, 04:11:37 pm
content: Quote from: bktemp on June 26, 2017, 03:22:50 pmThanks! I went and had a bit more of a dig and found thishttp://archive.siliconchip.com.au/cms/A_112809/article.htmland it mentions SMPS too. Whats interesting is I recently moved from Scotland to England, and when I used the transormer in scotland it was indeed a perfect sinewave. Perhaps its the case that the local electrical situation where I live now is radically different?

author: Kleinstein
author_rank: Super Contributor
timestamp: June 26, 2017, 07:37:50 pm
content: The deformed voltage can happen if other houses at the same mains transformer use a lot of power from poor power supplies. So it does not have to be you house, it could be something like the street. In many areas it was even worse before - it got better the last 15 years with more modern PCs that have PFC.Usually I won't worry much about this - there is not much one can do about it anyway.