Mains solder smoking? - Page 1
Forum section: electronics / projects

author: BoomBrush
author_rank: Contributor
timestamp: December 15, 2015, 04:59:21 am
content: Hi, I have a very unusual situation which I don't know the problem to except for a few ideas. I live in Australia with 240 volts.I'm trying to power the primary of a microwave oven transformer, so I soldered some wires onto the mains cables and that all worked fine. After about 10 seconds of use, my solder joints started to smoke. Is there something I am doing wrong or is it normal...? I suspect either its just the flux burning away, or maybe the solder is beginning to melt? Something else really bad? I haven't turned it back on since. Afterwards, the primary coil was quite warm but not concerning hot so there was significant current flow. Perhaps too much current caused the smoking to occur?I will give some further context. I am actually trying to make a high current 12v power supply, and so removed all the coils keeping the original primary. I wound my own secondary with about ~18 turns and was measuring the voltage to see what I was getting. When I turned it on, I was getting about 9v so I was almost there! It was making the 50hz humming sound which makes sense, but then it began to smoke so I disconnected immediately.I would appreciate some feedback into the solder problem - was it the flux in the solder, or it melting from too much current flow, or normal? I don't know.Solder I was using: 50G 63/37 Rosin Core Flux 1.8% Tin Lead Melt Roll Soldering Solder Parts Wire (from ebay)

author: calzap
author_rank: Frequent Contributor
timestamp: December 15, 2015, 05:29:28 am
content: The flux shouldn't smoke unless it's in contact with something far too hot. If the coil and cord are not hot, then something's wrong at the solder joint itself. Could be too little solder, cold joint, conductor stands missing at the joint, gap within the joint, etc. Consider using a terminal block. Have someone who is experienced with mains wiring look at the setup before you start a fire or get a shock.Mike in California

author: Ian.M
author_rank: Super Contributor
timestamp: December 15, 2015, 06:19:38 am
content: Sounds like its FUBARed. It shouldn't make much noise on no load, and the only way a conventional transformer would get hot enough to smoke with no load, would be over-voltage or a shorted turn. Are you sure it wasn't an inverter microwave as even a 340V chopper transformer would certainly smoke if fed 240V 50Hz due to the frequency being about 3 orders of magnitude lower than its designed for.

author: BoomBrush
author_rank: Contributor
timestamp: December 15, 2015, 06:54:44 am
content: Thanks for the responses.Quote from: calzap on December 15, 2015, 05:29:28 amI am not sure if the wire was hot from having a large amount of current or not. I want to avoid measuring current because I don't have a clamp current meter. It could be any of those things, my grandfather was a radio technician during the war (he is actually as qualified as an electrical engineer) and I wanted to ask him but his wife is in the hospital due to low blood pressure. I didn't want to disturb him, so I came on this forum for help. I also have a fire extinguisher within about 10 meters from me plus a fire blanket.Quote from: Ian.M on December 15, 2015, 06:19:38 amIt's probably making noise because the layering in the metal of the transformer is coming apart when I was grinding the top off (seen below in the picture, I circled it). The primary coil wasn't smoking, it was the solder joint between the mains and the wire going to the transformer. A shorted turn is a possibility, but I don't think that is the case here. There are places where the insulation of the wire is slightly scraped off, but it isn't touching any other part of any other wire. I got no idea about the difference of an inverter microwave and a normal microwave so it could very well be the transformer doesn't like the 50hz.I also didn't mention that I am using a PCB to get the mains voltage from. This is because it includes an isolation transformer (and other stuff) so I do not believe it would make a difference. Or maybe I am wrong?Some pictures:<-- These solder joins are the ones that began to smoke, the ones attached to the red and white wires.

author: mij59
author_rank: Frequent Contributor
timestamp: December 15, 2015, 07:46:35 am
content: There is blue tape on the top of the E-core, this will create an air gap between the I-core and the E-core.An air gap will reduce the inductance of primary winding leading to higher primary current.The soldering of the red and white wire in the fist picture looks different from the second picture, did you resolder the wires ?What's on the other side of the pcb ?Maybe the source of the smoke is a common mode choke.

author: BoomBrush
author_rank: Contributor
timestamp: December 15, 2015, 08:04:38 am
content: Quote from: mij59 on December 15, 2015, 07:46:35 amI don't plan on using the I section of the transformer, as it doesn't fit in the container I wish to put it in. I will lose a large amount in inductance due to that, but I am okay with that. Do you think perhaps since the I section was not put on, the unit is drawing more current than normal? I hadn't considered that.The soldering is the same, lighting I guess caused difference appearances. My soldering isn't the best either, perhaps it was cold joints or something.Some yellow thing (cap?), the isolation transformer and some things I think start with M. Mots or moths or something like that. I've heard Dave say them before.I wasn't too worried about these effecting the unit so I didn't worry about removing it, plus it provides a good earth. Do you think it may be the cause?One conclusion I suspect is because the original PCB was for a computer power supply, designed to output a set amount of current max. Maybe my new configuration is drawing more than it was intended to handle. It came from a 400 or 500 watt power supply so I was sure the pcb would be able to handle it fine. Maybe not...

author: vk6zgo
author_rank: Super Contributor
timestamp: December 15, 2015, 08:07:18 am
content: Pleasetell me you put the other laminations back!It won't work,if it is still in the same condition as in the pictures.Apart from that,your "jumble wound" secondary doesn't instill much confidence.Another thing is,that microwave oven transformers include what are called,if I remember correctly,"magnetic shunts",which have to be removed if you are using it for continuous service.http://www.users.on.net/~endsodds/psrewind.htm

author: calzap
author_rank: Frequent Contributor
timestamp: December 15, 2015, 08:14:49 am
content: Using green/yellow wire is not a good choice for a transformer winding; that color combo is reserved for earth/ground.Blue objects are likely MOVs, and yellow object is probably a cap.I'm starting to think this is a prank, but if it isn't, please have someone with experience look it over in person before applying power again.Mike in California

author: Ian.M
author_rank: Super Contributor
timestamp: December 15, 2015, 08:25:51 am
content: Its FUBARed. You broke the magnetic circuit so it couldn't cope with 240VAC any more, saturated and drew a grossly excessive current. *IF* you had replaced the I section and clamped it tightly in place before connecting it, it would have been OK, but now the primary has been overheated to the smoking point it can never be trusted again. Chuck it out with the rubbish (or strip and chuck the primary and keep the core for winding a low voltage choke).If at all possible DO NOT break the welds when rewinding an ex-microwave transformer - see vk6zgo's link. Its too much of a PITA to get the core back together properly. Many budget microwave transformers have been cost-cut to the limit, designed for 220V and are unsuitable for continuous 240V operation anyway.

author: BoomBrush
author_rank: Contributor
timestamp: December 15, 2015, 08:45:03 am
content: Quote from: calzap on December 15, 2015, 08:14:49 amIt wasn't meant to be permanent, it was the highest gauge wire I had at the time and I just wanted to see if it worked. I am very aware of the confusion that may be caused by using earth wire for non earth purposes, but I was hoping to replace that with some red or blue wire some day.I have no intention of applying power after smoke came out, generally a bad sign >_>. It can still be used as a learning tool however. I ideally want to ask my grandfather, but he is occupied with his wife at the hospital. Maybe tomorrow I will ask him about it.Quote from: Ian.M on December 15, 2015, 08:25:51 amI saw that upon reading the page, I had not even considered removing the coil without splitting apart the transformer. It looks much nicer too, shame it wouldn't fit in the housing I was planning to put it in though. I guess it doesn't matter anyway since it has already been cut open.I don't think the primary is ruined however, because before I even applied any power, the resistance was 2 ohms. Afterwards, the resistance is still 2 ohms. I think if there was a fault, the resistance would be different (or...?). Perhaps the inductance would have changed though, I never got to measure that (I don't have a meter for inductance).

author: Ian.M
author_rank: Super Contributor
timestamp: December 15, 2015, 08:55:08 am
content: The winding wire is (probably) copper which melts at >1000 deg C so the wire itself survived without its resistance changing noticably. However its insulation is plastic varnish and that's what you FUBARed. It will now have weak spots where it partially melted which sooner or later will break down resulting in further overheating and shorted turns.

author: Psi
author_rank: Super Contributor
timestamp: December 15, 2015, 09:01:51 am
content: As others have said, you are missing part of the transformer. There should be a metal top section.Without this it behaves less like a transformer and more like a dead short across the mainsDo a google image search for microwave oven transformer and compare what its suppose to look like to your one.

author: matseng
author_rank: Frequent Contributor
timestamp: December 15, 2015, 09:06:56 am
content: Quote from: calzap on December 15, 2015, 08:14:49 amFunnily enough the linked "safely use a MOT" -site in the post prior to yours was using a green/yellow wire as well :-)http://www.users.on.net/~endsodds/psrewind.htm

author: Ian.M
author_rank: Super Contributor
timestamp: December 15, 2015, 09:19:35 am
content: The electrons don't care what colour the wire is. Its on the low voltage side, and you should try finding anything other than earthing wirre that's heavy enough in a big box DIY store. Tape or sleeve the ends for identification, and call it good as a pro electrician will have fits anyway if they see a home-wound transformer!

author: BoomBrush
author_rank: Contributor
timestamp: December 15, 2015, 09:27:46 am
content: Hmm, these ideas have given me a lot to think about. Perhaps with my grandfather, we could rewind another transformer without breaking the top off with a brand new primary and see if that gives better results.Quote from: Ian.M on December 15, 2015, 08:55:08 amThat makes sense, thanks for inform me, which seems supported by the heat generated by the primary.Quote from: Psi on December 15, 2015, 09:01:51 amTotally agree, I have the top piece, I don't know why I didnt add it for testing outside of the desired case. Probably because I was hoping to use it without the top, I guess that isn't gonna happen.Quote from: matseng on December 15, 2015, 09:06:56 amHaha that makes me smile... Maybe the only wire they had lying around was the earth wire too.Quote from: Ian.M on December 15, 2015, 09:19:35 amMade me giggle! Yeah, if I decide to continue this project with my grandfather I hope we will use some wire other than earthing wire unless I reuse the secondary. In that event, I will definitely put some tape or something on the ends to identify that those points are not earth.

author: Ian.M
author_rank: Super Contributor
timestamp: December 15, 2015, 10:15:43 am
content: Just be really careful not to scratch the primary insulation next time. I'd even go as far as protecting it with masking tape while chiselling out the unwanted secondarys..

author: DanielS
author_rank: Frequent Contributor
timestamp: December 15, 2015, 08:16:44 pm
content: Quote from: BoomBrush on December 15, 2015, 09:27:46 amYou didn't add it because you thought it was not necessary.A simple rule to keep in mind next time you want to remove something from a device: it costs manufacturers something to put the parts there and manufacturers are usually not in the business of adding unnecessary costs to their designs. If something is there, you need to know why before you can remove or replace it because there will be a reason, however obscure it might be.In the case of an EI-core transformer, the I-core closes the magnetic circuit across the E-core, which is "kind of" important... the critical kind of 'kind of" since an open core loses most of its inductance. Even a microscopic gap considerably reduces inductance, which is why others mentioned that transformers are difficult to get working right after breaking the core open.

author: Zero999
author_rank: Super Contributor
timestamp: December 15, 2015, 08:58:47 pm
content: I agree with the other people here: the transformer will not work with a bit part of the core removed and will overheat very quickly if connected to the mains.The PCB in the picture does not contain an isolating transformer. The transformer-like object is a common mode choke and the other components are capacitors. It's an RF filter.It's possible the insulation on the primary may not be that badly damaged. If it was, then it would've smelled much worse than solder flux.When correctly assembled, the transformer core should be connected to earth, as there will not be sufficient insulation to provide adequate protection against electric shock.

author: Seekonk
author_rank: Super Contributor
timestamp: December 15, 2015, 10:51:33 pm
content: I'm still impressed he got the open that cleanly. I'll have to pick up the next microwave I see curbside.

author: Monittosan
author_rank: Regular Contributor
timestamp: December 16, 2015, 07:03:57 am
content: where a you located ? Iv got a few 200va toroidal transformers with 2x 12 volt primary's some caps and bridge rectifiers for next to nothing.Im in Melb

author: BoomBrush
author_rank: Contributor
timestamp: December 18, 2015, 02:53:12 am
content: Thanks for the advise, I can't reply to you all but I have read all the responses and I will take them into consideration.Quote from: Monittosan on December 16, 2015, 07:03:57 amSoz, I'm in Brisbane. I'm very interested in the bridge rectifiers though, what are the current capabilities? Maybe you could post them if it wasn't too much to ask.

author: Delta
author_rank: Super Contributor
timestamp: December 18, 2015, 04:21:16 am
content: Mate, please take your hobby in another, safer direction. Play with some opamps or MCUs and LEDs. You are going to be joining your Gran in hospital if you continue playing with shit like this about which you clearly have no clue.I really don't mean to be harsh, but planning to use half a core, or mistaking a choke for an isolation transformer? You are really not ready for messing with the mains!Stay safe dude!

author: BoomBrush
author_rank: Contributor
timestamp: December 19, 2015, 02:44:48 am
content: Quote from: Delta on December 18, 2015, 04:21:16 amI don't want to be ready for `messing with the mains`. Messing would involve carelessly doing things without a care in the world for safety.Rather than `messing` with the mains, I would much prefer to use it in a respectful safe manor, of which I have been doing. Some pages on mains safety I have read and intend to follow arehere,hereandhere. Plus:Quote from: BoomBrush on December 15, 2015, 08:45:03 amQuote from: Delta on December 18, 2015, 04:21:16 amDon't worry I intend to.

author: vk6zgo
author_rank: Super Contributor
timestamp: December 19, 2015, 04:05:15 am
content: Quote from: BoomBrush on December 19, 2015, 02:44:48 amGeneral rule:- If a manufacturer makes a device which has a particular construction,they probably did it because it is necessary to enable the device to function.The links you gave are all necessary stuff but they are more designed to prevent personal injury from inadvertent contact with dangerous voltages,& various other workshop hazards.Nobody even dreamed of addressing the situation you created.The people who write Safety Procedures are not mindreaders,& cannot anticipate everything.Someone who does not know any Electrical Theory,can learn & follow Safety Procedures assiduously,but can get caught out by trying something which"everybody knows won't work".A knowledge of Theory,including how transformers work,would have prevented you from trying to run a transformer with its magnetic circuit incomplete.To a large extent,you are a victim of your time---back in the day,transformers were common items,the only way you could,as a hobbyist, safely power Electronics off the Mains was to use one.Thus,beginners became familiar with such devices,including their construction early in their careers.After all,if you had seen dozens of transformers,& never seen one with the "I" laminations missing,you would assume it was necessary.By the way,I do not subscribe to the theory that "the way to learn is by making mistakes".On that basis,we would have never got much past the wheel in technology.Other people made the mistakes,so you don't have to!

author: seancsnm
author_rank: Regular Contributor
timestamp: December 20, 2015, 02:17:51 am
content: I did a similar MOT re-wind a while back. I cut the welds at the top of the transformer so I could get a new winding on more easily, and was pretty careful not to let the laminations separate too much. Correct me if I'm wrong, but I believe the laminations are supposed to be separated from each other by coats of insulation so that the magnetic flux doesn't move laterally through the core. Something about reducing eddy current losses. I wound it to about 6 V using 8 Ga wire and welded the top of the transformer back on. There was a fair amount of no-load vibration in the transformer and I don't know if this was because I shorted some of the laminated sheets together near the cut/weld points or if the transformer was inherently like that. I'd show an image if I had one but If I remember right it was a pretty good, tight weld (at least for an amateur).Anyways, it's capable of dumping a lot of current - probably 50-100 amps by the looks of things I made glow.

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: December 20, 2015, 02:43:34 am
content: And while we're here.What are you trying to run off the transformer? How much current are you aiming for?A 12volt AC transformer, once rectified, will be closer to 16volts.However, a transformer that does 12volts ac with no load, will drop when a load is applied...

author: Ian.M
author_rank: Super Contributor
timestamp: December 20, 2015, 02:45:33 am
content: Transformer laminations *may* be insulated with lacquer, but are often insulated with a black or blue oxide layer. That's to avoid circulating currents round the perimeter of any core cross section normal to the flux acting as a shoted turn.Welded transformers work because the flux at the outer surface of the core near an outer corner is a lot smaller - it concentrates towards the inner corner (unless there is a badly fitted gap). Its probable that your weld went deeper than the original, (unless of course you used an ultra-thin slitting disk to cut the old weld). That would increase the loop area of the partial shorted turn that each weld forms in the outer arm of the core, increasing the losses. Its also possible that it was a noisy b----r to begin with. I would try letting cheap runny superglue wick into the joint between the E and the I, as it may just be due to the I flexing under the forces involved.

author: seancsnm
author_rank: Regular Contributor
timestamp: December 22, 2015, 07:35:53 pm
content: Quote from: Ian.M on December 20, 2015, 02:45:33 amThanks for the clarification. Also, you might be right in that it was noisy to begin with. I'll fire it up again in a couple days if I get a chance.