Salvaged transformer issues - Page 1
Forum section: electronics / projects

author: jaycee
author_rank: Regular Contributor
timestamp: October 13, 2017, 08:24:46 pm
content: So I've salvaged a 150VA (approx) transformer from an old stereo system. It's quite an odd one, as it is not an EI core.. rather more like two C's with an I in the middle ! It delaminated nicely so I stripped the old secondaries off it and wound some test winding with thick wire in order to figure out the turns per volt ratio. The plan is to use the transformer in a bench power supply.However when reassembled, and with a 40W incandescent light bulb in series with the primary as a safety precaution, it buzzes very loudly and the bulb lights. Im measuring only 170V on the primary which means the rest is being dropped across the bulb.. i guess this means excessive current is being drawn.Im wondering what the problem could be - the transformer did work correctly before I delaminated it. The new secondary is 22 turns of ~1.48mm dia enamelled wire (salvaged from another transformer) and as far as I can tell, there arent any shorts in the turns.Im wondering if perhaps the laminations are now shorting and so there's a huge eddy current.. i haven't revarnished them or anything since delaminating it. I did the delamination by knocking one lamination loose and then sliding a Stanley knife blade between the others to separate them. The transformer is clamped together with some plastic clamps for testing.edit: Anyone got any ideas what could be wrong? I dont really want to cause a terrible fire

author: Gyro
author_rank: Super Contributor
timestamp: October 13, 2017, 08:51:20 pm
content: That's an unusual core lamination makup, not at all like the standard E I type. My thinking is that, on reassembly, you've ended up with lots of air gaps between the ends of the laminations which has significantly reduced the primary inductance. This isn't a problem with standard E and I laminations because they are interleaved, minimising and distributing any gap.In production, the transformer was probably assembled underconsiderableclamping pressure to ensure that the ends of the laminations butted together closely.You might be able to get sufficiently close coupling between the laminations by varnishing them and clamping the whole thing tightly in a vice wile it sets but my feeling is that what you've got there now is a very good gapped inductor for DC use!The other possibility is that youdohave a shorted secondary turn but my bet is on the core assembly.

author: jaycee
author_rank: Regular Contributor
timestamp: October 13, 2017, 09:03:00 pm
content: So the gap between the C's and the middle I piece is going to be a problem ? You're right in that it is difficult to get them lined up nicely and as tight as they were when it was first assembled. EI's never go back together perfectly either but as you say, because they are interleaved I guess it is much less of an issue.It'll be a shame if I have to scrap the transformer completely!

author: Gyro
author_rank: Super Contributor
timestamp: October 13, 2017, 09:11:16 pm
content: Yes, I think air gaps of that size are going to significantly reduce inductance - you could try different orientations of the C sections to try to get back to the original (presumably close) arrangement but that's going to be a problem if you've split a lot of the laminations. They definitely need to be clamped together under pressure though.I doubt if you've created enough shorts between them to cause significant increase in eddy currents - I could be wrong of course.P.S. I'm still trying to get my head around why they would construct a transformer like that - it might be marginally easier to slot the laminations into the bobbin but the added difficulty of getting decent joints would more than offset it. The aim of core construction is to minimise and distribute any gap as much as possible, initially E-I and ultimately toroidal. The other thing you try to minimise the amount of flux going 'across the grain' of the steel but there's no less of that in this arrangement either.

author: jaycee
author_rank: Regular Contributor
timestamp: October 13, 2017, 09:19:12 pm
content: I'll try some variations on how I'm clamping it up and see if things get better.The problem with most of the EI's ive salvaged is that they tend to be welded together. I can of course cut them apart or grind the welding out, but then the problem is putting them back together ! Here's an earlier attempt at doing that... looks incredibly messy though. Not energised this one yet to know if it's even working right.I'm now wondering if this is even worth the effort.. how salvageable are such transformers ? I appear to be having no luck so far!

author: Gyro
author_rank: Super Contributor
timestamp: October 13, 2017, 09:46:10 pm
content: It's certainly worth giving that one a try (lamp again). I'd be tempted to put the laminations back the same way as they were when you ground out the welds though. The laminations were already shorted by welding at those points, if you reverse half of them you are in danger of creating twice as many shorts between them with the due to the sharp edges. That would definitely increase your eddy current losses.Unfortunately decent bolted or clamped together transformers are getting harder to find. If your secondary isn't many turns then you might be better leaving the core intact and unwinding / removing the secondary (without damaging the primary insulation) and adding your winding through the slots instead. Use something to prevent the laminations from scraping the wire insulation. I've done that with toroidals - no choice there anyway, you end up with quite a length of wire trailing across the room!

author: jaycee
author_rank: Regular Contributor
timestamp: October 13, 2017, 09:53:31 pm
content: Well the second one.. the bulb lit again though i measured 190V across the transformer this time. It also didnt buzz whatsoever. I think I managed to get all the laminations in tightly, which is probably why.Is it safe to run a transformer with no secondary winding at all ? It'd certainly make initial testing easier before i bother trying to wind new secondaries!edit: I think this second transformer had VERY thin anodizing on its laminations, which is now compromised. I can make my meter's continuity tester beep in places with the probes across it. So maybe some varnish is what this one needs!

author: Gyro
author_rank: Super Contributor
timestamp: October 14, 2017, 09:48:47 am
content: Hi jaycee, yes you're absolutely fine running without any secondary winding. It will certainly save you some time and rule out the possibility of it being a shorted secondary turn at the same time.Yes I think the laminations are insulated by some sort of oxidising or passivation process (not rust). The eddy current voltage between adjacent laminations is extremely low (far less than one winding turn, or your meter's continuity test) even though the circulating current can be quite high. That means that the insulation can actually be quite low resistance and still be fine. The odd shorted one won't make a significant difference. Varnish will certainly help. It's probably best to remove any obvious burrs that you've created too - if they are chamfered below the surface then they can't make contact with adjacent laminations.P.S. You will never get to full mains voltage across the primary with the lamp in series (though it sounds as if you're still a bit far off yet). At some point you end up switching to a low rated fuse and measuring the core temperature rise over a reasonable period of time. Core losses will vary with the quality of the transformer. For instance, Microwave oven transformers tend to be very lossy.P.P.S. Just to be sure on safety... Make sure you have the earth wire connected to the core clamp and properly insulate you primary side connections. Also make sure the mains outlet you're using is RCD protected.

author: jaycee
author_rank: Regular Contributor
timestamp: October 14, 2017, 06:50:34 pm
content: Thanks for all the good advice so far !These are not huge transformers, 120-150VA at the most. While I expect some drop across the bulb I also know it shouldnt be drawing enough current on the primary unloaded to do anything more than light the bulb a tiny bit. I've just tested another 120VA transformer (welded, havent taken it apart yet) and the filament doesnt even glow when its on. It's the fairly bright glowing that is making me think something is wrong.

author: Gyro
author_rank: Super Contributor
timestamp: October 14, 2017, 07:24:45 pm
content: You're welcome.Yes, you're right, it was difficult to judge the transformer sizes from the photos, the mgnetising current of a 150VA transformer definitely shouldn't cause a 40W bulb to light. A microwave oven transformer primary, on the other hand, might well light it.It's good that you have confirmed this on a still-assembed transformer, all you've got to do now is get a secondary winding onto it!Oh, I had a thought earlier - you might want to try some ptfe plumbing thread sealing tape. It can be stretched really thin (it is thin anyway). You might be able to use it to isolate laminations that you know are going to short. Your main challenge is still to restore the original magnetic coupling though.P.S. It's a shame RS no longer sell their 100VA transformer kits. They had a pre-wound primary on a split bobbin and fresh laminations and clamp. Very handy. Worth keeping an eye on ebay.

author: jaycee
author_rank: Regular Contributor
timestamp: October 14, 2017, 10:46:44 pm
content: Guess I'm making a pigs ear of this. This transformer was fine, I cut it down the welds and clamped it back together with some G cramps (with plastic ends) and nope - drawing excessive current now. I guess those scuff marks on the ends of the I's are to blame.Those old transformer kits, I remember seeing them in Maplin about 20-25 years ago. Even if I could find them now, they'd be prohibitively expensive... cost is one big reason I'm trying to use salvaged transformers. I think ultimately I'm just going to have to buy a suitable toroid, and rewind that.I think I'll try cleaning the laminations from the earlier transformer with acetone, and then varnishing one side. I can then restack them such that the varnished side is always touching a bare side. If that doesn't work, then I'm totally out of ideas.

author: cdev
author_rank: Super Contributor
timestamp: October 14, 2017, 11:48:56 pm
content: On audio sites I have seen photos where people varnished the pieces of a transformer individually and hung them on a clothes line to dry, then put them back together.I don't know if that is what people are supposed to do but some people do it.Quote from: jaycee on October 14, 2017, 10:46:44 pmYou likely could find and buy a new core on Taobao or aliexpress..

author: jaycee
author_rank: Regular Contributor
timestamp: October 15, 2017, 12:17:26 pm
content: To be honest if I have to start buying parts, it would be easier and probably more cost effective to buy an off the shelf 160VA toroid, and rewind that. I'm currently out of work, and I like to recycle junk electronics anyway. I have successfully delaminated smaller (5VA-20VA) transformers and rewound them before, it's just the big ones I'm having no luck with!So I guess the next thing to try is varnishing the recovered laminations and see i that works.The goal is to make a transformer suitable for a bench power supply. I'll be looking to do 0-32V (for a 0-30VDC power supply) at a bit over 3A, and then a pair of 9V windings to create the bias supply for the control opamps. I already know the circuit I've got in mind works as I have both prototyped it in LTSPICE, and tested elements of it on a breadboard using separate transformers at lower overall voltage.Yep I know I could get the KORAD supply for about £50-£60 but I have most of the parts I would need to build one, and building things is more fun anyway

author: cdev
author_rank: Super Contributor
timestamp: October 15, 2017, 03:51:15 pm
content: Why don't you make a post in the buy sell wanted area asking if anybody has and might be willing to sell or trade something for the kind of magnetics or transformer you could use.Many people (like me) hate to throw working parts away. When I lived in the Bay Area over several decades I had build up a stash of parts which included several big transformers.When I moved away, I ended up putting all but one of the transformers in a free box. Luckily somebody did take them.As transformers are kind of bulky, I bet at least one of the many UK people here likely has a transformer which would work and be overjoyed to find a good home for it.Its worth trying.That said, your idea of buying a suitable toroid prewound and then unwinding and re-winding it to fit your needs makes sense for multiple reasons.It shouldn't be that expensive done that way.A toroid will contain its magnetic field naturally. Which is a plus.It sounds to me like you might learn a lot as well as get a substantially better power supply doing what you propose than you could ever buy new on a tight budget.