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Old 23rd Dec 2019, 11:18 pm   #21
David Simpson
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Default Re: Transformer Isolation

Quite frankly, WD & folk, in general regarding mains T/F's in vintage radios & equipment, "if in doubt, throw it out" is the safest policy. We all seem to have plenty of decent surplus ones lying around. If I haven't got an exact replacement, I ken folk who I could scrounge one off, I'm sure. Also, if its a very special T/F, just contact Ed Dinning who offers an excellent rewinding service.

Regards, david
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Old 24th Dec 2019, 5:26 pm   #22
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Default Re: Transformer Isolation

Quote:
Originally Posted by McMurdo View Post
begs the question if the interwinding screen is in good nick, how can leakage current get from the primary to the secondary?

DC Insulation tests should generally be done to earth with the L&N shorted together.
That was one of the things that puzzled me. Swapping the connections so that the neutral is at the screen end seemed to influence the amount of crossover and a voltage as high as 160VAC could only come from the primary. It is possible, I suppose that the screen connecting wire has become detached, but I don't suppose that there is any way to tell without unwinding?

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Originally Posted by McMurdo View Post
Be interesting to see how many uA or mA are flowing to earth from your dc terminals.
About a milliamp AC, and just a couple of microamps for DC.

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Originally Posted by David Simpson View Post
Quite frankly, WD & folk, in general regarding mains T/F's in vintage radios & equipment, "if in doubt, throw it out" is the safest policy. We all seem to have plenty of decent surplus ones lying around. If I haven't got an exact replacement, I ken folk who I could scrounge one off, I'm sure. Also, if its a very special T/F, just contact Ed Dinning who offers an excellent rewinding service.

Regards, david
Not sure how 'special' it is. I think it is probably standard spec for all 2A models of that era.

It would seem that just 24hrs indoors in the warm and dry has made a difference. I am now seeing just ~1.6VAC between earth and negative output. I am still seeing approximately 4.25VDC for a 5V setting so the latter may be down to something else. This DC voltage very gradually and slowly sinks if I hold the probes in position. Over a couple of minutes it dropped from 4.3VDC to 3.98VDC. If I remove the probes it quickly recovers and it does move up and down with the output. It remains present even with a load attached to the output. I don't suppose that 2 microamps is unlikely to cause any problem and might even be considered residual, but I am curious as to why it is there and why it varies with the output setting?

In any case, I guess this means that I can no longer use this PSU in the shed and maybe that David's "if in doubt thow it out" advice is the best course to follow. I had been toying with putting an offer in on a Siglent SPD330C (quick, easy) and was undecided as to whether to buy new or another TTi/Thurlby (might need some attention). Of course, if I could source another TX then that might be the cheapest option. I do have a TX rated 25V 100VA on the output, but it has no auxiliary secondary to drive the PSU internals from. I might not get 30VDC out of it, but might be able to get a little more current out of it. I would have to test it first though.

Last edited by WaveyDipole; 24th Dec 2019 at 5:49 pm.
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Old 25th Dec 2019, 10:33 am   #23
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Default Re: Transformer Isolation

I tested the resistances on my old, badly stored, PL320 transformer. I pulled it from a very damp garage yesterday & tested it this morning. All tests done using my Megger@1000V.
I double checked it with a few test resistors:
1 Gohm measured at 985 MOhm
100 MOhm measured at 100 MOhm.

Transformer results:
Primary to transformer clamp & ground 4.5 GOhm
Primary to 36V secondary 8.0 GOhm
Primary to 10-0-10 secondary 8.0 GOhm
Secondary to secondary 10.0 GOhm

All measurements taken with the primary & each secondary shorted at the terminals.
Just as a final check, I tested the secondaries (off load) & all were within 10% of nominal
I think my transformer is fine. With luck these measurements will be useful.If you decide that you need a transformer then drop me a message.

Just a comment but I had a serious leakage issue with some kit I bought from the USA recently. !60V @<1mA leaking across the ground between equipment. Problem showed up because the earth on the distribution socket was bad on just one socket. I know that I ought to check the mains i/p filters & change the leaky one. Well perhaps one day............

Happy Christmas.

Phil
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Old 25th Dec 2019, 12:00 pm   #24
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Phil, thank you for taking the time and trouble to do those tests. My megger seems no better after having been in the dry and warm over the past three days to I guess its toast. I have another one on the way and I am hoping to be able to take some sensible measurements when it arrives.

In the meantime, thinking about your last statement and earlier comments about the Earth fault on the socket, I checked the suspect PSU indoors this morning and the AC rerading between Earth and Negative was about 2.2VAC, so slightly higher than the day before, but still quite low. To rule out the socket I took it to the shed and plugged it into the socket where I had previously read the high AC voltage across those points, but still got around 2.2VAC. All the same, I think I am going to check the Earth to all sockets just to be sure.
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Old 25th Dec 2019, 1:22 pm   #25
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Default Re: Transformer Isolation

Incidentally, this discussion has been very useful. The "good" PL320 had around 10VAC between Negative and Earth as well as between positive and Earth. Reversing the neutral and live on the primary reduced this to around 2VAC. Placing a 0.1 between the negative of the rectifier and Earth further reduces this to negligible levels.

I am curious as to why the primary was wired with the live nearest the screen on both PSU's? It now looks odd to have live on the 0V and neutral on the 120V terminal (2 x 120V primaries in series), yet if having neutral next to the screen makes the PSU quieter then surely that would have been a desirable outcome for the design?
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Old 25th Dec 2019, 4:44 pm   #26
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Default Re: Transformer Isolation

Another trick that might null out more of the voltage would be to swap over the two 120V halves of the primary. This will change the capacitance distribution along the windings
Be careful of the phase of the windings.
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Old 25th Dec 2019, 9:00 pm   #27
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Default Re: Transformer Isolation

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Originally Posted by WaveyDipole View Post
I am curious as to why the primary was wired with the live nearest the screen on both PSU's? It now looks odd to have live on the 0V and neutral on the 120V terminal (2 x 120V primaries in series), yet if having neutral next to the screen makes the PSU quieter then surely that would have been a desirable outcome for the design?
The UK is - in a global sense - odd. We're one of relatively few countries where mains-plug polarity is enforced by default.
Much of the world uses plugs that are unpolarised, so in the context of the wiring of a transformer, there's no 'live' or 'neutral', just two active wires - even though one may be coloured brown and the other blue.
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Old 25th Dec 2019, 9:55 pm   #28
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Default Re: Transformer Isolation

Transformers in intermittently heated buildings are moisture magnets. As the temperature cycles, condensation is deposited on the exposed iron, whence it wicks down into the edge of the windings. I can well imagine that much of the leakage was over the surface and through the very edges, where it would miss the interwinding screen and where reversing the polarity would affect the leakage path length.

Taking AC voltage measurements on the secondary side w.r.t. earth is somewhat arbitrary because the leakage behaves much like a current source, being a relatively higher voltage in series with a high impedance. If the leakage is 1mA, it matters little whether that results in 10V or 100V w.r.t earth at a given moment, that is as much a function of the impedance from the secondary side to earth, meter included. It is the currrent value that determines whether it will or will not cause a tingle or damage electronics.

I don't think the earthing of your sockets is implicated, since the stray voltage was between the supposedly floating secondary side and the earth obtained from the socket via the case. A disconnected socket earth would, if anything have minimised this voltage. Although, it's not a bad idea to check the loop impedance or continuity from time to time.

A side note re. RCD sensitivities; many typical EN61008 RCDs trip at around 20-24 mA AC. The EN requires the threshold to lie between 15 and 30mA,
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Old 26th Dec 2019, 1:09 pm   #29
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Default Re: Transformer Isolation

As LN says, domestic RCD's are required to trip in the region of just under 30mA. They are also required to trip within 30mS - which is 1 & a 1/2 pulses of 50Hz peaks through ones heart. So the medical experts have decided for safety purposes.
Mains transformers - Jesus, we've all come across grotty old radios which have lain in folk's damp lofts & garages. Me - non get powered up without a Variac, and with always an AVO8 in place of a fuse in the 3 pin plug. Note - not a DMM, which are notorious for the .1 or .01 lcd's sometimes randomly drifting over several numbers whilst testing. I was told many years ago by an old hand to watch the meter very carefully, on the current range, for intermittent wee jigging about which could indicate imminent breakdown of a component or transformer winding. Likewise - a decent analogue Megger. Tell you what else I use - a medical stethoscope(wrapped in clingfilm) - if I suspect some sort of insulation breakdown. They are ideal for hearing the tiniest of wee wee internal sparking.
I repeat - if in doubt - throw it out(or get it rewound), regarding transformers, chokes, etc.

Regards, David
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Old 26th Dec 2019, 6:19 pm   #30
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Originally Posted by G6Tanuki View Post
The UK is - in a global sense - odd. We're one of relatively few countries where mains-plug polarity is enforced by default.
Much of the world uses plugs that are unpolarised, so in the context of the wiring of a transformer, there's no 'live' or 'neutral', just two active wires - even though one may be coloured brown and the other blue.
Agreed, re. no live or neutral. There is no polarity its just the order in which it is printed on the tag board. We are just used to having the blue wire going to the 0V (or 'N') tag. Now I have the brown one going there it looks odd, but to all intents and purposes does not make much difference, except to reduces stray AC voltage considerably. I was just wanting to be sure that there was no other reason that I am unaware of that this might have been wired in that way other than the tag board being printed in that particular order.

Quote:
Originally Posted by Lucien Nunes View Post
Taking AC voltage measurements on the secondary side w.r.t. earth is somewhat arbitrary because the leakage behaves much like a current source, being a relatively higher voltage in series with a high impedance. If the leakage is 1mA, it matters little whether that results in 10V or 100V w.r.t earth at a given moment, that is as much a function of the impedance from the secondary side to earth, meter included. It is the currrent value that determines whether it will or will not cause a tingle or damage electronics.
Thank you and I do take your point. I guess that I focussed on taking AC voltage measurements at these particular points because that is where the source of the tingle ulitmately tracked back to. I was then asked to take current readings which I did. Whether 1mA @ 160VAC or even 30VAC read on a DMM is sufficient to damage anything I am not sure. Naturally the AVO, being of lower impedance, reads much less. However it seemed to me that a well defined tingle across 3-4in of my hand (between index finger and thumb) was not something to be ignored.... By reversing the wires I now have it down to 30VAC and no tingle, but that is still much more than I am getting from the good unit.

Quote:
Originally Posted by Lucien Nunes View Post
I don't think the earthing of your sockets is implicated, since the stray voltage was between the supposedly floating secondary side and the earth obtained from the socket via the case. A disconnected socket earth would, if anything have minimised this voltage. Although, it's not a bad idea to check the loop impedance or continuity from time to time.
Also a good point.

Meanwhile, just a day back in the shed and the stray AC voltage level and current are back up to previous levels so damp ingress does seem to be a factor, whatever path the "leakage" may be taking. In addition, whereas a 0.1 cap between Earth and negative practically got rid off any stay voltages on the other good unit, on this one it does not. My "gut feeling" is that this may be an early sign of trouble so I think it will have to go. Just waiting on the megger to do some proper insulation tests.

Quote:
Originally Posted by David Simpson View Post
Likewise - a decent analogue Megger.
Funnily enough I was thinking about buying a used Fluke 1520, but now that you mention analogue I guess I will have to re-think that idea! The one I have coming is a Metrohm 251a. Doesn't read into the Gohm range, just to 200MΩ like the Robin. It only cost me a tenner and is supposed to be working so no big deal if it turns out to be a dud. If it proves to be working properly, then perhaps no other purchase necessary.

Incidentally, in view of a thread I saw on EEVblog on the subject of Siglent, have decided to ditch the idea of purchasing the Siglent SPD3303C. If it comes to it, then I will look out for another, perhaps more recent Thandar PSU.
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Old 26th Dec 2019, 6:45 pm   #31
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Default Re: Transformer Isolation

Just a thought WD, you mentioned a "shed" - a wooden shed perhaps ?
Some folk in the past have mentioned their shed's damp/condensation problems, and in fact there was a long running(and at times heated - excuse the pun) thread about a year or so back on decent lining for a new shed(see "Search"). Also, there have been a couple of past in-depth threads on outdoor shed's earthing & recommended safety precautions. Perhaps a moderator might kindly supply the links.

Regards, David
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Old 26th Dec 2019, 7:26 pm   #32
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David, no, its a plastic shed. The floor construction is such that the top surface is raised slightly (an inch or so) off the slabs, but that is all. I have the heater on while I'm there and have not noticed any obvious signs of condensation. I would certainly be interested in past discussions on safety. Had a search and found this which applies to wooden sheds, but nothing on safety yet. Still digging....

https://www.vintage-radio.net/forum/...ight=shed+damp

Incidentally a dehumidifier sounds like an interesting idea.

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Old 28th Dec 2019, 11:31 am   #33
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Well the megger turned out to be a dud. The description implied that it came from a working stock of a defunct company, but it is completely dead (nothing at all on the display) and clearly been 'got at' as the screws retaining the back were loose and one was even rattling around in the battery compartment. Only fit for the bin I think.
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Old 11th Jan 2020, 8:35 pm   #34
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Default Re: Transformer Isolation

A useful way to dry out a transformer is to short circuit the LV windings, whilst energising the HV windings at a very much reduced voltage.
The reduced voltage (from a variac) should be adjusted such that the normal full load current flows (you can check primary load current using an Avo on ac amps). The primary voltage will only be a few percent of its normal value. The resistance losses in the windings will heat them, and dry from within. External heat is far less effective. A friend of mine who used this trick on a monstrous 3KV transformer left it warming for a week, and after that all was sweetness and light! Expect the transformer to be very hot to the touch.

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Old 12th Jan 2020, 11:03 am   #35
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Not come across that trick before but it gives me a good excuse to get cracking on with repairing the variac. In the meantime, I have another tested and good transformer on the way along with another insulation tester. I do have a couple of questions though: doesn't the damp ingress indicate a compromised seal? Therefore, even if dried out wouldn't the internals get damp again if exposed to the same shed conditions? I will definitely give it a try though.
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Old 12th Jan 2020, 11:26 am   #36
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Only fit for the bin I think.
It may be useful to someone else for spare parts if you did a post in the 'offered' section with a few photos.

There's a lively group of Avo/Megger/Evershed & Vignoles collectors hereabouts, and if they found you'd binned even a dead one, they'd be sticking hot pins in wax models of your shed

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Old 13th Jan 2020, 3:30 pm   #37
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Default Re: Transformer Isolation

Sorry, I had used the term ‚megger’ rather loosely on this occasion. The tester was not actually a Megger or AVO branded instrument. Its possible that there may have been some salvegeable items in it, but it really was in a horribly battered condition, both inside and out, and has now gone back to the seller. Hopefully my shed is now safe!
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Old 15th Jan 2020, 7:12 pm   #38
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Today I received a replacement insulation tester, batteries included. Its a Newlec Nlins3. The display looks almost identical to a Megger, except that it not marked beyond 1Gohm which is consistent with the specification which state that it measures up to 1Gohm. Interestingly though although the display will not go above 999MΩ, the pointer does still indicate into well over that 1Gohm region. The other difference is that it has no backlight although apparently there is a hack for that. Photos of the PCB showed a Japanese HV relay as well as a number of Japanese and known brand ICs e.g. NEC, Philips etc. so I figured it should be worthwhile purchase and probably a better bet that these cheap far east models.

I tested with a 10MΩ resistor with the following result: Fluke 187->10.67MΩ; Nlis3@250v->10.8MΩ; Nlis3@500v->10.8MΩ. Readings on the Nlis3 at both voltages are consistent if a touch higher than the Fluke 187, but overall within 2 counts of the Fluke so that gave me confidence that the readings should be reasonably accurate.

Yesterday I received a replacement transformer from a member who was upfront about the fact that it had been stored in very damp shed conditions but was able to show measurements of at least 1 Gohm at 500 V between all windings and ground. It has been sat in my shed for over 24hrs and I have now been able to measure it and can confirm with my Nlins3 that all tests were indeed over 1Gohm. I was confident that they would be but wanted to see whether my new meter concurred, which it did.

So, then I moved on to testing my existing transformer. Here are the readings:

Primary to earth clamp: 0.22MΩ
Primary to 36v secondary: 0.33MΩ
Primary to 10-0-10 secondary: 0.17MΩ
36V secondary to 10-0-10 secondary: 0.19MΩ

Clearly very different and obviously well below 2MΩ.

I will be replacing the transformer tomorrow.

I'm not sure whether as a matter of interest it would be worth doing another test after it has been in the airing cupboard for a couple of days and again after the dry-out method mentioned by Pamphonica? I'm thinking that if its OK when dried out than perhaps I re-purpose it to use it indoors and build a PSU to power my Ardiuno projects? The other option is perhaps to dismantle it and keep the bobbin for enamelled copper wire? Or i could perhaps ask whether Ed Dinning or someone else might be interested in it for the iron and former?
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Old 16th Jan 2020, 2:18 pm   #39
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I replaced the TX today. First indications were that there were no significant AC voltages or currents at the output except the residual voltages that were consistent with the other good PSU, which was good news. The PSU was then soak tested for a while and seemed stable so I performed the calibration steps. After that I left it on soak test again for 30mins or so with a 1A load and still looked stable, so courtesy of a forum member, this PSU has been saved and will hopefully provide good service for a few more years to come!
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Old 18th May 2020, 9:42 am   #40
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Now that the variac has been repaired, I was able to use it to try Pamphonica's idea to dry out the transformer internally. This seems to have been at least partially successful. The resistance measured at 500V between any winding and the ground as well as primary and secondary rose to the Gohm range. However, the resistance between the two secondary windings remained relatively low at a little over 20Mohm (when compared with the 8Gohm reading on the replacement obtained thanks to Oldtestgear) or so despite applying the heating for some 10 days. I performed the insulation test several times during this period and although that 20Mohm reading varied considerably it never rose beyond 50Mohm or so.

Unfortunately this morning I also found that alethough the transformer had been left standing in my home "office" room over the weekend, the insulation resistances had dropped again. Windings to ground are now around 300Mohms, primary to secondary about 120Mohm and secondary to secondary is just 10Mohm.

I wasn't sure how hard to push the heating, but started winding it up gradually over the first couple of hours until the current reached about 150-180mA on the AVO attached on the primary side. I kept it at that rate so that the transformer remained hot to touch but could still be handled. I didn't push it to the full 250mA or so it would have taken to run it at the full 60VA (30V @ 2A).

I'm not sure whether it is worth risking pushing it a bit harder, but it would seem from the above that although the drying out by internal heating did help, the gains are being lost relatively quickly (i.e. in just a couple of days) once the transformer is left standing again, even though it was located indoors and in a relatively dry place.

I am happy to report that the Thurlby power supply continues to work well with the replacement transformer in place and I am pleased with that, but it does unfortunately look like the original transformer cannot be salvaged.

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