Transformer Isolation

[WaveyDipole]

I have a thread about one of my Thurlby PL320's on EEVblog but I wanted to specifically ask about the transformer. Today I noticed a tingling when my hand made contact with the negative sense bar and the case at the same time. When I checked, I found 160VAC across those points. Digging deeper, I checked the earth to line resistance with a megger and this tested good - i.e. no resistance registered. With a DMM I then checked the resistance between windings. This was 8.1MΩ between pimary and the 30V secondary, but 0.6MΩ between primary and the 10.5-0-10.5V secondary. With the unit switched on, there is approximately 160VAC between Earth and the 10.5-0-10.5V secondary. A 'lower Z' measurement with the AVO shows between 50V and 100V depending on range.

Is the insulation between the transformer windings breaking/broken down?

I am trying to understand whether this is normal/salvegeable/toast, and whether this means that I will need to replace the transformer or even get another PSU.

I have a second almost identical unit. I must admit that I haven't opened it and tested that transformer, but it does not exhibit this problem. There is slight AC voltage between ground and negative (approx 5VAC) sense but nowhere near the reading I get on this unit.

[Herald1360]

The transformer ac " leakage" may be capacitive.

It may be awkward, but a dc megger test with the windings disconnected from anything else would be informative.

[WaveyDipole]

Forgot to mention, but all above measurements were done with the secondaries disconnected. I can megger between windings was well if it helps.

[WaveyDipole]

It would seem that the readings on my megger can be relied upon. I have a Robin model 3001 and it reads above 200MΩ on everything. Even measuring a resistor on the 'Ω' setting was well adrift of its actual value. The one reliable indication that I have is the resistance reading from the Fluke 187 DMM, which read 8.1MΩ between primary and the 30V secondary and 0.6MΩ between primary and the 10.5-0-10.5V secondary. The connections to the secondary windings were disconnected when the readings were taken. There are no filter caps on the mains, just the power on/off switch, mains flex, plug and fuses.

[Herald1360]

Sounds like the megger is dead. Does it read sensibly on a known 1meg resistor?

[joebog1]

Is the transformer dry? By that I mean is it stored in a damp shed?. I have seen paper interleaved transformers leak when damp, especially if they are not varnished or at least wax dipped.

Joe

[WaveyDipole]

Originally Posted by Herald1360

Sounds like the megger is dead. Does it read sensibly on a known 1meg resistor?

I don't have a 1MΩ resistor to hand at the moment, but I did test a variety of others:

Resistance -> Reading

3.3k -> 1.108
390k -> 1.
3.3MΩ -> 1.
10MΩ -> 1.

A reading of 1. means that the resistance is over 200MΩ. Incidentally, the batteries are fresh - I checked just to be sure and all cells read at least 1.6V. The resistance readings just do not seem to make any sense.

Originally Posted by joebog1

Is the transformer dry? By that I mean is it stored in a damp shed?. I have seen paper interleaved transformers leak when damp, especially if they are not varnished or at least wax dipped.

Joe

Both are stored in the shed as that is my 'workshop'. The shed being of plastic construction is not unduly damp and quite waterproof, but of course at this time of year - it being mid-winter and cold - and with all the rain that we have been having in the UK, the conditions outdoors are very wet. There is some kind of a coating on the windings but as to whether the seal has been computerised somewhere is anyone's guess. It may be, of course, that the other unit has not suffered the same problem due to the transformer being sealed better.

Whatever the case, would brining it into a warm dry environment (i.e. somewhere indoors) help? If so, then how long to wait before testing it again?

[Ed_Dinning]

Hi, a transformer may read a lower resistance value when meggered than when checked with a DVM as a higher voltage is applied by the megger.
Try leaving the meter in an airing cupboard for a few hours/ days, opened up and see if things improve.
As mentioned earlier there is always the question of interwinding capacitance and the 50Hz current flowing through it. This is not leakage, but can be reduced by transformer design.

Check your meter to see if it has suppression caps fitted anywhere that may pass AC current.

You can also check the magnitude of the current by using a sensitive milliamp meter between common bar and earth. Measure both AC and Dc currents


Ed

[WaveyDipole]

Originally Posted by Ed_Dinning

Hi, a transformer may read a lower resistance value when meggered than when checked with a DVM as a higher voltage is applied by the megger.
Try leaving the meter in an airing cupboard for a few hours/ days, opened up and see if things improve.
As mentioned earlier there is always the question of interwinding capacitance and the 50Hz current flowing through it. This is not leakage, but can be reduced by transformer design.

Check your meter to see if it has suppression caps fitted anywhere that may pass AC current.

You can also check the magnitude of the current by using a sensitive milliamp meter between common bar and earth. Measure both AC and Dc currents

Ed

Ed, thanks. I will have a closer look at the meter in due course. In the meantime, the current readings on the PSU between common bar and earth was about 1mA AC and 0.038mA DC. On the good unit, its 0.024mA AC, and difficult to tell on DC as it barely registers.

if its down to interwinding capacitance, then how to deal with it?

[Silicon]

Is the laminated core of the mains transformer earthed?

Does the transformer have an electrostatic screen?

[WaveyDipole]

Originally Posted by Silicon

Is the laminated core of the mains transformer earthed?

Yes, in as much as the TX is bolted directly to the chassis floor, which, in turn has an Earth point bolted to it.

Originally Posted by Silicon

Does the transformer have an electrostatic screen?

Yes, I think so. There is a yellow/green wire spot welded to the TX frame which then goes to roughly the middle of the winding bobbin. if that is the case, then should there be any capacitive coupling from primary to secondary?

[Refugee]

Try reversing the mains so that the start of the primary in the middle of the coil is connected to neutral.

[WaveyDipole]

Hmmm.... an interesting idea! Just checked the wiring and live is connected to the side nearest the screen and neutral the furthest. I swapped them around as suggested so that neutral is nearest the screen and this has reduced the voltage to about 30VAC and the current from 1mA to 0.2mA.

I also checked the Earth from Earth pin in the mains plug to chassis, TX and Earth connector on the front panel. Readings were between 0.2Ω and 0.4Ω taking into account test lead resistance using 'Rel' mode.

[Herald1360]

Originally Posted by WaveyDipole

......


There is some kind of a coating on the windings but as to whether the seal has been computerised somewhere is anyone's guess.

......

Love it!


"To err is human, to really foul things up, you need a computer"

[julie_m]

Obviously the spelunker chicken found and substantiated the nearest read worm in the dictatorship, even although that was nit the actuarial work you Maentwrog!

[WaveyDipole]

Ah yes! The computerised spell checker has been compromised!

I thought maybe a 0.1 between said terminal and Earth might help. It does but only slightly. I also discovered that there is a DC voltage between said negative connector/link and Earth of approximately the same, but slightly higher, voltage than on the DC output. Neither output terminal is linked to ground and there is about 3MΩ between either terminal and Earth. Puzzled.

[Oldtestgear]

I have a few of the brown PL320s that I keep for spares, including displays & single transformers. These are kept in a spectacularly damp garage. I have to go there tomorrow & will try to remember to dig out one from the box. Happy to check the transformer as collected plus dry it out over Christmas & check it again. Does depend on on:
(a) me remembering to collect it
(b) #1 son returning my Megger that he borrowed a few days ago.

This may be of some help.

Happy Christmas to all.

Phil

[David Simpson]

Aye WD, 160V AC floating about a chassis & giving you a "Tingle", oh dear ! Even if the T/F is on the blink(probably is), you have an earthing &/or fusing problem. I'm surprised your domestic RCD hasn't tripped ?
With mains T/F insulation testing, don't fanny about with DMM's. I'd be looking at least at a 1000V Megger, and pretty high M ohm or infinity resistance readings results.
Guys have already advised you about damp ingress in T/F's, and if after several days in your airing cupboard, low readings are still being shown - scrap it.
Me, I'm lucky in having a modernish Megger SL Series 1, which has ranges up to 2.5KV & a discharge button for capacitor testing.
Folks views on workshop & equipment safety, protection, earthing, etc. have filled many thread pages in recent years. (See "Search").

Regards, David

[WaveyDipole]

Originally Posted by Oldtestgear

I have a few of the brown PL320s that I keep for spares, including displays & single transformers. These are kept in a spectacularly damp garage. I have to go there tomorrow & will try to remember to dig out one from the box. Happy to check the transformer as collected plus dry it out over Christmas & check it again. Does depend on on:
(a) me remembering to collect it
(b) #1 son returning my Megger that he borrowed a few days ago.

This may be of some help.

Phil, thank you for being willing to do that. If you do manage to get around to it, then I'm sure it will be interesting to note whether a bit of damp does cause any significant issue with these PL320's. Of course, please don't feel pressured and if you don't get a chance to do this then no problem.

Originally Posted by David Simpson

Aye WD, 160V AC floating about a chassis & giving you a "Tingle", oh dear ! Even if the T/F is on the blink(probably is), you have an earthing &/or fusing problem. I'm surprised your domestic RCD hasn't tripped ?

Just to confirm, the domestic RCD hasn't tripped at all and I do know that this, at least, is working OK as it was tested only a couple of months ago. The currents I was measuring were well below the 30mA threshold, but even so, getting a tingle was indeed a concern, as was the possibility that even now with only 30VAC floating around, this might still be enough to cause damage low voltage boards.

Just as a comparison, the second PL320, which is of identical type and has sat next to it, has only around 5VAC floating around and negligible/residual DC.

Originally Posted by David Simpson

With mains T/F insulation testing, don't fanny about with DMM's. I'd be looking at least at a 1000V Megger, and pretty high M ohm or infinity resistance readings results.

Unfortunately the megger was giving me silly results so is currently in the process of "drying out" as per Ed Dinning's suggestion.

Originally Posted by David Simpson

Guys have already advised you about damp ingress in T/F's, and if after several days in your airing cupboard, low readings are still being shown - scrap it.

I checked all the Earth points. The two bolts holding the TX together were a little slack and tightening it up by a third of a turn or so brought the resistance from plug Earth pin to chassis to Earth socket on the front plate down from 0.3Ω-0.4Ω to approximately 0.1Ω, but this has made no difference to the problem. I prodded around the rectifier board, the auxiliary rectifier and the 5v regulator and all seems good. There seems little else that can be done for now so I am now following your advice and have brought it inside into warn and dry conditions. I will check it in a couple of days or so to see whether there is any change.

[McMurdo]

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.

Be interesting to see how many uA or mA are flowing to earth from your dc terminals.