Hi all,

A Power transformer exhibits a different "fault" issue than I have come across before .... and would like your input before my next step which is to peel back the covering .... and could cause unnecessary damage.

There are 4 inputs 115, 120, 230, 245v .... 10 outputs of various voltages ... Shield connection to Chassis ... all fairly standard.
It is 'donut' shaped .... very heavy ... was well shielded, protected etc .. German manufacturer.
All connections from the power supply circuitry have been removed ... it sits alone on my Work bench.
It shows signs of heat stress ... brown stripes on outside of covering.
There are no 'open' circuits ... but some unexpected connections between Secondaries ... so I am not 100% certain of the geometry of the secondary circuits as yet.

However, my key problem is when I connect a low level ac signal into the Primary circuits, my Scope shows a half wave rectified waveform ... that is, positive sine waves only, with low voltage on negative swings of the input signal.

The ac signal input is definitely a sine wave. I have checked my testing technique on two other power transformers and always see the expected sine wave across the primary and secondaries ... so I am sure that this is some issue with the Power transformer under test.

Previously, I have found some sort of heat sensitive resistor(?) buried under the protective covering of power transformers ... they are designed to 'open' if the current is too high ... anyone know what they are called????

Can one of these produce a diode action?????

I am not keen to cause any damage to this transformer by opening it up if I am on the wrong track. Any thoughts appreciated.

Sounds like a winding has shorted.

I think you are referring to a thermal fuse, which is non-resetting.

Positive Temperature Coefficient (PTC) devices go high in resistance to limit current when their temperature rises and resistance lowers again when the temperature falls, however those would not be generally used inside transformers.

At a guess, you are possibly seeing diode-like action from corrosion between windings.

Time to measure winding resistances

A high voltage insulation test would also be advisable. I do that with any old transformer before deciding to use it or not.

Do you have an RMS voltage meter as back up to the scope voltage measurement?

If you have a variac and ac ammeter, or even better a scope view of the ac current, you can increase the driving voltage on the full primary winding and monitor the magnetising current. Even better if you do that first with a known good transformer, to appreciate the current waveform shape and RMS current level as the excitation voltage is increased up to rated level. If there was breakdown across secondary turns then you will easily notice a change to the primary current.

Thermal Fuse was one .... Thermistor for In-rush current was the other device ..... couldn't remember its name.

Resistances of Secondary coils were very close to zero ... but it was the lack of resistance between the 'supposed' separate coils that worried me most.

Bit the bullet today and peeled off the surrounding tape and found what I dreaded ... insulation cooked off some windings. Will strip it down further over coming days.

Thanks for the input.

Hi,

I have unwound all the Secondary coils. They were on the outside of the Core and all have melted insulation.

However, the good news is that the Primary windings appear okay. I peeled off some of the protective covering and the heat stress is only on the outside and all windings show no sign of damage. Furthermore, the 'diode action' that I indicated earlier that was occurring when I applied an a/c signal to the Primary coils has gone ... they now respond with a clean sine wave and have amplitudes representative of the various voltage input options ... still do not understand why!!!!

So, what to do with it .....?????

I would like to rewind the Secondary coils. I have appropriate wire and know the number of turns, length of wire etc..

If anyone has done this before, that is, rewind the Secondaries of a Power Transformer, I would appreciate any advice or do's / dont's etc.. I am thinking here about insulating the Seccondary coils from each other with electricians tape ... what about heat dissipation etc ... ????

Hi Ian read some of my radio repairs in the special projects section!

I have wound PT from scratch or used a existing primary and rewound secondaries.
I could go on and on as I have been winding trannies all my working life but wont!
Some of my rewinds in radios I have gone into some detail.

You have to work out a bunch of details such as wire sizes v amp ratings, number of turns, and see if that will fit in the space available.
I would assume you know the turns per volt by having counted how many turns was on a known voltage winding, like the heater volts.
that gives you a rough figure of so many turns per volt and so the number of turns required for the other heaters and HT windings.
Then if the wire and the insulation dont exceed the winding space exisiting you cram it all in.

Most radio PT's used fragile enamel wire and had to be layer wound with interlayer insulation.
I scramble wind and hammer the turns down because I use modern poly coated wire.
I dont use wax to dip the windings in!
Any modern polyester "laquer" stuff is far superior to wax or old style laquer.
Any modern "sticky tape" has far superior voltage rating than the paper used.
I put in layer insulation every couple of hundred volts, not each layer..
You can spray on laquer as you go, or , dip the lot in floor varnish or similar when finished.
A layer of 2KV insulation between primary and earth and primary and secondaries is a MUST.
Could be plastic bobbin, "winding" type insulation material to get the dielectric rating and spacing.

I use a simple chuck and spindle winder with a counter and use my fingers for tension and guiding.
The wire spools run on a simple trestle.
Your fingers develop thick skin real fast.

now i'm going on and on..............................

Have a look at what I do in practice in the projects and ask me any questions!

Cheers, Fred.

Do not use old fashioned winding wire. One of our members has set up a lathe for rewinding and that older insulation just does not cut it.

Those non resetting thermal fuses can be annoying and can cause an otherwise perfect transformer to get tossed. Some can be replaced. I have actually put a reset type in the poorly thought out UPS, after its attempt at melting down its batteries.

Marc

Makers of toroidal transformers love fitting microtemps to them which have a rating that borders on the transformer's normal operating temperature. Bloody useless in this day and age because a power surge can and will send them over the edge. If I was given two bob for each one I have replaced or just simply removed and left the transformer in service for years afterward with no problems....

This kind of set up is an arse-covering exercise where transformers are installed inside plastic cases that come with no ventilation holes.

I have seen a video showing how to calculate & wind a torroidal transformer:

https://www.youtube.com/watch?v=lyZ7nM6Fo94&list=FL3lEQ32FQEeGv_Sg32FBJOg&index=40&t=0s

Coupla questions, I see this chap uses a shuttle ho hold the wire - looks practical - is there a better way?
Also, where would one go to get a core?
Best,
John

Hi 6A8, the next thing up from a hand shuttle is to make a machine to do the job.
Google winding toroids and marvel at some of the machines!

Get a core? I just strip old ones out of dead gear and rewind as necessary.

Fred.

The C-core, clamp and bobbin method (as used in 70s Pye TVs for example) is a MUCH easier way to make a toroidal transformer.

Hi all,

Rewound, by hand, all the Secondary windings .... used a 'test' rectifying / filtering circuit to test each Winding and get the appropriate voltages required.

I am now testing all the key components in the PSU ...4 different supplies .... to find why the Secondaries burnt out in the first place.

Fred ... thanks, read your articles ... very helpful.