Mistery Transformer

[augustol]

Hey guys and girls,

So, I got a transformer laying around here but it has no markings at all in it. The original appliance that had it was powered in/by 12V. So, I do know it's primary is 110V and secondary is 12V center tapped. It's perfectly working. Since it has no markings at all, I would like to find a way to discover it's current rating to see if I could make a linear power supply out of it. My multimeter cannot measure such low DC resistances but it can measure inductance. I've seen some people on the internet measuring ESR of the winding in order to determine some ratings. Maybe I can use inductance to get to ESR at 60/50Hz?

There's one thing I've never seen or heard of, though. One of the taps in the secondary (not the center one) has a thin wire next to it that measures continuity to the the primary. At first I thought the transformer was shorted, but than I realised it's just this very thin wire that has continuity to the primary and it does no measure any continuity to any of the secondary taps. What could that be? Also, I can't find the other end of this wire, I can only measure continuity from the primary to it. Anyone know what that is?

I'm going to take a picture of it and post in here soon.

[WattsThat]

If it’s connected to the primary, it is a tap of some kind, might be for higher or lower than normal expected voltage.

The capacity of a mains transformer can be estimated by the volume the core material, relatively easy to measure and calculate if the core slots are visible.

[bsfeechannel]

A rule of thumb to determine the power rating of a transformer in watts is to measure the cross sectional area of the center leg of the EI core in centimeters squared and square it.

So for example, let's suppose the central leg is 2 x 2 cm. The area is 4 cm², so the power rating will be 16 W.

[bob91343]

I suggest an easier way.  Find a catalog of power transformers that use similar construction and find one that weighs about the same.  Power and weight are closely related.  So a one pound transformer will have about the same power rating as the next one pound transformer, if they are designed for the same frequency.

[Ian.M]

As the thin wire connect to the primary and is in close contact to one of the secondary leads, emerging from the same sleeving, you cannot and must not trust the transformer to provide any isolation.

It is absolutely *NOT* suitable for repurposing in a general purpose PSU.

*IF* it was designed with that wire connected to the primary, just about the only thing you could sensibly do with it would be to use it as the step-up output transformer for a lowish VA inverter, as it is generally desirable to have the line voltage secondary ground referenced so GFCI/RCD protection can be used on the output.  However I fear that it may be the termination of an inter-winding screen, and that the primary to screen insulation has broken down, which would mean its only fit for scrap.

[augustol]

As the thin wire connect to the primary and is in close contact to one of the secondary leads, emerging from the same sleeving, you cannot and must not trust the transformer to provide any isolation.

It is absolutely *NOT* suitable for repurposing in a general purpose PSU.

*IF* it was designed with that wire connected to the primary, just about the only thing you could sensibly do with it would be to use it as the step-up output transformer for a lowish VA inverter, as it is generally desirable to have the line voltage secondary ground referenced so GFCI/RCD protection can be used on the output.  However I fear that it may be the termination of an inter-winding screen, and that the primary to screen insulation has broken down, which would mean its only fit for scrap.

I actually thought it could be something like that.

I don't know exactly how it's called in english, but in a literal translation, the original appliance it was used in would be called a "stabilizer". This thing is actually commonly used around here to protect other electronics from voltage spikes coming from the mains. It senses these spikes and very rapidly switches a relay off and back on to try to prevent unwanted voltages to get to your electronics. I don't actually know if it is effective and I have once heard this is a mith and it's no recommended. It was very common around here in the early 2000's to use it to protect computers. Almost everybody had one.

This extra wire might do just that, sense this spikes. I'm not very interested in the design of the original appliance, so I haven't given it any time to study the circuit. I just wanted to reuse the transformer and casing.

I guess I'll have to try to remove this strange tap from the transformer or maybe re-do the transformer from scratch without this strange tap.

I do think this wire is supposed to be connected to the primary since it has no continuity to any other winding and it kind of makes some sense for the use. The "stabilizer" just had a shorted transistor that would keep the relay on, with the output off for the electronic that was powered through it (a computer, actually).

[george.b]

I don't know exactly how it's called in english, but in a literal translation, the original appliance it was used in would be called a "stabilizer".

Those used autotransformers with multiple taps.

[bsfeechannel]

I actually thought it could be something like that.

I don't know exactly how it's called in english, but in a literal translation, the original appliance it was used in would be called a "stabilizer".

In English it is automatic voltage stabilizer.

[augustol]

I did open the transformer and was able to separate the thin wire from the secondary.

Just out of curiosity, the primary is a center tapped winding and this wire connected to the secondary has a lot more turns (don't know how many, but from center to each tap on the primary I measure 2mH and 13 mH). I didn't have to completely unwind it, just removing the core and the first paper isolation was enough to be able to access it.

The thing is: I'm inclined to just get this tap and isolate it and leave it there safely isolated. I've measured all the taps from primary and secondary to see if I was able to measure any continuity, even in high ohms there is no measurement. How safe is that? I'm not very fond of the idea of unwinding the secondary in order to get to the primary just to remove this tap. Although this extra wire might come in handy sometime.

[Doctorandus_P]

With a bit of patience, transformers are quite easy to measure.

If you have a variac, you can gradually increment the voltage (on any winding) and when you see the current increasing disproportianally, then the core is getting saturated and the voltage is getting a bit too high.

Similarly, if proper voltages are applied and you draw too much current then the transformer becomes hot after a while. If the current is only a bit too much, then it can take a while (half an hour or so) before it overheats.

[Ian.M]

A bench PSU's transformer *NEEDS* either reinforced insulation between the primary and secondaries, or basic insulation either side^* of a grounded interwinding screen.  Simple basic insulation isn't sufficient as for a bench PSU, the secondary wont be grounded.  Your transformer is almost certainly not built that way as the presence of the wire internally connecting the primary and secondary sides indicates its likely to have no more than functional insulation.

The *ONLY* way to use it safely in a bench PSU circuit would be to fully rewind at least its secondaries to implement either a reinforced insulation barrier between the primaries and secondaries, or two basic insulation barriers with a full width grounded foil interwinding screen^#, with proper creepage and clearance distances (which would require either a bonded built-up bobbin to achieve the creepage distance requirements at the edges of the winding, or rewinding the primary to implement margining), with significant loss of winding area and thus VA capability.  You'd then still have to soak test it at full load and Hi-Pot test it at 1KV to see if its actually fit for purpose.

If I were you, I certainly wouldn't want to tackle the job without a well equipped transformer winding shop and prior experience or a mentor, to guide me away from the possible safety pitfalls.  Also its *NOT* worth doing unless you need to reproduce a transformer with an unusual custom set of secondaries, and cant simply order one from a custom transformers specialist.

TLDR: Its not fit for your purpose nor can it be made so without several times more materials and labor than its worth.

* In theory you only need functional insulation between the screen and the secondaries, but that would be unsafe if the supply ground was ever lost.

# The foil screen must *NOT* form a shorted turn.  It needs an insulated overlap of at least the minimum creepage distance.  Also bringing out the ground maintaining basic insulation between it and the secondaries may be problematic.   

[augustol]

A bench PSU's transformer *NEEDS* either reinforced insulation between the primary and secondaries, or basic insulation either side^* of a grounded interwinding screen.  Simple basic insulation isn't sufficient as for a bench PSU, the secondary wont be grounded.  Your transformer is almost certainly not built that way as the presence of the wire internally connecting the primary and secondary sides indicates its likely to have no more than functional insulation.

The *ONLY* way to use it safely in a bench PSU circuit would be to fully rewind at least its secondaries to implement either a reinforced insulation barrier between the primaries and secondaries, or two basic insulation barriers with a full width grounded foil interwinding screen^#, with proper creepage and clearance distances (which would require either a bonded built-up bobbin to achieve the creepage distance requirements at the edges of the winding, or rewinding the primary to implement margining), with significant loss of winding area and thus VA capability.  You'd then still have to soak test it at full load and Hi-Pot test it at 1KV to see if its actually fit for purpose.

If I were you, I certainly wouldn't want to tackle the job without a well equipped transformer winding shop and prior experience or a mentor, to guide me away from the possible safety pitfalls.  Also its *NOT* worth doing unless you need to reproduce a transformer with an unusual custom set of secondaries, and cant simply order one from a custom transformers specialist.

TLDR: Its not fit for your purpose nor can it be made so without several times more materials and labor than its worth.

* In theory you only need functional insulation between the screen and the secondaries, but that would be unsafe if the supply ground was ever lost.

# The foil screen must *NOT* form a shorted turn.  It needs an insulated overlap of at least the minimum creepage distance.  Also bringing out the ground maintaining basic insulation between it and the secondaries may be problematic.   

Cool, I'm going to take a look at the resource and investigate a bit further, make some research on the topic. Thanks!