TRANSFORMER LOOK UP

[kinarfi]

When I come up with a transistor I don't know, it's easy to find it's data sheet, in most cases,
Is there such a database for Transformers or is it possible to figure out what you have with an ohm meter and / or an inductance meter?
Thanks,
Jeff

 

[Colin]

Yes. You weigh it. 500gm = 15VA

 

[jpanhalt]

That reminds me so much of Jim Williams' quip about how to pick the inductor for a SMPS. He showed a food market scale.

John

 

[kinarfi]

Actually, that's a look down, not a look up, you have to put the xformer on the scale and look down to see what you have, Thanks

 

[crutschow]

Transformers are so variable, depending upon the application, that it's not easy to characterize them without significant testing for things like inductance, core saturation level, turns ratio, winding resistance, etc.

 

[kinarfi]

I was just hoping that the manufacturers were compiling a list of what was what and some one was collecting the info, guess I'll just buy some very accurate scales, or I could just develop psoriasis and use the scales from that.

 

[Colin]

Yes. You can look at the thickness of the wires and count the number wires and look at the colours, feel the weight and look at the type of construction, but you cannot determine anything accurate. If you know where the transformer came from and have a similar one in a project and have a lot of comparisons, you can say two transformers are similar or equal or identical.
But in 50 years I have never "messed around." You don't know if the transformer has been thrown out because of leakage, spikes, a shorted turn or hum and buzz.
This type of list has never been prepared because it has never been needed. Transformers rarely fail and when they do, the product gets thrown out too.
Out of 35,000 TV repairs maybe 1 or two transformers were replaced and these were from voltage doubling TV sets where the transformers ran very hot.
And only one stereo amplifier transformer.
If you want to test a "power transformer" Connect 2 x 30watt transformers back-to-back to produce a 30w isolating transformer and connect the primary of the transformer under test. If the transformer draws current, it has a shorted turn or is not designed for say 240v. You can read the output voltage and fit a globe up to about 20watts to determine the current.

 

[Superdat]

Surely measuring the coils with a multimeter will give you a starting point. The size of the transformer will also give some clues.
E.g. if it's tiny and the primary reads 10 ohms, 220vac/10 = 22 Amps, not very likely!
Once you've figured out if it can work with a given primary, measure the output. Again size and resistance will give some clues.
Not an exact science since there's a fair bit of guess work about tolerances etc.

 

[Colin]

As the thickness of the primary winding changes a very small amount, the resistance changes enormously.
The primary can be from 7 ohms to 2,000 ohms.
If you know what you are doing, and you know where the transformer came from, the wire size will help you a small amount. But you really want to know if an unloaded transformer can be connected to 240v AC and remain COLD. That's why you start with the isolation transformer.

 

[RODALCO]

Firstly check the overall condition of the transformer first. No signs of overheating or burn marks.
Use an ohmmeter and check continuity of the windings.
The higher resistance windings are often the mains wiring.
As Colin suggested with 2 TX's back to back
or
put a 40 Watt lamp in series with the winding and do measurements.
The lamp will limit the current. it will light up briefly on the TX inrush current and should glow dim on the magnetising current.

 

[dr pepper]

The above might work for a iron laminate mains freq transformer, for a smps tranny though it would be tricky without pulling the thing apart and sussing out the ferrite's properties what it would be used for.
That said if its a standard etd core or something you can get ideas from datasheets: https://en.tdk.eu/download/519704/0...2ba503/ferrites-and-accessories-db-130501.pdf
Airgap is also soemthing you cant really tell by looking at a tranny, and is has a big effect on things.

 

[kubeek]

You can get a good idea on a normal low frequency transformer by judging the size of the core and measuring the voltages. SMPS transformer are a different beast and I'd say 99% of them are custom wound.