Is this transformer shot ?

[ThermallyFrigid]

Hello,

    I saw a video on YT that showed how to test a small transformer such as a common SMPS transformer using a powered wall wort AC adapter.
https://youtu.be/kZZRGDTgZdY

    I have this transformer......and when I place it on top of an AC adapter it does not respond to the magnetic flux when I connect a VOM, set to AC voltage with (clip on leads) on pins 10 & 11 or on pins 2 & 3 or 4 & 5. Does that mean it is bad? Shouldn't any transformer like this respond to magnetic flux?

    In the video his transformer responds nicely and you can determine the Primary from the Secondary sides. His also reveals the turns ratio. It is quite amazing. Mine won't give up ANY secrets. It's like a dead blob.

    Thanks



     

[Ian.M]

I skimmed a few seconds of that video and its absolute bullsh!t.  Well designed and built transformers have small to negligible external magnetic field so for the coupling between two of them to induce significant voltage in the windings of the second, both must be crappy minimum cost transformers and carefully positioned for good coupling.   Also a conventional line frequency transformer will have negligible effective coupling to the transformer or inductors in a SMPSU, as the transformer is designed to operate at line frequency, and the thickness of its laminations are such that eddy current losses will damp any coupling at frequencies above a few KHz, but modern SMPSUs operate at switching frequencies of at a minimum several tens of KHz up to low MHz frequencies.  Its about as likely to work as trying to get a steam engine to turn over by farting in its general direction rather than connecting a high volume source of compressed air in place of its steam feed!

If your transformer has a laminated core and you are certain its a mains transformer, measure all the windings on an ohmmeter to find the highest resistance one.   That's probably the primary.  connect it to low voltage AC (e.g. from a traditional line frequency bell transformer) via a 10W car bulb.  You can then measure the voltage on the suspected primary and all the other windings, calculate turns ratios and see if they make sense with that winding as the primary.

If its got a ferrite core or you suspect its an audio transformer, use a signal generator as the excitation source, set to about 1V output, with a resistor in series equal to the signal generator's nominal output impedance to limit the current.  start at 1KHz for suspected audio transformers and 100KHz (or the highest frequency less than that your meter is calibrated for, but use a scope if you have one) for SMPSU transformers.

[ThermallyFrigid]

Thanks Ian.
That seemed like such a super easy great way to see if a transformer was good or bad <sigh>

It is in a DC to AC inverter (300watt)
Connected as follows......



Appreciate the advice.  Will follow it.

Best Regards

[Audiorepair]

I suspect the YouTube demo actually WILL work with "crappy" transformers, that's why they are crappy and very cheap.

In fact I will try it tomorrow.



It is very unlikely your inverter transformer will do this, however, because it isn't a crappy mains transformer that does this.

[Ian.M]

Oh yes, and any pair of small wallwart transformers crappy enough to disturb a CRT monitor or TV would be good candidates to try it with.  However as I pointed out above the coupling between a line frequency transformer, even the crappiest piece of shonky sh!t from an 80's wallwart, and even the nastiest 'China Export' modern switching wallwart is going to be negligible, and between two reasonably decent line frequency transformers is going to be pretty small even if you clamp them core to core.  Unless you've got a *good* DMM, it may well be too small to read anything on its lowest AC range.  Therefore its a gimmick (like using a small mains transformer as a 1 bit non-volatile magnetic memory), not a practical method of testing transformers.

[ThermallyFrigid]

So I eventually got the circuit repaired and working and as suggested, the transformer was perfectly fine.

The ONLY measurements I was able to get from it was with my DE-5000 inductance meter.  But I'm not sure what those readings tell me.
I "think" it tells me the Primary is on the side with pins 10 and 11 ?
Can anyone confirm that?

I got 3.376 mH across pins 10 and 11
28.9uH between 5/6 and 3/4
28.9uH between 1/2 and 3/4
116uH between 1/2 and 5/6

[testpoint1]

if you have a good one, you can test the inductor value as your referenece via the LCR meter, then compare with the suspect one's result.