Power calculations for transformer

[ants]

I have an audio transformer that is rated on the datasheet with primary at 3.2 ohms and secondary at 1.2Kohms. I use it at 100Khz and it works fine. The thing is how can I work out the power on the primary. I can't accurately measure current because my multimeter seems to change measured current by a lot, lets say 0.4A. So I can only accurately measure the voltage with my scope.

The inductances of the primary is 0.83mH and secondary 590mH. At 100KHz the primary should have an impedance of 500 ohms. At 10Khz it should be 50 ohms.

Which impedance do I use, the datasheet one? Or the 100KHz impedance one, which just seems too big.

Thanks.

 

[FvM]

The specification seems to fit an ultrasonic rather than an "audio" transformer. Or perhaps audio for bats...

I don't understand, what's the meaning of the said 3.2ohm/1.2kohm specification. Is it a winding DC resistance (possibly) or a nominal load impedance? In any case, it doesn't say anything about rated power. Rated current (defined by resistive losses and permissible overtemperature) and saturation flux are mainly determining transformer power capabilities.

Actual tranferred power in your circuit is a different thing. At first, are you talking about real or apparent power? There's a big difference with a piezo transducer load.

To understand, what's actually going on in your circuit, you may want to identify the elements of a transformer equivalent circuit. Beisdes DC windings resistance, windings ratio and main inductance, the leak inductance is an important parameter.

 

[ants]

The transformer is listed on the webiste I bought it as a miniature audio transformer but I've just looked at the packaging itself and it doesn't mention the word audio, it says 'miniature output transformer'. I think the 3.2ohm refers to impedance of the load and the website is implying it be used with 4 ohm speakers. I have tried to find out more about the transformer and I think it is a simple forward transformer, I've in the past bought flyback and other varieties which don't work well with my piezo at 100KHz.

**broken link removed**

I've taken measurements based upon vibration of the piezo and I'm happy with 12Vpp or 4Vrms on the 3.2ohm coil, on the other coil I get around 200Vpp.

I'm not sure whether I need real or apparent power. My thought and this is only a guess if I assume power = (V * V)/R

Then it is (4 * 4)/3.2 ~ 5Watts

But I'm not sure how the 3.2ohm coil reacts to different frequencies

Thanks,

Ant.

 

[FvM]

V²/R would refer to real power, but there is no real impedance R of 3.2 ohm present in your circuit - and no 5W real power.
A shunt resistor, either at the primary or secondary, can be used to measure the load current.

The transformer looks like the output transformer of 60th pocket radio, but it should have a higher main inductance
to achieve at least a telephone frequency range. How did you get the measurements?

 

[ants]

I just measured the inductance of the primary and secondary again using an LCR meter. It is the same transformer type but not the exact one from when I wrote the values above. It was 1.6mH and 800mH so similar.

I also put a 0.05 ohm resistor in series with the input of the 3.2ohm coil and measured the current. I had 10mA across the resistor and 100mA across the transformer coil with about 180Vpp on the other coil at 97KHz.

I understand the concept of real and apparent power, but not the practicalities. Something else springs to mind in that I am currently testing the piezo with PP3 batteries. In the past if I attached the setup to a mains power supply I got 10Vpp and 0.6A across the 3.2ohm coil.

Regards,

Ant.

 

[karesz]

Hi,
I can not see a datasheet with measures, its for me not real that it can work on 5W_seems to be ca. 20mm; (as FvM said too) a pocket radio transformer...
K.

 

[ants]

The transformer measures 20mm x 15mm x 15mm. It is larger in size than some other coilcraft transformers I have that are 5W but I wouldn't know what the output of this one is. Maybe I've been happy with 0.5W all along but have been unable to do the power calculations.

Regards,

Ant.

 

[karesz]

Ant,
Are you sure pls that the Coil Craft trafos are specified for 5W at audio frequencies?
Greetings!
K.

 

[ants]

I don't know but you are implying they won't, so I won't disagree

I bought a lot of them to try, probably 20. But I ended up not using any of them. The logic I am using is that the displacement of the piezo is proportional to the voltage and the current isn't important. So as this transformer ups voltage by 19 it is better than one that ups voltage by 3 or 4.

I'm still not sure what power I need or how to work out the power of a transformer.

Regards,

Ant.

 

[FvM]

The transformator specification mentioned 200 mW, if I remember right. But the power transfer capability increases with frequency.

The windings current is a limiting factor however. 0.6 A seems too much. Measuring the DC resistance can give a first estimation of permissible current.

I don't exactly understand your above desription:

I had 10mA across the resistor and 100mA across the transformer coil.

Resistor and coil current should be identical with a series connection.

 

[ants]

I took one of the transformers apart to look at the windings of the 3.2 ohm coil and then compared it to some inductors I have with different current ratings. The closest I get seems to be 0.7A, I was looking by eye so there would be room for error.

I was wondering about the 10 and 100mA measurements I took. I thought perhaps there was some real/apparent current effect going on. Now I've realised my multimeter just won't give reliable readings.

Thanks,

Ant.

 

[FvM]

Now I've realised my multimeter just won't give reliable readings

I was thinking of measuring the shunt voltage with an oscilloscope. That's the only way to determine v to i phase shift and possibly estimate real power.

 

[ants]

I put a 1ohm resistor in series with the transformer primary (3.2ohm) at 100KHz. The voltage across it was 1.05Vrms. I put a 0.39ohm resistor in series and got 0.55Vrms.

I know from experience that this isn't enough current for my purposes if i use 8 PP3 batteries (rather than the 2 I test with) or a mains supply the piezo works much better and how I want. For my final build which is some time away I'll be using a lithium battery and DC booster which hopefully will give me enough current. How much I'll be supplying then and now remains a bit of a mystery to me.

Regards,

Ant.

 

[ants]

I can have a guess.

The voltage across the primary coil was 4Vrms and the current looks to be 1V/1ohm = 1A
This implies I had 4 Watts of power, which isn't so far from my original guess and a lot above the 0.25W datasheet rating. I can put 4 times the current through it, seems a lot.

 

[FvM]

You can determine from the transformer temperature if it can stand the current.

The said 4 W is most likely rather 4 VA (apparent power). The difference doesn't count for the transformer rating,
but it does for the transducer output power.

 

[ants]

That stands to reason, I think it will be 4VA that I had. The transformer is very robust I will push it to see what it can do. Thanks.