Homebrew Toroid O/P Tranny

[boiss]

I have recently wound a toroid for use as an O/P transformer and did some simple testing on my toroid. I wanted to get some idea of the inductance but have only a scope, so I made a little hartley osc with a BC177. With some caps across the transformer I got a resonant freq of 28Hz giving me an inductance of 45H. Removing the caps the resonant frequency rose to about 60KHz giving a total capacitance in circuit of about 20pF, this includes the winding capacitance, the series 10pF in the base circuit, the collector capacitance and the scope probe capacitance.
The winding resistance anode to anode is 40 Ohms.
I'm not an expert on transformers so what are your thoughts?
Boiss.

[EC8010]

Thoughts? Well, changing capacitors changed a resonant frequency from 28Hz to 60kHz? Typo? What were you looking for? Iron-cored inductors are full of traps for the unwary experimenter. Almost everything affects the answer you get for inductance. Frankly, if you want to know if it's suitable as an output transformer, then try it and measure the results - it will be quicker than trying to predict results.

[Claus]

This is not a typo, Boiss are measuring the inductance of the transformer with a known capacitor, and after knowing the indutance he can conclude the intenal capacitance of the transformer, by measuring the new resonant frequency. Im not a expert either, but as far as i can see this pretty straight forward.

Best Regards
Claus

[boiss]

Claus, That's correct, first test was with a parallel 0.69uF cap in order to calc the Inductance. My worries are that there are not thousands of turns on this core but the inductance is already very high.
Boiss.

[Jeb-D.]

There doesn't have to be a lot of turns. Core material and cross-sectional area has as much to do with the inductance as the number of turns.

I'm no pro with magnetics but from what I do know it seems that pretty much everything is a variable.

[EC8010]

28Hz to 60kHz is more than three orders of magnitude. Adding capacitance to change a resonant frequency as a means of determining inductance etc will not work over this large a range.

[boiss]

EC8010, Why not, The same formula will apply, with 45H and 0.69uF resonance is at @28Hz and 45H with 20pF gives around 60KHz. It is L and C with negligible R at resonance. Only Q will change. This was carried with an oscillator passing only a couple of mA in order not to cause major changes in inductance.
Boiss

[boiss]

EC8010, WHOOPS Sorry!!! just ran it on an on line calculator, I made a mistake.
the capacitance at 60KHz would only be 0.15pF an impossible figure. Something doesn't tie up very well.
I guess I'll have to knock up a rough and ready amp to test it as you suggested.
Boiss.

[EC8010]

Glad you realised the results didn't tie up. The fact of the matter is that you need a complex model for a transformer, whereas your initial experiments assumed a very simple model. A rough and ready amplifier will give you proper results. Good luck.

[schiller]

Originally posted by boiss
EC8010, WHOOPS Sorry!!! just ran it on an on line calculator, I made a mistake.
the capacitance at 60KHz would only be 0.15pF an impossible figure. Something doesn't tie up very well.
I guess I'll have to knock up a rough and ready amp to test it as you suggested.
Boiss.

20pF was equally impossible. Why bother with resonances?

Simulate your output device with a resistor in the series with the primary, load the secondary properly and measure the frequency responce.
That is how the manufacturers of transformes determine small-signal specification.

Greetings
Konstantinos

PS: Capacitance in audio transformers is in the order of some nanofarads.