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Old 16th February 2013, 04:06 PM   #11
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Btw. in your diagram i guess you meant 0.01uF for Cx.

Cheers,
Chris
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Old 16th February 2013, 05:29 PM   #12
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Default FirstTry

I just performed a first measurment w/o the extra capacitor.

For my testing setup, i fed the signal generator directly into one channel of my oscilloscope and the other into the transformers secondary winding. See first pic.

The basic idea being that the ref signal and the measurement signal align or in phase at the resonant frequency.

Unfortunately this wasn't the case. I could see the amplitude of the meas. signal rise to a maximum at around 172kHz. See second pic.

I found another resonant frequency at around 933kHz. In this case the wave forms aligned as expected. See third pic.


Should i just accept the first measurement or is something wrong here?

Thanx,
Chris
Attached Images
File Type: jpg test_setup.jpg (55.0 KB, 92 views)
File Type: jpg 171kHz_Peak.jpg (94.8 KB, 92 views)
File Type: jpg 933kHz_Peak.jpg (92.8 KB, 92 views)
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Old 16th February 2013, 06:20 PM   #13
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Location: Silicon Valley
Hm, the transformer I measured most recently was an 80 VA model, one of these: http://www.mouser.com/ProductDetail/...252bpne6dXjQ==

and its secondary resonated at 155 kHz (Cx=0) with the primary shorted. So I am inclined to think your 172 kHz measurement is probably the one to trust. Hagerman says (page 5, paragraph 4):

Measure the voltage across the coil while varying frequency. The lowest frequency which gives a peaked reading is probably the natural frequency of the coil.
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Old 16th February 2013, 07:13 PM   #14
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Hi,

mine is a 25VA talema, one of these:
RKPT 25218 - Ringkerntrafo, Print, 25 VA, 2x 18 V, 694 mA bei reichelt elektronik

I might try another test probe on my scope, since the one I'm using is a bit worn out and the tip falls off easily.

Cheers,
Chris
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Old 16th February 2013, 08:30 PM   #15
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Location: Canandaigua, NY USA
If you grab my doc (Word) file of handy electronic formulas here, the first section covers coupling and leakage. IMO, it's useful to measure coupling because that which isn't mutual inductance must be leakage inductance.
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Old 17th February 2013, 04:06 AM   #16
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Location: Jackson,michigan
Here is a cool little spreadsheet for excell that I was introduced too in this thread,

How to construct a cube louver (Acoustat)

For calculating leakage inductance and stray transformer capacitance a while back.
You can also enter in the transformers natural resonate frequency and enter 0pf in the first line and then add your extra capacitance and enter the second frequency in the next.
Either way works.

This will help you from doing the long hand method of the calculations.

The only thing I find confusing is that in his instructions jelanier says to find a dip in the voltage and this is the resonate frequency of the transformer.

I typically use the peak voltage.

I have found that using the dip voltage is usually a much higher frequency and is so small that it is hard to find at times.

Either method seems to produce the same results as long as you take very accurate readings of the frequency's.

Maybe some one can clarify this for me.
I just started getting back into this since three years ago so I am a little vauge on the proper methods.

I do know that leaving the secondary open is actually shorted by the stray transformer capacitance.
Thus adding more will produce a second lower frequency to be used in the equation to find the actual secondary leakage inductance without physically shorting the winding with a wire.

Cheers.

jer
Attached Files
File Type: zip Leakage indutance caculator.zip (4.2 KB, 12 views)

Last edited by geraldfryjr; 17th February 2013 at 04:10 AM.
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Old 17th February 2013, 08:32 AM   #17
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Hi jer,

I'm not 100% sure what u mean by the following:
Quote:
I do know that leaving the secondary open is actually shorted by the stray transformer capacitance.
Thus adding more will produce a second lower frequency to be used in the equation to find the actual secondary leakage inductance without physically shorting the winding with a wire.
Isn't that what I'm doing?

Maybe its not clear but I'm modeling this according to the transistormarkj's sketch.
HexFred vs. Hyperfast Diode (1200v)

E.g. with the stray and extra capacitance connected across the secondary winding.


Regards,
Chris
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Old 17th February 2013, 09:21 AM   #18
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Yes, it is actually.

jer
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Old 17th February 2013, 12:31 PM   #19
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Default Calc. now with Cx or extra cap

Thanx jer, also for the excel sheet.

It made me realize that my equations post had some error's, forgot to square pie in all of the formula's.

I added now the extra capacitor with value 0,033uF. My new resonant frequency was around 105.660 kHz. This freq. was indeed lower than the measurement w/o it! I also wasn't able to discover any higher resonant frequencies with it installed, at least up to 2 MHz!

But it wasn't at least 1/3 of the nat. freq. of the transformer, e.g. of 171.239 kHz, so i used both measurements to calculate the inductance.

Here's what i got:
L Leakage = 0.00424 H or 4.24 mH (Btw. this agrees with the excel sheets calculation!)

For C, I'm still not so sure, when I plug it in I get:
C Stray = 0.00000536 uF or 5.36pF

The excel sheet calculates a value of:
20600pF

For the calculation of my C Stray value i just substituted the L value into my second equation, e.g.:
C = 1 / (freq2 ^2 * 4 * pie^2 * L) or
C = 1 / (105.660^2 * 4 pie^2 * 0.00424)

The excel sheet calculates it a bit different. Will have investigate that to fig out what its doing.

Maybe I will choose and even larger Cx and retest s.th. new resonant freq. is < 1/3 of nat. freq..


Cheers,
Chris
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Old 17th February 2013, 01:24 PM   #20
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Location: Silicon Valley
Chris, I would have expected that if (Ls , Cs) resonates at 172 kHz, then (Ls, (Cs || 33nF)) would resonate at about 30 kHz. Perhaps if you started your sinewave generator at a frequency of 3 kHz, and then dialled it up very slowly, you might see a resonance in the low tens of kilohertz (?)

Does your capacitance meter agree that the extra capacitor is indeed 33 nF?

By the way, you can double-check the results of your hand calculations, or your Excel spreadsheet, using LTspice. Simulate the circuit that you're measuring, and see whether it resonates at XX kilohertz with no extra capacitor. See whether it resonates at YY kilohertz with a 33nF extra capacitor. If the resinant frequencies in simulation don't match the resonant frequencies in the lab, then your calculated L and C values aren't correct.
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