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Old 17th February 2013, 01:45 PM   #21
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Join Date: Aug 2012
Hi,

I made some more measurements, with various Cx caps.
My prev. measurements were incorrect.

Nat. freq. of trany approx. 169.503 kHz


1. Ceramic 0.01uF, resonant freq.= 155.038 kHz
L Leakage = 0.0172 mH

2. Not sure of type, 0.018uF, resonant freq.= 134.344 kHz
L Leakage = 0.029 mH

3. MKP 0.033uF, resonant freq.= 105.500 kHz
L Leakage = 0.0422 mH


The leakage values seem to vary a bit. The bigger the capacitor the larger the calc. leakage. Should i keep increasing the value till the leakage result converges hopefully?

Regards,
Chris
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Old 17th February 2013, 07:14 PM   #22
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Location: Silicon Valley
It sounds like your results are diverging rather than converging. I think it may be prudent to simply give up on this type of measurement and try something else.

You could try to find someone near you who owns an LCR meter, and ask them to measure the secondary inductance twice; once with primary open and again with primary shorted. Perhaps you could find such a person by posting a message here on the diyaudio site.

Or you could use the approximate rule of thumb that transformer leakage inductance is very roughly 5% of total inductance (secondary inductance with primary open-circuited). Measure secondary inductance, either with an LCR meter or with a sinewave generator and a resistor, then multiply by 5%. By the way, since the L is ~ 20X greater, the resonant frequency will be about sqrt(20) = 4.5X lower. Perhaps the sinewave generator measurement approach will work better for you around 38 kHz than it did around 170 kHz.

If the sinewave generator approach does work well for you with the primary open circuited, i.e., if measurements with 3 different external capacitors all give the same extracted Ltotal, then you can also extract an approximate value of secondary interwinding capacitance from these measurements. Because secondary interwinding capacitance doesn't change much when you short the primary vs. open circuit the primary.

I'd like to call your attention to pages 8-11 of the Hagerman paper. Here he discusses using a capacitor (with no resistor) as a "snubber". This doesn't eliminate oscillatory ringing, but it does lower the resonant frequency. Next he talks about using this parallel capacitor AND a conventional RC snubber. The parallel capacitor lowers the resonant frequency, and the RC snubber kills the ringing. This 3 component snubbing network has the great advantage that it frees you from measuring the secondary interwinding capacitance. Just make the parallel capacitor much much larger than any possible secondary capacitance, and voila, Ctotal = Cparallel.

The 13 VA transformer that Hagerman measured, had a secondary capacitance of 550 picofarads. The 80 VA transformer that I measured, had a secondary capacitance of 1300 picofarads. I'm pretty confident that yout transformer's secondary capacitance is within a factor of 3, either way, from that. So if you installed a 150,000 pF (0.15 uF) capacator in parallel with your RC snubber, you would now be absolutely certain that the LC resonant circuit has C = 0.15 uF exactly. And the resonant frequency would fall by about sqrt(150000 / 1300) = 11X.

Finally, keeping in mind that your end goal is to design a snubber, you could decide to build yourself a little test board that lets you directly observe snubber performance and lets you easily change the snubber resistance. Something along the lines of Bob Cordell's book Fig 16.10; it's designed to let you easily monitor the rectifier voltage (including all of the oscillatory ringing due to poor snubbing) without the headache of common mode voltage. This will let you empirically select the snubber resistance that gives a damping "zeta" which pleases you. It'll probably take 2 hours to design the board, 2 hours to build it, and 1 hour to run the experiments with different values of snubber resistance. And now you've got a jig that lets you do the same measurements again, some day in the future, when you want to fiddle with a different transformer.
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Old 17th February 2013, 07:41 PM   #23
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Yes,I typically use about 5 or 6 values in a wide range from 47 pf to 680pf and then find the approximate value that is common to all combinations of the values I used.

A fellow DIYer plots them on a chart and finds the crossing point on a chart as shown here,

Antek Toroidal power transformer for Step-up, Measurements (part 1/2).

If your transformer has any electrostatic shields this will complicate matters even more.

I am working with two transformers right now in a step-up configuration.
One that I have had laying around as in my earlier tests here,

Step-up transformer design

And an Antek AS-1206 that has shielding in its construction.

Both transformers exhibit quadrupling of the leakage inductance as they should when the HV winding's (now secondary) are connected in series as the leakage inductance is proportional to to the square of the number of turns.
Another words if the turns are doubled the leakage inductance quadruples.
As described here,

OTimpedance

Now for the confusion.
My core shows also the doubling of the stray capacitance when the secondary are connected in series.
This should cut the impedance seen by the driving amp in half and is confirmed by the doubling of the of the primary current using just the transformer self capacitance as a load at 10Khz and 20Khz

The Antek on the other hand shows the same actions for the leakage inductance as expected.
But the capacitance on the other hand and is halved ifor the series connection of the secondary winding.

In both situations I was using the same resonate mode of measurement with a 1 ohm resistor feeding the primary's.

But upon measuring the primary current for both transformers clearly shows the doubling of the primary current for the series connection of the secondary.

Keep in mind that this is a doubling of the transformation ratio as well.

Completely opposite results between the two transformers with the very same setup using this technique.
Except for the results of measuring the primary current is exactly the same for both in the same situations.

Now since then during a core saturation test I had burned a few winding's on the Antek AS-1206 and I had stripped it down to the to the 115v winding and removed the electrostatic and outside magnetic shields.
Fixed and repaired the burn't and shorted winding and rewound a new primary with the same number of turns that was on it.
And now I still get the very same opposite results compared to my other transformer with whatever shields that I could remove, removed!

I have stripped off the extra winding's on my first core and this lowered my capacitance a good deal.
But I haven't gotten down to the 115V winding's yet in order to see if they are maybe wound very differently causing these very strange measurements.

Sorry,I am not trying to go off topic or hijack your thread.
I am just expressing my recent discovery's using this method and now I am thoroughly confused of which is the correct way and what measures that I may not be accounting for, As my application is different than yours.

FWIW

jer

Last edited by geraldfryjr; 17th February 2013 at 07:44 PM.
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Old 18th February 2013, 10:32 AM   #24
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Thank you transistormarkj for your detailed analysis, I really appreciate it! So the basic message is, well actually there's a few here but the first one is:Measure the secondaries inductance with primary-opened instead of the leakage, cause its 20x larger and approx. the leakage by taking 5% of it. Yes, I will try it with my 3 capacitors and post the results later. Thanx for the tip! As far as the snubber network is concerned, I was planning on using 1 parallel capacitor and 1 in series with a resistor, e.g. a la Hagerman. For me, I could prob. assume that my leakage lies in the middle of Haggerman's value and yours, e.g. 925pF. Your assumption of 3x 550, e.g. ~ 1500pF is a bit more. Is bigger better or just safer when one cannot measure it exactly?What about the capacitance of the diodes in a fullwave rectifier? Haberman, mentions it and approximates 50 pF according to the datasheet. For two in series he says its half, so wouldn't that be than 25pf? Also I looked up a std. 1N4007, I found it to be 15pF, which != Haberman's 50pf?? Mayabe I shouldn't be so picky about these little values but I would like to understand it....I may start off with 0.1uF values, since I have a few. Maybe they may fit with my L measurments above, let's c. I Plan to measure the ringing later when the power supply is built. Any tips on measureing this, e.g. hook the scope up to the output of the bridge?It shouldn't be that hard to change a few caps and resistors if its still bad. My main objective here is more of an educational one. I just wanted to understand how snubbers work on a learning by doing, instead of copying blindly the schematics of others.Also thanx for the tip about Bob Cordell, he seems to quite a knowledgable guy! I will def. check his book out.Best regards,Chris

Last edited by chris2; 18th February 2013 at 10:38 AM.
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Old 18th February 2013, 11:10 AM   #25
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Hi sorry about the prev. post. my browser didn't allow me to edit it correctly!

Here's a more readable version:

Thank you transistormarkj for your detailed analysis, I really appreciate it!

So the basic message is, well actually there's a few here but the first one is:
Measure the secondaries inductance with primary-opened instead of the leakage, cause its 20x larger and approx. the leakage by taking 5% of it.

Yes, I will try it with my 3 capacitors and post the results later. Thanx for the tip!

As far as the snubber network is concerned, I was planning on using 1 parallel capacitor and 1 in series with a resistor, e.g. a la Hagerman. For me, I could prob. assume that my leakage lies in the middle of Haggerman's value and yours, e.g. 925pF. Your assumption of 3x 550, e.g. ~ 1500pF is a bit more. Is bigger better or just safer when one cannot measure it exactly?

What about the capacitance of the diodes in a fullwave rectifier? Haberman, mentions it and approximates 50 pF according to the datasheet. For two in series he says its half, so wouldn't that be than 25pf?

Also I looked up a std. 1N4007, I found it to be 15pF, which doesn't equal Haberman's 50pf??

Maybe I shouldn't be so picky about these little values but I would like to understand it.

I may start off with 0.1uF values, since I have a few. Maybe they will fit with my L measurments above, let's c.

I Plan to measure the ringing later when the power supply is built. Any tips on measureing this, e.g. hook the scope up to the output of the bridge?

It shouldn't be that hard to change a few caps and resistors if its required.

My main objective here is more of an educational one. I just wanted to understand how snubbers work on a learning by doing example, instead of copying blindly the schematics of others.

Also thanx for the tip about Bob Cordell, he seems to quite a knowledgable guy! I will def. check his book out.

Best regards,
Chris
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Old 18th February 2013, 03:19 PM   #26
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Hi,

ok i made some induction measurements on the secondary with various capacitors.

assumptions:
-primary opened
-used prev. resonance freq. equations to det. it

1. without any Cx or extra capacitor:
resonant freq.: 9.2031 kHz

2. with Cx=0.01uF:
resonant freq.: 5.326 kHz, calc. inductance: 0.06 mH

3. with Cx=0.018:
resonant freq.: 4.0453 kHz, calc. inductance: 0.07 mH

4. with Cx=0.033:
resonant freq.: 3.0247 kHz, calc. inductance: 0.07 mH

The values look a lot better now.

Not sure of the units, are they mH's or H's?

Guess if all is ok, I should go with the 0.07 value. An approx. value for the leakage could be 5% of that value or: 0.0035.

Cheers,
Chris
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Old 18th February 2013, 03:26 PM   #27
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Its a good idea to find the inductance using a capacitor signal generator and a scope.

I have 2 LC meters and neither were every accurate. One meter told me the inductor was 100uH when it was really 10uH ! The other was a bit closer with 60uH when it should be 10uH. This really screwed up the calculations for my half bridge switch mode power supply. Due to the 10uH my power supply just went into over current mode and locked out.
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Old 18th February 2013, 04:14 PM   #28
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Those measurements suggest that Ltotal is about 0.07 Henries, i.e. 70 milliHenries. Using the 5% guesstimate, secondary leakage inductance is likely to be somewhere in the vicinity of 0.0035 Henries, i.e. 3.5 milliHenries.
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Old 2nd March 2013, 01:07 PM   #29
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Hello everyone,

since my measurements seemed to be so random and I was quite unsure of myself or my methods, i wrote a letter to Talema to ask them if they would make some measurements for me.

See the attached file, its an excel sheet in open office format

Transformer is a Talema:
model: RKPT 25218, 25 VA
primaries: 2x 115v can be wired in series for 230v
secondaries: 2x 18v

Equipment used for the measurements:
Waynekerr 3260B

NOTE: the secondary measurements were made with the windings connected together in series, so one must divide those values by 4 to get a per winding value. I've done that for some of them, e.g. Sec. + Sec. Leakage Inductance. See also (Per Winding) caption.

My induction measurements of 0.07mH@3 kHz were "way" off compared to his. He measured 0.41H@1kHz or 410mH!?

Maybe for such transformers with 2x Primareis + 2x Secondaries, both Secondaries must be tied together in order to get correct measurements?


I will remeasure with both secondaries connected to see if my results are more comparable to his!

Cheers,
Chris
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Old 2nd March 2013, 01:12 PM   #30
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Sorry, guess it won't accept my file.

Here's a screen shot...


Cheers,
Chris
Attached Images
File Type: jpg talema_measurements.jpg (83.3 KB, 66 views)
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