John Curl's Blowtorch preamplifier part II

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#4,305
I have experimented with "Hybrid RIAA" in the past. The concept is that the loop feedback is constant across the band and that the phase relationships across the input differential are constant. On paper it looks good but it did nothing useful for me. I prefer to put the 2.2 KHz pole as a passive network after the active section.
 
#4,308
janneman said:
OK I understand that, there's some hysteresis on the BH curve. So switching off at say 1 or 1.5 mS after zero crossing, that would get me close to the point of zero remanence.

Then next switch on at zero crossing. Hmm. Easy, just to change a few constants in my Flowcode program 😉 .

Need to do some measurements though....

jd
Click to expand...

Coming back to an earlier discussion on transformer inrush current, did some measurements with a PIC-driven switcher (which is part of an ongoing super-secret project - stay tuned 😉 ).

The DUT is a 2*200W power amp with a 1.2kVA xformer with 4*33.000uF loading the rectifier. The amp is not driven, so it's only the supply inrush you see.
Two curves, in one I switch on the amplifier at a mains zero-crossing, in the other at the top of the mains waveform. Don't see any significant difference; in each case the inrush peak is around 30 Amps (this is 230V mains of course).
Comments?

jd
 

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#4,310
janneman said:
Coming back to an earlier discussion on transformer inrush current, did some measurements with a PIC-driven switcher (which is part of an ongoing super-secret project - stay tuned 😉 ).

The DUT is a 2*200W power amp with a 1.2kVA xformer with 4*33.000uF loading the rectifier. The amp is not driven, so it's only the supply inrush you see.
Two curves, in one I switch on the amplifier at a mains zero-crossing, in the other at the top of the mains waveform. Don't see any significant difference; in each case the inrush peak is around 30 Amps (this is 230V mains of course).
Comments?

jd
Click to expand...

If I recall correctly, part of the issue is what part of the cycle the transformer was switched off before applying power to it again, as that sets the residual flux condition before turning it back on.
 
#4,311
pooge said:
If I recall correctly, part of the issue is what part of the cycle the transformer was switched off before applying power to it again, as that sets the residual flux condition before turning it back on.
Click to expand...

Yes indeed, good point! In my test case, I always switched the xformer off on a mains zero-crossing. I probably did each test 20 times, probably more, never saw a significant difference, maybe an amp or so.

Please note that in a power amp there is always zero load current at mains zero crossing; the rectifiers are always turned off at that point, except during the first few turn-on cycles as you can see.

jd
 
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#4,312
janneman said:
Yes indeed, good point! In my test case, I always switched the xformer off on a mains zero-crossing. I probably did each test 20 times, probably more, never saw a significant difference, maybe an amp or so.

Please note that in a power amp there is always zero load current at mains zero crossing; the rectifiers are always turned off at that point, except during the first few turn-on cycles as you can see.

jd
Click to expand...

I don't think the issue is the charging of the caps at turn on. It's the current in the transformer, itself. The residual flux creates a condition of high inrush. If I recall, the condition is worse if turned off near top of cycle. Also, turn on at zero point of cycle is at highest slope of waveform.
 
#4,314
pooge said:
I didn't mean to imply that capacitor charging doesn't matter. However, in looking at the transformer, if the transformer core is left near saturation at turn off, turn on can push it into saturation.
Click to expand...

Well, unfortunately (😉 ) that doesn't happen in my case.
If I find some more time, I will experiment by moving the switch-off point along the wave. See how that impacts the inrush.

jd
 
#4,315
janneman said:
Yes indeed, good point! In my test case, I always switched the xformer off on a mains zero-crossing. I probably did each test 20 times, probably more, never saw a significant difference, maybe an amp or so.

Please note that in a power amp there is always zero load current at mains zero crossing; the rectifiers are always turned off at that point, except during the first few turn-on cycles as you can see.

jd
Click to expand...

Question directed in general, not specifically to janneman..

Does anybody have links to papers of any kind which explains why a transformer core inrush current is dependent on the core residual? Measurements would be good, theory would be better..

Cheers, John
 
#4,316
jneutron said:
Question directed in general, not specifically to janneman..

Does anybody have links to papers of any kind which explains why a transformer core inrush current is dependent on the core residual? Measurements would be good, theory would be better..

Cheers, John
Click to expand...

Yes someone posted a paper on that - I'll try to find it.

Edit: it is in German, but has a few understandable figures, even if you can't decipher the text & captions:

http://www.kupferinstitut.de/front_frame/pdf/s193.pdf

.... but this is better:

http://www.imeko.org/publications/wc-2003/PWC-2003-TC4-117.pdf


jd
 
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#4,319
pooge said:
Google is your friend.

Type: transformer inrush saturation

Plenty of papers.

Note that toroids are worse because they saturate easier, as usually built.
Click to expand...
Yes, google is indeed my friend.

janneman said:
Yes someone posted a paper on that - I'll try to find it.

Edit: it is in German, but has a few understandable figures, even if you can't decipher the text & captions:

http://www.kupferinstitut.de/front_frame/pdf/s193.pdf

.... but this is better:

http://www.imeko.org/publications/wc-2003/PWC-2003-TC4-117.pdf


jd
Click to expand...

Thank you. Nice articles.

I will be pulling all the reference articles as well.. an interesting topic, this is....I have 90 75kva 3 phase 480 transformers and 150 45kva transformers to worry about, their reaction to a simple cycle or two dropout would be of interest..😱

Cheers, John
 
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