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Old 15th April 2010, 11:49 AM   #4121
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Joachim, your message box is full, please empty it, so I can send messages, or send me an email with your email address (preferred).

(Sorry for being of topic, John)


Regards,
Sigurd

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Here is a picture of how the LC Audio DC is actually build.
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Old 15th April 2010, 12:18 PM   #4122
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I emptied my box.
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Old 15th April 2010, 02:49 PM   #4123
Jakob2 is offline Jakob2  Germany
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The material for mains transformer cores is magnetically soft, but that only means that the coercive field strength needed is low to keep the losses in each cycle low.

But nevertheless the core stays at a certain remanence level after switching off the mains voltage. And it depends on the point on the cycle where this level would be.

The mains voltage waveform droves the core through the complete hystheresis curve and the absolute worst case is if the mains voltage during the switch on tries to further magnetise the core in the same direction where it did remain during the switch off.
The core will quickly saturate in this case.

Combine that with a transformer with very low losses (i.e. a toroid transformer) and youŽll get the highest possible inrush current if switched on in the zero crossing moment.

@ janneman,

there is a nice pdf from the german "kupferinstitut" which described the situation in the case for mains voltage dc levels; unfortunately written in german, but even so the graphs are very informative:

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

The interesting part starts at page 11/12 in the pdf.

If the transformer core saturates partly during the mains voltage cycle the transformer normally not only starts to hum but the stray fields become very disturbing too.

Wishes

Last edited by Jakob2; 15th April 2010 at 02:52 PM.
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Old 15th April 2010, 04:11 PM   #4124
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Quote:
Originally Posted by Jakob2 View Post
[snip]@ janneman,

there is a nice pdf from the german "kupferinstitut" which described the situation in the case for mains voltage dc levels; unfortunately written in german, but even so the graphs are very informative:

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

The interesting part starts at page 11/12 in the pdf.

If the transformer core saturates partly during the mains voltage cycle the transformer normally not only starts to hum but the stray fields become very disturbing too.

Wishes
German is my 2nd language (after English), thanks for the link.

Another question, if I may:

Power transformers for amplifiers only deliver power to a load at or near the top of the mains voltage; that's when the rectifier diodes start to conduct, and they switch off again after a small part of the cycle (40-60 degrees max).
So, doesn't that mean that at the mains zero crossing, by definition the load on the transformer is infinite and there's no secondary (and thus no primary) current?

jd
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Old 16th April 2010, 10:20 AM   #4125
Jakob2 is offline Jakob2  Germany
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Quote:
Originally Posted by janneman View Post
German is my 2nd language (after English), thanks for the link.
I remembered that from the earlier exchange.

Quote:
Another question, if I may:

Power transformers for amplifiers only deliver power to a load at or near the top of the mains voltage; that's when the rectifier diodes start to conduct, and they switch off again after a small part of the cycle (40-60 degrees max).
So, doesn't that mean that at the mains zero crossing, by definition the load on the transformer is infinite and there's no secondary (and thus no primary) current?

jd
Do you mean that question in relation to the "dc problem" or the "inrush current case" or more for transformer in use with conventional rectifying and filter circuits in general?

Wishes
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Old 16th April 2010, 10:44 AM   #4126
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Quote:
Originally Posted by Jakob2 View Post
Do you mean that question in relation to the "dc problem" or the "inrush current case" or more for transformer in use with conventional rectifying and filter circuits in general?

Wishes
The context is remote switch-on of power amps. I also used to do that at mains zero-crossing, because I know that the transformer secondary current is zero at that point. Not sure what the primary current is at that point.

jd
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Old 16th April 2010, 12:09 PM   #4127
Jakob2 is offline Jakob2  Germany
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Quote:
Originally Posted by janneman View Post
The context is remote switch-on of power amps. I also used to do that at mains zero-crossing, because I know that the transformer secondary current is zero at that point. Not sure what the primary current is at that point.

jd
I see, that relates directly to the worst case i did mention before.
If the core was left for example halfways magnetised (the remanence) in the same direction in which it will be further magnetised during the first part of the mains cycle that follows the switch on in the zero crossing, then the core will saturate and left is approximately an air coil with the dc resistance of the primary winding.
Not a problem with small transfomers but if it gets to big toroidals with just a dc resistance of the primary of ~2-3 Ohms.....

Even switching on at the peak of the mains voltage does not fully solve the problem but betters the peak of the inrush current.

If you have not to take into account short drop outs of the mains, then a ntc in combination with a relay still works (despite the problem that it needs some time to cool down, so you have to wait between sequential switching).

Wishes
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Old 17th April 2010, 04:08 PM   #4128
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@Jakob2: thanks for the link to this pdf, great reading!

Have fun, Hannes
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Old 17th April 2010, 05:30 PM   #4129
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Quote:
Originally Posted by Jakob2 View Post
I see, that relates directly to the worst case i did mention before.
If the core was left for example halfways magnetised (the remanence) in the same direction in which it will be further magnetised during the first part of the mains cycle that follows the switch on in the zero crossing, then the core will saturate and left is approximately an air coil with the dc resistance of the primary winding.
Not a problem with small transfomers but if it gets to big toroidals with just a dc resistance of the primary of ~2-3 Ohms.....

Even switching on at the peak of the mains voltage does not fully solve the problem but betters the peak of the inrush current.

If you have not to take into account short drop outs of the mains, then a ntc in combination with a relay still works (despite the problem that it needs some time to cool down, so you have to wait between sequential switching).

Wishes
OK, but how do I know what the remanence and direction is at switch-off? If I also swicht off always at zero crossing, when the load current is zero, does that not solve the problem?

jd
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Old 17th April 2010, 06:46 PM   #4130
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Originally Posted by janneman View Post
OK, but how do I know what the remanence and direction is at switch-off? If I also swicht off always at zero crossing, when the load current is zero, does that not solve the problem?

jd
No,

The BH loop does not drop to zero when you remove the drive. It actually requires the drive to drop a bit in the other direction before it erases the remnant field. However it is lower than if you switched off at maximum drive.
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