inrush current

[Wiebe Cazemier]

Hi,

A friend of mine is trying to repair an active woofer of a speaker set, which
started to blow out the primary fuse suddenly. The problem would appear to be
in the torroidal transformer, because even when nothing is connected, the fuse
blows. We decided to be stupid and put in a 2A instead of 1A fuse. It didn't
blow, and the primary current was only 20 mA. All secondary voltages were
normal. (two assymetric outputs, and one symmetric with center tap).

So, it would appear the inrush current is very high. But, because the device
worked normally in the past, it would appear that the inrush current suddenly
increased. Is this possible? To be more precise, is this possible when all the
secondary voltages are normal? If so, how?

Thanks in advance.

 

Wiebe Cazemier wrote:

Hi,

A friend of mine is trying to repair an active woofer of a speaker set, which
started to blow out the primary fuse suddenly. The problem would appear to be
in the torroidal transformer, because even when nothing is connected, the fuse
blows. We decided to be stupid and put in a 2A instead of 1A fuse. It didn't
blow, and the primary current was only 20 mA. All secondary voltages were
normal. (two assymetric outputs, and one symmetric with center tap).

So, it would appear the inrush current is very high. But, because the device
worked normally in the past, it would appear that the inrush current suddenly
increased. Is this possible? To be more precise, is this possible when all the
secondary voltages are normal? If so, how?

Possibility: original fuse was 1A slow-blow. It died because of
"normal" fatigue. Or of some temporary overload that cleared.

Then it was replaced with a 1A fast-blow. This seems consistent with
your description of the problem (especially as you leave out the very
important facts of slow-blow vs fast-blow).

Slow-blow fuses for capacitor-input DC filters are the norm, not the
exception.

Look carefully with the factory fuse or if not available the markings
saying what sort of fuse is supposed to go there.

Also, even though the average primary current may be 20mA, what
actually matters to the fuse is something closer to the RMS current,
and the current waveform is often wildly non-sinusoidal. A factor of 50
to 100 between average and RMS seems implausable, though with no load
(capacitor charging only on the very tip of waveform) it may actually
be the ratio.

Tim.

 

[Phil]

Wiebe Cazemier wrote:

Hi,

A friend of mine is trying to repair an active woofer of a speaker set, which
started to blow out the primary fuse suddenly. The problem would appear to be
in the torroidal transformer, because even when nothing is connected, the fuse
blows. We decided to be stupid and put in a 2A instead of 1A fuse. It didn't
blow, and the primary current was only 20 mA. All secondary voltages were
normal. (two assymetric outputs, and one symmetric with center tap).

So, it would appear the inrush current is very high. But, because the device
worked normally in the past, it would appear that the inrush current suddenly
increased. Is this possible? To be more precise, is this possible when all the
secondary voltages are normal? If so, how?

Thanks in advance.

Apart from the slow/fast blow advice given elsewhere, check that the
mounting bolt etc for the toroidal isn't causing a 'shorted turn' by
touching the case on both sides.....if the case is metal then any bolt
through the middle of the toroid must only connect with the case on one
side.

Phil.

 

On Mon, 10 Apr 2006 14:24:50 +0200, Wiebe Cazemier <halfgaar@gmx.net>
wrote:

Hi,

A friend of mine is trying to repair an active woofer of a speaker set, which
started to blow out the primary fuse suddenly. The problem would appear to be
in the torroidal transformer, because even when nothing is connected, the fuse
blows. We decided to be stupid and put in a 2A instead of 1A fuse. It didn't
blow, and the primary current was only 20 mA. All secondary voltages were
normal. (two assymetric outputs, and one symmetric with center tap).

So, it would appear the inrush current is very high. But, because the device
worked normally in the past, it would appear that the inrush current suddenly
increased. Is this possible? To be more precise, is this possible when all the
secondary voltages are normal? If so, how?

Thanks in advance.

Seems very unlikely inrush current increased. I use torroids in my
amps and find they do use lots of inrush current (particularly with a
stiff power supply) I fuse them with slow blow fuses and had to
install two power switches - one for each pair of amps, or my clock
and modem reset themselves when I turn on the stereo.

Torroids have better regulation as a rule - so the current required to
charge the supply caps is reflected in the inrush current to a greater
degree than with E-I core transformers.

Does it require a slow blow fuse?

Sounds like you already eliminated the filter caps as a direct source
of the problem?

If you're using the correct fuse type and it still blows, and its been
working all along prior to this, check the amp. Ideally, look at the
current with a scope.. You may have a problem in the bias supply -
coming up unbalanced or very high current due to a bad cap.

A scope on the output terminals will show it coming on in an
unbalanced condition (and a loud speaker thump) - but depending on the
design, that could be normal.
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[Wiebe Cazemier]

On Monday 10 April 2006 15:18, shoppa@trailing-edge.com wrote:

Possibility: original fuse was 1A slow-blow. It died because of
"normal" fatigue. Or of some temporary overload that cleared.

Then it was replaced with a 1A fast-blow. This seems consistent with
your description of the problem (especially as you leave out the very
important facts of slow-blow vs fast-blow).

Slow-blow fuses for capacitor-input DC filters are the norm, not the
exception.

Look carefully with the factory fuse or if not available the markings
saying what sort of fuse is supposed to go there.

I have been thinking along these lines. But because we tried slow blows, the
only thing I can imagine is that the original fuse is extra slow blow, but I
don't know if those things even exist. I'll ask him if the transformer has any
info written on it about special kinds of fuses.

Also, even though the average primary current may be 20mA, what
actually matters to the fuse is something closer to the RMS current,
and the current waveform is often wildly non-sinusoidal. A factor of 50
to 100 between average and RMS seems implausable, though with no load
(capacitor charging only on the very tip of waveform) it may actually
be the ratio.

Perhaps, but there were no filtercaps connected. Going with the "extra slow
blow" theory, perhaps this transformer has some kind of very deformed current
waveform, which the original fuse was designed for.

He doesn't have a scope, but I do. If we can't find the problem, we could bring
it here, and put it on my scope. But I don't think I'm gonna measure the
primary current with it, because I don't know enough about putting scopes on
the mains.

 

[Wiebe Cazemier]

On Monday 10 April 2006 15:35, Phil wrote:

Apart from the slow/fast blow advice given elsewhere, check that the
mounting bolt etc for the toroidal isn't causing a 'shorted turn' by
touching the case on both sides.....if the case is metal then any bolt
through the middle of the toroid must only connect with the case on one
side.

We/he tried with the tranny in the free. It was just lying there on the table.
And, the metal plate to hold it in place in the (wooden) case, has a rubber
sealing of course, so there cannot be any shorts.

 

[Wiebe Cazemier]

On Monday 10 April 2006 15:39, default wrote:

Seems very unlikely inrush current increased. I use torroids in my
amps and find they do use lots of inrush current (particularly with a
stiff power supply) I fuse them with slow blow fuses and had to
install two power switches - one for each pair of amps, or my clock
and modem reset themselves when I turn on the stereo.

I have contructed a four channel power amp with a 600 VA torodial myself. I use
two special NTC's in series with the primary coil to limit inrush current. Is
that no solution for you?

Torroids have better regulation as a rule - so the current required to
charge the supply caps is reflected in the inrush current to a greater
degree than with E-I core transformers.

Does it require a slow blow fuse?

Sounds like you already eliminated the filter caps as a direct source
of the problem?

If you're using the correct fuse type and it still blows, and its been
working all along prior to this, check the amp. Ideally, look at the
current with a scope.. You may have a problem in the bias supply -
coming up unbalanced or very high current due to a bad cap.

As I said, the fuse also blows with nothing connected to the transformer. It's
tjust the tranny itself which is causing it to blow, not even filter caps.

A scope on the output terminals will show it coming on in an
unbalanced condition (and a loud speaker thump) - but depending on the
design, that could be normal.

I thought speaker thumps were the result of the amp's inability to cope with
low voltages. A trick in poweramp design is employing a constant current
source to avoid that, if I'm not mistaken.

 

On Mon, 10 Apr 2006 16:01:43 +0200, Wiebe Cazemier <halfgaar@gmx.net>
wrote:

On Monday 10 April 2006 15:39, default wrote:

Seems very unlikely inrush current increased. I use torroids in my
amps and find they do use lots of inrush current (particularly with a
stiff power supply) I fuse them with slow blow fuses and had to
install two power switches - one for each pair of amps, or my clock
and modem reset themselves when I turn on the stereo.

I have contructed a four channel power amp with a 600 VA torodial myself. I use
two special NTC's in series with the primary coil to limit inrush current. Is
that no solution for you?

That would work, but sort of defeats the purpose of a super stiff

supply. I have four 100 watt amps with banks of computer grade caps
feeding photo flash low ESR caps in the amps. Four transformers, one
per channel.

I could use some NTC thermistors with time delay relays to short them
after the supplies charged - just never got around to it. The two
switch technique works well enough.

It would be an excuse to add some more pilot lights and if I used a
DPDT relays, I could shunt the caps to pairs of light bulbs to show
the caps charge and discharge . . . "but that way insanity lies," and
it wouldn't do anything for the sound.

Torroids have better regulation as a rule - so the current required to
charge the supply caps is reflected in the inrush current to a greater
degree than with E-I core transformers.

Does it require a slow blow fuse?

Sounds like you already eliminated the filter caps as a direct source
of the problem?

If you're using the correct fuse type and it still blows, and its been
working all along prior to this, check the amp. Ideally, look at the
current with a scope.. You may have a problem in the bias supply -
coming up unbalanced or very high current due to a bad cap.

As I said, the fuse also blows with nothing connected to the transformer. It's
tjust the tranny itself which is causing it to blow, not even filter caps.

I still can't conceive of a way that a transformer can suddenly

develop a problem with inrush current without increasing the
excitation current. 20 milliamps sounds high for a torroid, but still
reasonable. Do you have another torroid to compare it to? Leave it
powered up and check the heat? Shorted turn perhaps?

I have a small 120 VA supply with a torroidal transformer on my
workbench. The inrush current is well over an amp and excitation
current, with just the filter caps is ~ point four milliamps.

My supply has a mil spec 1 amp magnetic circuit breaker on the 120
input and it will pop that about half the time from a cold start -
it has a variac input so if I turn it up from zero the circuit breaker
holds. I'm sure a 1 amp slow blow would work since this magnetic
breaker is very fast and holds only to 1.2 amps. 20 volt no load
output with a 10,000 mfd cap.

A scope on the output terminals will show it coming on in an
unbalanced condition (and a loud speaker thump) - but depending on the
design, that could be normal.

I thought speaker thumps were the result of the amp's inability to cope with
low voltages. A trick in poweramp design is employing a constant current
source to avoid that, if I'm not mistaken.

Constant current in the diff amp helps if that's the cause, but bias

supplies and mismatched output transistors are another cause. With a
one amp fuse in a subwoofer system I figure we're talking about a
relatively inexpensive amp in something like a computer system.

But you eliminated the amp already so that only leaves the
transformer. Right?
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[I.F.]

<default> wrote in message
news:1etk32lhc3baudc23grqb8p824g3bp0bi4@4ax.com...

On Mon, 10 Apr 2006 16:01:43 +0200, Wiebe Cazemier <halfgaar@gmx.net
wrote:

On Monday 10 April 2006 15:39, default wrote:

Seems very unlikely inrush current increased. I use torroids in my
amps and find they do use lots of inrush current (particularly with a
stiff power supply) I fuse them with slow blow fuses and had to
install two power switches - one for each pair of amps, or my clock
and modem reset themselves when I turn on the stereo.

I have contructed a four channel power amp with a 600 VA torodial myself.
I use
two special NTC's in series with the primary coil to limit inrush current.
Is
that no solution for you?

That would work, but sort of defeats the purpose of a super stiff
supply. I have four 100 watt amps with banks of computer grade caps
feeding photo flash low ESR caps in the amps. Four transformers, one
per channel.

I could use some NTC thermistors with time delay relays to short them
after the supplies charged - just never got around to it. The two
switch technique works well enough.

It would be an excuse to add some more pilot lights and if I used a
DPDT relays, I could shunt the caps to pairs of light bulbs to show
the caps charge and discharge . . . "but that way insanity lies," and
it wouldn't do anything for the sound.


Torroids have better regulation as a rule - so the current required to
charge the supply caps is reflected in the inrush current to a greater
degree than with E-I core transformers.

Does it require a slow blow fuse?

Sounds like you already eliminated the filter caps as a direct source
of the problem?

If you're using the correct fuse type and it still blows, and its been
working all along prior to this, check the amp. Ideally, look at the
current with a scope.. You may have a problem in the bias supply -
coming up unbalanced or very high current due to a bad cap.

As I said, the fuse also blows with nothing connected to the transformer.
It's
tjust the tranny itself which is causing it to blow, not even filter caps.

I still can't conceive of a way that a transformer can suddenly
develop a problem with inrush current without increasing the
excitation current. 20 milliamps sounds high for a torroid, but still
reasonable. Do you have another torroid to compare it to? Leave it
powered up and check the heat? Shorted turn perhaps?

I have a small 120 VA supply with a torroidal transformer on my
workbench. The inrush current is well over an amp and excitation
current, with just the filter caps is ~ point four milliamps.

My supply has a mil spec 1 amp magnetic circuit breaker on the 120
input and it will pop that about half the time from a cold start -
it has a variac input so if I turn it up from zero the circuit breaker
holds. I'm sure a 1 amp slow blow would work since this magnetic
breaker is very fast and holds only to 1.2 amps. 20 volt no load
output with a 10,000 mfd cap.


A scope on the output terminals will show it coming on in an
unbalanced condition (and a loud speaker thump) - but depending on the
design, that could be normal.

I thought speaker thumps were the result of the amp's inability to cope
with
low voltages. A trick in poweramp design is employing a constant current
source to avoid that, if I'm not mistaken.

Constant current in the diff amp helps if that's the cause, but bias
supplies and mismatched output transistors are another cause. With a
one amp fuse in a subwoofer system I figure we're talking about a
relatively inexpensive amp in something like a computer system.

But you eliminated the amp already so that only leaves the
transformer. Right?
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There is in fact a commercially available chip that switches out an inrush
limiting resistor/NTC after a delay.

 

[Scott Lane]

"Wiebe Cazemier" <halfgaar@gmx.net> wrote in message
news:e1diqi$47a$1@news4.zwoll1.ov.home.nl...

Hi,

A friend of mine is trying to repair an active woofer of a speaker set,
which
started to blow out the primary fuse suddenly. The problem would appear to
be
in the torroidal transformer, because even when nothing is connected, the
fuse
blows. We decided to be stupid and put in a 2A instead of 1A fuse. It
didn't
blow, and the primary current was only 20 mA. All secondary voltages were
normal. (two assymetric outputs, and one symmetric with center tap).

So, it would appear the inrush current is very high. But, because the
device
worked normally in the past, it would appear that the inrush current
suddenly
increased. Is this possible? To be more precise, is this possible when all
the
secondary voltages are normal? If so, how?

Thanks in advance.
Sounds like a shorted woofer coil. Sometimes they won't show a short until

you 'push' on the cone, or until it moves when you apply signal to it.
Scott

 

[Arfa Daily]

"Scott Lane" <fcctech@yahoo.com> wrote in message
news:Blx_f.29$Ar5.23@fe02.lga...

"Wiebe Cazemier" <halfgaar@gmx.net> wrote in message
news:e1diqi$47a$1@news4.zwoll1.ov.home.nl...
Hi,

A friend of mine is trying to repair an active woofer of a speaker set,
which
started to blow out the primary fuse suddenly. The problem would appear
to be
in the torroidal transformer, because even when nothing is connected, the
fuse
blows. We decided to be stupid and put in a 2A instead of 1A fuse. It
didn't
blow, and the primary current was only 20 mA. All secondary voltages were
normal. (two assymetric outputs, and one symmetric with center tap).

So, it would appear the inrush current is very high. But, because the
device
worked normally in the past, it would appear that the inrush current
suddenly
increased. Is this possible? To be more precise, is this possible when
all the
secondary voltages are normal? If so, how?

Thanks in advance.
Sounds like a shorted woofer coil. Sometimes they won't show a short until
you 'push' on the cone, or until it moves when you apply signal to it.
Scott

I rather think not, as if you read the whole thread, you will see that the

OP has carefully stated, on more than one occasion, that the fuse is blowing
with absolutely nothing connected, except the transformer primary ...

Arfa

 

[Ross Herbert]

On Mon, 10 Apr 2006 14:24:50 +0200, Wiebe Cazemier <halfgaar@gmx.net>
wrote:

Hi,

A friend of mine is trying to repair an active woofer of a speaker set, which
started to blow out the primary fuse suddenly. The problem would appear to be
in the torroidal transformer, because even when nothing is connected, the fuse
blows. We decided to be stupid and put in a 2A instead of 1A fuse. It didn't
blow, and the primary current was only 20 mA. All secondary voltages were
normal. (two assymetric outputs, and one symmetric with center tap).

So, it would appear the inrush current is very high. But, because the device
worked normally in the past, it would appear that the inrush current suddenly
increased. Is this possible? To be more precise, is this possible when all the
secondary voltages are normal? If so, how?

Thanks in advance.

Depending upon the manufacturing method and materials used the inrush
current can be as much as 50 times the normal primary current for 1/2
cycle averaging 15 times for about 2 cycles.

See this paper
http://www.emeko.de/en/pdf/02-01-Introduction-reasons-for-transformer-inrush/02-01-10-01.pdf

 

[Wiebe Cazemier]

On Monday 10 April 2006 17:54, default wrote:

I have contructed a four channel power amp with a 600 VA torodial myself. I
use two special NTC's in series with the primary coil to limit inrush
current. Is that no solution for you?

That would work, but sort of defeats the purpose of a super stiff
supply.

These NTC's get a very low resistance once they are warm. In believe it was in
the order of 0.02 Ohms or something. The only thing I don't understand is,
that they don't feel warm, even when the amp has been on for a few minutes.
And, when the entire casing of the amp has warmed up, therefore the NTC's as
well, and I turn it off and back on immediatly, there is nothing that
indicates a high inrush current. Even my bench power supply has a higher
inrush current, because sometimes one of my computer resets when I turn it on
(and sometimes even when I turn it off...)

This reasoning has me doubting a bit if the resistance really does get very
low. Also because perhaps in most devices, a 600VA transformer normally has a
higher minimal/standy current than what I'm using it for (the amp in idle only
pulls about 25 watts of the mains).

I'll see if I can measure the resitance when they're cold, and when the amp has
been warmed up.

I could use some NTC thermistors with time delay relays to short them
after the supplies charged - just never got around to it. The two
switch technique works well enough.

Until someone who doesn't know how to operate it comes along...

I still can't conceive of a way that a transformer can suddenly
develop a problem with inrush current without increasing the
excitation current. 20 milliamps sounds high for a torroid, but still
reasonable. Do you have another torroid to compare it to? Leave it
powered up and check the heat? Shorted turn perhaps?

I don't have a torroidal to test with, but I'll ask if he does. I'll ask if he
can measure the idle current, and check for heat on the "broken" one.

I have a small 120 VA supply with a torroidal transformer on my
workbench. The inrush current is well over an amp and excitation
current, with just the filter caps is ~ point four milliamps.

0.4 mA? That is 0.0004 A. My DM can't even measure that, because you have to
use the 10A unfused connection, which is only accurate to the mA.

BTW, I got a reply to my question I asked him if the transformer said anything
about what kind of fuse it required, other than the fact it needs to be slow
blow. It doesn't say anything. But, he also mentioned that Amplimo
(transformer manufacturer) requires fuses with a high I˛t (I*i*t for those who
cannot see the squared sign). And, also according to
http://www.circuitprotection.ca/fuseology.html there are different kind of
slow blows. He's asking the local electronics shop for advice, perhaps they
indeed have a fuse for him with an extra high I˛t.

 

[Wiebe Cazemier]

On Monday 10 April 2006 18:52, I.F. wrote:

There is in fact a commercially available chip that switches out an inrush
limiting resistor/NTC after a delay.

Do you also know it's typenumber? I think that would be a great solution.

 

[Wiebe Cazemier]

On Tuesday 11 April 2006 08:57, Ross Herbert wrote:

Depending upon the manufacturing method and materials used the inrush
current can be as much as 50 times the normal primary current for 1/2
cycle averaging 15 times for about 2 cycles.

See this paper

http://www.emeko.de/en/pdf/02-01-Introduction-reasons-for-transformer-inrush/02-01-10-01.pdf

Thanks, I'll have a look at it.

 

[default]

On Tue, 11 Apr 2006 11:59:42 +0200, Wiebe Cazemier <halfgaar@gmx.net>
wrote:

0.4 mA? That is 0.0004 A. My DM can't even measure that, because you have to
use the 10A unfused connection, which is only accurate to the mA.

I have to creep up with the voltage or it eats the fuse in my meter.
Two of the supposed advantages of torroidal cores is high efficiency
and good regulation - but all torroids aren't created equal, and
industry can pervert anything to save a buck

I can't see the label without disassembling the supply but I can see
that it was made in the US. I have another torroidal in a DC supply
that runs some fans - came out of a surplus motorized wheelchair
charger. It sucks down 25 watts or so just sitting idle - guessing
from the heat (idle current isn't a great predictor of power used
since it is reactive)

I prefer torroidal transformers for my tinkering - easy to add a buck
or boost winding or whole new secondary to tweak it for a particular
application.

BTW, I got a reply to my question I asked him if the transformer said anything
about what kind of fuse it required, other than the fact it needs to be slow
blow. It doesn't say anything. But, he also mentioned that Amplimo
(transformer manufacturer) requires fuses with a high I˛t (I*i*t for those who
cannot see the squared sign). And, also according to
http://www.circuitprotection.ca/fuseology.html there are different kind of
slow blows. He's asking the local electronics shop for advice, perhaps they
indeed have a fuse for him with an extra high I˛t.

Interesting site. I didn't know about the time delay distinction
and just lumped them with the slow blow types. Or at least didn't
know about it in electronics applications - the motor starters I'm
used to usually employ specific time delay fuses for their application
and environment.



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[Wiebe Cazemier]

On Tuesday 11 April 2006 15:48, default wrote:

I can't see the label without disassembling the supply but I can see
that it was made in the US. I have another torroidal in a DC supply
that runs some fans - came out of a surplus motorized wheelchair
charger. It sucks down 25 watts or so just sitting idle - guessing
from the heat (idle current isn't a great predictor of power used
since it is reactive)

The 25 watts I determined my poweramp uses, was determined with a special
device, capable of keeping the angle between current and voltage into account,
so I guess that's acurate. But, it's not the transformer alone which does
that, it's the quiescent current of four amps.

I prefer torroidal transformers for my tinkering - easy to add a buck
or boost winding or whole new secondary to tweak it for a particular
application.

My electronics is mainly focussed on audio, and the small magnetic field of
torroids is a good advantage for that. And, they are easily mountable.

Interesting site. I didn't know about the time delay distinction
and just lumped them with the slow blow types. Or at least didn't
know about it in electronics applications - the motor starters I'm
used to usually employ specific time delay fuses for their application
and environment.

I guess there is a normal type of slow blow, because I hardly ever see/hear
anybody talk about the I˛t requirements of fuses.

 

[Wiebe Cazemier]

On Tuesday 11 April 2006 11:59, Wiebe Cazemier wrote:

These NTC's get a very low resistance once they are warm. In believe it was
in the order of 0.02 Ohms or something. The only thing I don't understand is,
that they don't feel warm, even when the amp has been on for a few minutes.
And, when the entire casing of the amp has warmed up, therefore the NTC's as
well, and I turn it off and back on immediatly, there is nothing that
indicates a high inrush current. Even my bench power supply has a higher
inrush current, because sometimes one of my computer resets when I turn it on
(and sometimes even when I turn it off...)

This reasoning has me doubting a bit if the resistance really does get very
low. Also because perhaps in most devices, a 600VA transformer normally has a
higher minimal/standy current than what I'm using it for (the amp in idle
only pulls about 25 watts of the mains).

I'll see if I can measure the resitance when they're cold, and when the amp
has been warmed up.

I have results. The NTC's are about 9 Ohms total when cold. And they don't
decrease by any usable amount when warm (well, as far as they get warm)... On
to find a better solution...

About this. Ever since the beginning, there is a slight 50 Hz hum in the
output, almost inaudible, you have to put your head against the speaker to
hear it. There are no harmonics, it's just 50 Hz. Could this be caused by
these NTC's, that the supply ripple is getting to high? How much ripple can an
amp with good common mode rejection ratio handle?

The filterbanks are 4x4700 ľF, positive ripple is about 300 mV, negative 260.
Transformer is 25-0-25, so DC is about +35 and -35. That ripple does seem
quite high. The difference in ripple can be explained because the positive
rail is used to drive 4 relays.

The hum is not present when the amp is powered from my bench supply.

 

[I.F.]

"Wiebe Cazemier" <halfgaar@gmx.net> wrote in message
news:e1guvs$inq$1@news5.zwoll1.ov.home.nl...

On Tuesday 11 April 2006 11:59, Wiebe Cazemier wrote:
These NTC's get a very low resistance once they are warm. In believe it
was
in the order of 0.02 Ohms or something. The only thing I don't understand
is,
that they don't feel warm, even when the amp has been on for a few
minutes.
And, when the entire casing of the amp has warmed up, therefore the NTC's
as
well, and I turn it off and back on immediatly, there is nothing that
indicates a high inrush current. Even my bench power supply has a higher
inrush current, because sometimes one of my computer resets when I turn
it on
(and sometimes even when I turn it off...)

This reasoning has me doubting a bit if the resistance really does get
very
low. Also because perhaps in most devices, a 600VA transformer normally
has a
higher minimal/standy current than what I'm using it for (the amp in idle
only pulls about 25 watts of the mains).

I'll see if I can measure the resitance when they're cold, and when the
amp
has been warmed up.

I have results. The NTC's are about 9 Ohms total when cold. And they don't
decrease by any usable amount when warm (well, as far as they get warm)...
On
to find a better solution...

About this. Ever since the beginning, there is a slight 50 Hz hum in the
output, almost inaudible, you have to put your head against the speaker to
hear it. There are no harmonics, it's just 50 Hz. Could this be caused by
these NTC's, that the supply ripple is getting to high? How much ripple
can an
amp with good common mode rejection ratio handle?

The filterbanks are 4x4700 ľF, positive ripple is about 300 mV, negative
260.
Transformer is 25-0-25, so DC is about +35 and -35. That ripple does seem
quite high. The difference in ripple can be explained because the positive
rail is used to drive 4 relays.

The hum is not present when the amp is powered from my bench supply.

The NTC thermistors I've salvaged from a whole range of scrap PC monitors
have ranged from a few hundred Ohms to as high as 12k at room temperature,
the running resistance is usually 5 Ohms or less.

 

[I.F.]

"Wiebe Cazemier" <halfgaar@gmx.net> wrote in message
news:e1fup5$94o$2@news3.zwoll1.ov.home.nl...

On Monday 10 April 2006 18:52, I.F. wrote:
There is in fact a commercially available chip that switches out an
inrush
limiting resistor/NTC after a delay.

Do you also know it's typenumber? I think that would be a great solution.

Its among 20 or so Gb of data sheets on my other PC, but next time I'm there
I'll see if I can remember which folder to look in!