On Mon, 10 Apr 2006 16:01:43 +0200, Wiebe Cazemier
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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