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 I²t (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 I²t.