On Fri, 14 Apr 2006 00:28:11 +0200, Wiebe Cazemier
wrote:

On Thursday 13 April 2006 08:23, Ross Herbert wrote:
Sorry, that url was not correct. This one appears to work (very long
url watch wrap)
[long url]

This URL contains a session which now longer exists. But, I found it by
clicking through from the first link you gave me.

Anyway, I have two of them in use now, but I'm gonna use a softstarter circuit,
because the NTC's don't reduce resistance... I guess the idle current is too
low.


The NTC will remain high resistance (if you call 5 or 10 ohms max
high) during the first few cycles of the inrush current period at
switch-on no matter what the load is. It's temperature will increase
and the resistance will fall due to the high surge current but if the
load on the DC supply is negligible or non existent then the NTC
resistance will rise again to almost its nominal cold value. However,
one would imagine that a 600VA transformer would normally have an idle
load current of several hundred milliamps (depends on the application
though) which would be sufficient to keep the NTC in its low
resistance range providing you choose the right NTC. The fact that the
idle current is low and the NTC resistance not at its minimum during
low idle current is not really a problem since it has already done its
job in protecting the fuse. When the normal load is subsequently
applied the current drawn will definitely cause the NTC to go to its
minimum value and that is what you want to happen.

If the NTC method isn't to your liking then you will have to use a
delayed switch-on circuit which uses a relay contact to short out a
resistor in the primary winding a couple of seconds after switch-on.

A typical delay circuit such as shown here
http://www.jcscript.de/projects/eaton.html could be used where the
relay contact will short circuit say a 10 ohm 10W resistor in the
primary circuit of the transformer shortly after switch-on thus
preventing the surge current from damaging anything.