Howdy,

An NTC (or two) should work well in the transformer primary.
It's a common means for reducing inrush at the input of SMPS.
Works fine for linear supplies too. There are numerous
manufacturers of these world wide.
http://www.rti-corp.com/Electronics/surggard.htm

Typical numbers for magnetizing current are 1-5% of the
rated load at steady state. And as much as 10-20 times
the load current for inrush. I stated 5 times for inrush
earlier. It's more than that for commodity (cheaply made)
type transformers.

You probably have some load on the transformer, bias power
for the regulator, bleeder resistors and so forth. So I'm
guessing the idling current is more than the magnetizing
current alone. If the transformer is designed for ~1KVA
50A*13.6V times some factor for the regulator efficiency
say 0.7 at full load, that's about 970W, then 28W loss at
no load isn't unreasonably high assuming the core loss equals
the copper loss at full load.

The iffy insulation is a good argument for having the
output return of the power supply securely bonded to
earth ground.

Fence wire in water should make an excellent load. I've
never heard of anyone testing an RS-50 at full load.
You might want to sneak up on it. A working 25A supply
is better than a dead 50A supply!


73
Grumpy



Highland Ham wrote in
:

=======================================
Haven't come across these ,at least not under this name. I understand
they are Power NTC resistors .
Wonder if these would heat-up quickly enough within the 'start-up
period' to prevent a current surge.

In the meantime I have tested my Astron RS-50A PSU.
When switched on at 230 V AC (50hz) it sometimes trips the circuit
breaker and is rather heavily buzzing ,but when the voltage is increased
to that value from zero with a variac the buzzing level is lower .
No-load current is 0.69 Ampere , Power is 28 watts as measured with a
multifunction energy monitor.
Having had the unit running no-load for 1 day the buzzing level is
acceptable with the transformer being luke warm.

I now intend to include a simple soft start unit ( 12V relay with 1-2
seconds switch-on delay which shorts a series 50 Ohms-50W resistor (from
junk box) in the primary transformer winding line.

Comments on quality of unit :
-Insulation between primary and secondary winding of transformer looks
rather 'iffy'

- Output connection posts are of the type normally specified for 30
Amperes max.
I shall replace these by more heavy duty posts.

Having done that I intend to test the unit on rated capacity with 4
(approx 1 Ohm) resistors made of iron fencing wire , put in a bucket
with water.

Frank GM0CSZ / KN6WH