15 May
2007
15 May
'07
1:10 a.m.
On 14 May 2007 at 20:27, Dan wrote:
Anytime you use a variac, it is wise to put a current
limiter in series.
This can be done easily with an incandescent bulb (about 60W or 75W).
This prevents any surge entering the circuit if there is a short. The
surge in power cause the lamp to glow very bright which creates high
resistance thus reducing the current. When a unit is functioning
properly without any shorts at nominal voltage, the bulb will just have
a faint dim.This is the oldest trick in the book.
Not so fast...
If you'll re-read my original post on this, my question related not
to using a current limiter in series with the variac, but using one
in lieu of the variac. As many have pointed out, running a SMPSU
from a variac may not produce the desired result.
In fact, I'm wondering if an incandescent with a tungsten filament
alone is best for this, rather than, say a carbon resistor with a
negative temperature coefficient of resistance, which might
ameliorate the inrush current. One could then use an incandescent
for current limiting once the inrush problem had been dealt with.
Any thoughts on this?
Cheers,
Chuck