[Amps] Step start

[David Cutter]

I have a 240V 15A variac which would consistently blow a 13A plug fuse at 
switch on, unloaded.  The spec for these plug fuses states they must blow in 
30s with 2.1 x nominal current applied, ie >27A: these always blew 
instantly.  Slow blow fuses did not work either.  The variac looks like a 
short across the mains until the magnetising current is established. 
(Large power supplies have large transformers which look similar). My simple 
cure was to fit a mains relay across the variac and 2W 1k resistor in series 
with the supply to both the relay and variac.  The relay contacts shorted 
the resistor.  The inherent time delay within the relay allowed the 
magnetising current to rise slowly enough to avoid that large surge.  I 
don't know how vulnerable filaments are to switch on surge, I don't recall 
reading anything about this in valve specs, but it's been a while.

David
G3UNA



> It does not matter for the HV caps how fast they are loaded.
> The reason for a step-start circuit is to limit the supply current to a
> value below that of a typical household circuit breaker.
> As long the step-start resistor is in circuit the HV cap will never reach
> full voltage.
>
> 73
> Peter