[Amps] Filament Inrush Control

[Phil Clements]

> The delay isn't really a key parameter. The big current surge is over
> within a 3-4 mains cycles. After that, the current slowly ramps down, so
> you can short out the series resistor as soon as you like. When the
> relay pulls in, there is a second current surge - a step-up in current
> to the normal standby level.

When I design from scratch with a typical step-start circuit, I adjust the
delay on the relay until the reading on the HV meter is 2/3 the way up
to normal value. By this time, all surges are over, and the filter cap(s)
are almost charged. This assures that the step-start is not operating too
fast, nor is it hanging in too long, heating up the resistor(s) in series
with the a.c. mains. This saves a lot of set-up time if the power source
for the relay comes from other than the transformer being step-started.

Just one of many ways to end up with an effective circuit.

(((73)))
Phil, K5PC