[Amps] Sort Start

[Ian White G/GM3SEK]

Larry Carman wrote:
>The Henry RF generator using a 4 KW power supply at 4000 VDC and a filter
>choke
>only use a .5 second soft start. The 1/2 second delay seems to be plenty
>even
>with 30 mf of caps and no filter choke.  I've been running this arrangement
>or years. Too much soft start in certain instances will cause the bypass
>relays
>to energize too slow and cause relay clattering amongst other problems.

Agreed - most step-starts take way too long.

When the step-start is about right, the first and second current steps 
will be about equal. You can tell this, near enough, by comparing the 
loudness of the two thumps from the transformer.

The relative size of the two current steps depends on both the delay 
time and the value of the series resistor.

If you don't have any startup current limiting at all, the current surge 
is big, but the filter caps are fully charged within the first 4-5 mains 
cycles (<100ms) and then it's all over. A step-start shouldn't take much 
more than twice that long, so a nice brisk 'ker-lick' from the switch 
and the relay (or two relays) is about right... so that's something like 
0.5s, or 1s tops.

Too long a delay will simply overheat the series resistor, but with no 
advantage.


-- 
73 from Ian G/GM3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek