[Amps] Step start resistors

[Ian White, G3SEK]

>>
>>Any ideas on what values to use for step start resistors? In the past, I've
>>tended to choose a value from the available stock - something like 50 ohms,
>>100 watts, which limits the surge current to just over 4 amps. I expect
>>that a value such that the current won't exceed the full load primary
>>current of the transformer is probably best, but any suggestions?
>>
>>(I'll probably still use what's available in my stock of vitreous enamelled
>>wirewound resistors about 10 inches long and an 1 inch in diameter,
>>though!)
>>
>>73
>>
>>Peter G3RZP


>**  I use a minimum value of R that - when added to the transformer
>primary R - does not exceed the mains circuit-breaker (or fuse) ampere
>rating.   For example, a Heath SB-220 has 10A breakers and c. 2-ohms of
>primary R when wired for 240V.   Thus, to limit inrush-I to 10A, the
>stepstart R needs be at least 22-ohms total..  More R adds step time.
>As stepstart-R keeps increasing, at some point, the HV will stop rising
>before it reaches the desired 60% level which closes the stepstart relay.
>  In other words, mo' is not always mo' betta.
>-   The typical delay time for a stepstart using a value of R that limits
>inrush-I to the circuit-breaker rating is a bit under 1-second -- which
>is about the same time it takes for a thoriated-tungsten filament/cathode
>to reach full emission.
>

We're trying to control two different current surges:

1. The initial current surge when the amp is powered-up through the 
resistor
2. The second current surge when resistor is shorted out.

My feeling is that the best compromise is to make those two peak 
currents roughly the same. If one is a lot larger than the other, the 
rating of the mains fuse or breaker is higher than it could be... which 
means that the amp is less well protected than it could be.

For example, choose the resistor so that the first surge almost reaches 
the maximum current rating of the mains fuse or circuit breaker. Then 
try to adjust the delay time so that the second surge peaks at about the 
same level.

The first peak you can calculate, but the second one you have to 
measure. Since it's not very critical, an ammeter with a reasonably 
lively movement should be enough to show whether the two peaks are 
roughly equal. Alternatively, you can listen to the transformer and 
watch the room lights!

(My own amp is sending clear signals that the second surge is quite a 
lot bigger than the first...)


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
                            Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek