[Amps] Step start resistors

[K7RDX]

Most ops here in the good ol USA don`t have a problem with house lights
diming when powering up our amps....We usually run the psu from a dedicated
240vac line....At least that`s my way here in Central WA....Also we have the
cheapest hydro power in the world!  We sell a lot of our  power to the boys
in Southern Ca.....Costs them a bit more than us. Incidentally, I step start
my 4kv supply with 25 ohms/200watt, and the 6700 supply with 50ohms/200watt
res. Been doing that for long time now and so far no bitching from either
supply...Good new year to all, Jim.
----- Original Message -----
From: "Ian White, G3SEK" <G3SEK at ifwtech.co.uk>
To: <amps at contesting.com>
Sent: Monday, January 05, 2004 9:07 AM
Subject: Re: [Amps] Step start resistors


> >>
> >>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
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