>>>
>>>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.
>
** Which is taken care of if relay pull-in is set so that it occurs when
the filter caps are at c. 2/3rd of max potential.
>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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