Phil Clements wrote:
>
>
Will had written:
>> The first 1 to 5 cycles is the worst part of the surge, but from tests
>> that was run by a ham on this mailer, the transformer didn't settle down
>> until 30 to 40 cycles. This was captured on a storage scope and I have the
>> results here somewhere saved as a jpeg photo. Anyhow, a 1/2 second delay
>> is about all that's needed on the step start. I'd say they used this
>> heater cone over its greater power rating than would be a 25 watt or so
>> resistor.
>
>The reason for the cone heater instead of a 25-50 watt resistor is that this
>is a "manual" step-start circuit on the Henrys. It only has three parts; a
>cone heater, a switch, and the operator's hand. As the switch is rotated
>from off to on, it allows the mains current to flow through the cone heater,
>step-starting the power supply. Since it is possible for the operator to
>screw up and leave the switch in a position that the cone heater is still in
>the circuit, it required a robust resistance in order to stand the
>continuous duty in a situation like this. Some folks who don't read the
>Henry manual think this is a "hi-lo" or "SSB-CW" power switch instead of a
>step-start, and try to operate the amp in the intermediate switch position.
>Even the robust cone heater has its limitations (300 watts-1000 watts,
>depending on the model) and will fail open after this abuse goes on for a
>period of time. A savvy operator, when turning on the amp, will pause in the
>intermediate switch position until the HV meter gets up to 3/4 of its final
>normal value. This will usually take less than one second. At any rate, when
>the HV meter has come to a stop during turn-on, the "step-start" function is
>complete, and the switch should be moved to the "on" position immediately to
>prevent over-heating of the coil on the cone heater.
>
Sounds a bit primitive - no wonder it needs that big resistor! With so
many well-known ways to make a completely automatic step-start, it seems
a really strange decision to use a manual switch and leave the whole
thing wide open to operator error.
Replying to Will's comment also, a step-start is really only about the
first few big cycles of inrush current. There is no point in waiting
until the current has "settled down" to the last few percent before
switching the surge resistor out. All it does is heat up the resistor,
with no real benefit to the power supply components.
--
73 from Ian GM3SEK
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