Tom W8JI wrote:
>> I tried the zero crossing switch on two HV power supplies (8877 and
>> GS35B), both power up just fine, nice and quiet, no blinking lights.
>>
>> Using a zero crossing switch makes a significant difference compared
>> to closing the switch when the line is not zero. Closing the switch
>> at zero allow the output capacitance to charge as the line voltage
>> rises to peak, following the curve of the sine wave.
>
> If your supply fully and smoothly charges in one quarter cycle and
> only draws 20 amps while doing it that is a miracle Jeff. You either
> have a supply with horrible ESR or you are missing something.
You clipped the test conditions from my original email
In some modeling with a 4000V supply with 50 Ohms equivalent series
secondary resistance and 20 uFd of capacitance, the single cycle
peak capacitor current hit 21 amps compared to 80 amps if switched
at the peak of the line. When drawing 1 Amp of plate current, peak
diode current ran about 4.5 Amps.
20 Amps on the _secondary_ side.
> Since the power factor of the supply is not zero, peak current does
> not occur on switch closure even at the sine wave crest. Peak surge
> would occur when you close the switch before the line voltage crest is
> reached.
The switch closes at 0V and the peak current occurs at the steepest part
of the sine curve, about 2 msec after switch closure.
> With a 2.4kV transformer having 15 ohms ESR, 20 uF charged to about
> 3300 volts calculates to be about 24 amperes first half cycle surge
> into the capacitor. Line current (assuming perfect power line) would
> be 10 times that value, or 240 amps.
Yes, 24 Amps on the plate side for a fraction of a cycle through diodes
rated for at least 30 Amps. And yes, 240 amps for a fraction of a cycle
through the power line. What's the problem with those numbers?
The single cycle start up surge only rises to about 4 times the current
of the repetitive peaks that occur at 120 Hz to keep the capacitor charged.
> We don't eliminate inrush by starting at zero. What we need to do is
> ramp it up with a gating system, or start it through a resistance and
> then switch the resistance out.
A zero crossing switch provides an intermediate approach between no
inrush protection and full inrush protection. And I think it limits the
inrush to reasonable an practical levels that meet the component
specifications.
jeff, wa1hco
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