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Re: [Amps] Current draw at 110 volts for SB-200 and SB-1000

To: Steve Thompson <g8gsq@ic24.net>
Subject: Re: [Amps] Current draw at 110 volts for SB-200 and SB-1000
From: Vic Rosenthal <vic@rakefet.com>
Date: Fri, 03 Dec 2004 11:02:32 -0800
List-post: <mailto:amps@contesting.com>
Steve Thompson wrote:

The high current 'gulps' I meant are the ones every line half cycle, not at
syllabic rate. The line voltage your meter measures tends to reflect a cycle
to cycle average, and doesn't indicate what happens during those few ms
whereas the psu feels the full effect.

An example (imprecise, so please don't shoot me down): If your amp is
pulling an average of, say, 10A from the line for something approaching a kW
input, the charge goes into the capacitors in a space of maybe 2ms during
the half cycle of 8.33ms. The average current during those 2ms has to be
40A, with the peak being 50, 60 or even higher. That sets the voltage drop
that the psu suffers whereas your voltmeter and lights don't react so much
to the short term disturbance.

If you put a scope across the primary of the transformer, you might see a distorted waveform due to the large current drawn during part of the cycle. But the output voltage of the supply is related to the voltage that appears across the filter capacitors. This total charge is (I would guess) proportional to the area under the curve of the distorted waveform I mentioned. So the resultant output voltage drop due to the resistance in the circuit feeding the PS is an average (of some kind) of the voltage drop across the primary over the period of the cycle multiplied by the square of the turns ratio.


Another way of putting this is that although the voltage drop due to the power line resistance varies greatly during the cycle, the capacitors even it out.

--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco

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