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# Re: Need a calculator that knows about coulombs

```On Wednesday 23 December 2015 06:47:39 Joe wrote:

> On Wed, 23 Dec 2015 05:09:18 -0500
>
> Gene Heskett <gheskett@shentel.net> wrote:
> > On Tuesday 22 December 2015 21:02:31 John Hasler wrote:
> > > Gene Heskett writes:
> > > > In this case, the size of a capacitor hooked to a square wave
> > > > source, and I want to know how much charge is transfered for
> > > > every full cycle of the input square wave.
> > >
> > > Zero assuming linear source and load impedence.
> >
> > Ahh, but its being fed to a voltage-doubler with schottky diodes, so
> > there should be a calculatable amount of charge delivered to the
> > output capacitor Cf, by the pump capacitor Cp, per cycle.  With Cp
> > being of a capacity such that with the usual logic circuits output
> > impedance, capable of delivering 10 to 20 milliamps into a resistive
> > load, IOW not a zero impedance source. Cf would be sized such as to
> > be about 50x Cp, and will be driving an enhancement mode fet which
> > will turn on a 40 amp SSR once the charge has built up on Cf to make
> > the fet conduct.
>
> Do it numerically with a spreadsheet.
>
> It takes a while with any reasonably-sized spreadsheet, but you can
> trust the result, you can see every stage and can spot anything
> obviously wrong.
>
> I once needed to analyse the start-up surge into a large capacitance
> using a toroidal mains transformer, and given the non-linearity of the
> rectifiers, I could see no other way. When results can reach hundreds
> of amps for a millisecond or so, you don't want large piecewise-linear
> approximations.

True, but all I am interested in, and the transformers involved are
toroids, 4 of them good for half a kw each, is limiting the input surge
to 127/51=2.49019607843 amps at the initial turn on with the 51 ohm R in
series with their combined primaries.  Then however many seconds later
when those caps are at 90% or better charge and long before I or the
code starts the spindle, shorts the 51 ohm resistor allowing full
current flow.  It could be in excess of a minute because the first
operation is homing the machine, one axis at a time, which depending on
where it was when last powered down, could take up to a full minute to
do. The spindle power at full song is not a requirement until that is
done.  And because of the acoustic noise from both the spindle and
vacuum, I don't normally start the spindle and vacuum until it is in
position to start cutting with the next commanded move. I am the one
writing the code.

So I don't need to analyse it microsecond by microsecond, just control it
to a defined peak current draw. In this case I could use a lower
resistor value R but thats what I have available in my "junk box"
without ordering online.  It will catch 316.254901961 instant watts when
the caps are at 0 volts, more than its 200 watt rating, but not for long
enough to blow the glaze off it over time as that instantaineous power
will drop rapidly as the caps charge up to working voltage.

Cheers, Gene Heskett
--
"There are four boxes to be used in defense of liberty:
soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>

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