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Re: Coil Efficiency (and true wattmeter)
From: David E. Sharpe[SMTP:sccr4us-at-erols-dot-com]
Sent: Thursday, June 26, 1997 12:45 AM
To: Tesla List
Subject: Re: Coil Efficiency (and true wattmeter)
Tesla List wrote:
>
> From: FutureT-at-aol-dot-com[SMTP:FutureT-at-aol-dot-com]
> Sent: Tuesday, June 24, 1997 2:39 PM
> To: tesla-at-pupman-dot-com
> Subject: Re: Coil Efficiency (and true wattmeter)
>
> In a message dated 97-06-21 07:13:23 EDT, you write:
>
> << Hi all,
> > I think we could be on the verge of making some real
> > progress in nailing down the merits of actual coil parameters. There
> > is much to be done in the wall socket -> coil efficiency stakes. But
> > if we look at the raw E x BPS vs spark length figures, we can start
> >to see where the coils themselves might be improved. I think there is
> > now a most fruitful area of research open to us. We might at last
> > have a platform from which to examine L/C ratios and the like. It
> >seems obvious now that when using current efficiency measurements we
> > are floundering around in the dark (with an arc to light the way :)
>
> > Malcolm
> >>
>
> Malcolm, All,
>
> I agree. Can you suggest a simple way to measure the capacitor voltage?
>
<SNIP> The max power input for the circuit is 1300 watts,
but maybe this can be "upped" using appropriate shunts. I haven't built
one yet, but I don't really think there's any choice, unless someone can
suggest something simpler of comparable accuracy.
Maybe Dave will add some more info here about his unit.
John Freau
-----------------
Chip, John, Malcolm, ALL
The wattmeter that I have previously posted can be upgraded
easily to at least 10,000 W by simply increasing the Vmultiplier
resistor from 100k, 1/2W to 220K, 1W (115 to 230VAC input),
and changing the Rshunt from 4.6" of #14 AWG THHN copper wire
(1.00 milli-ohm) to 3.8" #8 AWG THHN wire (250 micro-ohm).
The 0 and span of op-amp is adjusted for 0-10V = 0-10kW. Full
scale up to +rail of op-amp (about 14kW) is possible with
this arrangement. Higher current capability is possible,
Rshunt should provide 15 millivolts FS at IFS. The above 250 micro-ohm
shunt would be suitable to currents up to 60A. Large shunt wire should
use mechanical lugs, and sense wires tapped off lug to optocoupler
as a standard Kelvin voltage sense connection.
The 741 opamp can be replaced by a MAXIM MAX427CPA instrumentation
grade precision op amp (cost less than $5.00, Digikey, Jamesco,
MCM other in USA). This upgrade will improve accuracy to significantly
better than 1% across full span, and pin-outs are identical (direct
one-for-one swap).
Several indicating meter options are available, I like the Yokogawa
hybrid digital/baragraph analog 51 segment panel meter. Even though
pricey (about $200 through outlets for 0-10VDC input), can be scaled
at no cost to engineering units (0-10.00KW), is specifically designed
for switchgear service, and is a very professional indicator.
The only concern I have with this design is there is minimal circuit
board clearance through the optocoupler (0.1 and 0.3 inches). Fusing
of the V multiplier with 250V fast blow fuses (1/16A) is probably
a prudent thing to do. The optocoupler and opamp should be carefully
shielded from air currents, and should be allowed 5-10 minutes warm-up
(actually thermal equilibration) for max accuracy. Info for the
op-amp is available for download at the MAXIM WWW website. Several
multichannel optoisolators with up to 5kV isolation have recently
become available from several suppliers (Hewlett-Packard) which have
significantly wider clearances than above mentioned. I can not
infer performance of these HV devices as an analog pass-thru device
as the NEC 4 channel optocoupler part is used.
I will keep list informed as a large unit is built and tested.
DAVE SHARPE, TCBOR