# No Subject

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Firstly, wot i said yesterday applies: all the usual calibrations, rules
of thumbe ASSume a sine wave.  I have never scoped a tesla power
supply curcuit, but i wager a LARGE stack of cookies its not drawing a
sine wave.  Any meter will give more or less out of cal results, except
hotwire or thermocouple.  Usual meters SHOULD be OK for relative
indication tho i wouldnt bet on twice the reading meaning current.

Lessee wot i can come up with on calling this, ASSuming a sine wave:

D'arsonval (thats the descriptor for the usual "coil moves in fixed
magnetic field" type meter) fundamentally averages current flow thru
it.  BUT in the nature of a sine wave, averaging it ends up low on
the "Effective" value.  This is where the infamous sqrt 2 comes in.
(it can be derived, but its been decades since i've seen it done,
so i'll just take it as a given.)

What this means is that where a DC meter fed full wave rectified
AC will indicate the AVERAGE of the value (.5) the effective [1]
value is higher, the famous .707.  Put another way, the peak is
1.414 the average.

For a full wave rectified meter, the INDICATION must be multiplied
by 1.414 its value (1.0 volts means 1.414) to get the effective
value.  (in usual purchased VOMs this is all buried in the calibration
of the ranges, so invisible to the user.  BUT IT ASSumes a sine...)

Since Steve's proposed circuit bypasses half the current round the
meter, the meter will read half what it is supposed to.  So 2*(sqrt 2)
times the indication is ALMOST there.

(I've run thur this tep by each, if i'm wrong, sing out....)

A small (depending on the meter) effect is that the "bypass diode" will
leave 0.7V (coincidence, no relation to the .707 above) _backwards_
across the meter.  The meter will average this in, and read "a smidge"
lower.  Calculating the size of the "Smidge" needs knowing the
resistance of the meter & the normal currents, etc, etc.

[1]
Whats an Effective Value?
Same thing as an RMS value.  8)>>
It was found early in the transition from DC to AC that averaging did not get
it right.  (and was obvious to the mathematically inclided, anyway...).  The
"effective" value is that value of DC that is "effectively" the same as AC.
IF AC was triangular shape, the average WOULD be right.  From the math of a
sine wave, this cranks out the 1.41/.707 conversion.

====================
Caution:
Naught to do with tesla...
There was a whole family of "ionic" loudpspeakers in the 60s.  Notably used
for "tweeters" (HF end of ther ange, but i think a few "woofers" were built,
tho the power supplies got outrageous...

Years ago, i had a neat book, since lost by loaning, on loudspeakers written in
1925 !  A wondrous thing.  Recounted in there was a "Victorian" PA system:
A goodly sized gas jet at the small end of a horn.  Gas supply by a delicate
valve, whose position was modulated by the voice.  This varied the amount of
gas burnt & hence the volume of expanding gasses.  True Story, tho reference
is missing.  Low Fi, to be sure.
regards
dwp

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