DC meters with AC Re: Ignition coil (anti-parallel pairs) array and other questions

To: tesla-at-pupman-dot-com
Subject: DC meters with AC Re: Ignition coil (anti-parallel pairs) array and other questions
From: "Bill Noble" <william_b_noble-at-email.msn-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Wed, 21 Jul 1999 11:26:43 -0600
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

I think the original adivce is a bit overstated, I'll offer my suggestions

for both meters - a single diode will work, but I agree, a simple bridge
rectifier (full wave) is best - that is what you find inside analog VOMs.

for both - I have never seen a filter capacitor, certainly not a few
microfarads, used with a VOM type meter - if you find you want more damping,
then a RC circuit will do, but I have only seen this kind of circuit in a VU
meter, and that's not what you want.  So, no capacitor.

for voltmeter. .. in series with AC side of the rectifier, add a resistor.
you want to calculate the resistor value so that at the max voltage you wish
(for full scale on the meter) the current through the resistor equals the
current rating of the meter - as stated below, most analog meters tell you
the full scale current somewhere on the meter - if it's already set up as a
voltmeter, there will be an internal resistor - measure that value, and then
use E=IR to calculate what the max current is.

for ammeter - I have made the shunt resistor out of a short length of 14 or
12 gague wire - you didn't say what max currejn you wanted - I'll suppose
that given a 0-3 amp meter, you want 30 amps full scale - so use a light
bulb and your variac to set 3 amps of current through the meter without the
shunt.  Then hook about 6 inches of wire in parallel (note, parallel, not
series) with the meter terminals and see what the meter reads.  add or
subtract wire until you get it to read 0.3 amps.  you now have made a "times
10" multiplier for your meter.  DO NOT coil this wire up into an inductor -
if it's kinda long, make it into a zig/zag or sinewave pattern to avoid
making an inductor.  If the wire seems too short, go to a heavier gague.
----SNIP--------\
 I have bought an old volt and ampmeter (both
> > analogue). Only problem is they are both for dc. The voltmeter is up to
> 300
> > Volts and ampmeter up to 3 amps. How do I convert them to ac (240 volts
> > mains here) (and changing the scale the ampmeter uses (want to be able
to
>
> > read larger currents?)), if is is possible??
>
> For the Voltmeter:  Hook up a bridge rectifier (to turn AC into DC) and a
> suitable filter capacitor ( a few uF will do) across the meter.  Then,
> you'll have to put a series resistor in series with the meter to adjust
the
> range.  Does the meter have any markings indicating what it's full scale
> current is?  (Typically something like 1 mA, or 50 uA)
>
> For the Ammeter: You need to put a small value, high current resistor in
> parallel with the meter to adjust the range.  The value depends on the
> resistance of the ammeter. You'll also need to put a bridge rectifier and
> filter cap on.
>
>
>
>