Re: Identifying and Adapting Meters

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: David Speck <Dave@xxxxxxxxxxxxxxxx>

Paul.
There's nothing at all stupid about recycling meters at all. I do it often.

The meters are probably marked with a Plus sign on one terminal.  If not, 
the + terminal is probably the one that had the red, white or yellow, 
orange or brown wire on it (negative colors are usually black, blue, 
violet, or gray for American designed circuits).  If the unit came from 
Japan, then the white wire is probably the + lead, and any other color for 
the negative, but there is no absolute rule.

If you have an adjustable DC power supply, just connect the supply with the 
proper polarity in series with a 10K resistor, your meter under test,  and 
an inexpensive digital volt meter set on the milliampere range.  Start with 
the supply at 0 volts, and increase the output slowly.  If the meter 
deflects backward, you have the polarity wrong.
If it deflects forward, then advance the power till you have a full scale 
reading, and consult your handy DVM to see what the full scale current is.
You may need to increase or decrease the test 10K resistor to get a full 
scale deflection.  If you have to apply lots of voltage to get even a 
little movement, then there is probably an internal scaling resistor inside 
the meter.  With careful disassembly, this can be removed, or, more simply, 
shorted out to get the max sensitivity from the meter movement.
After you find out what current gives you a full scale deflection, you can 
apply your DVM across the meter terminals, adjust for a full scale 
deflection again with just the power supply and the temporary scaling 
resistor, and see the voltage drop across the meter.  Using R=I/E, you can 
then determine the internal resistance of the meter coil.  With these 
parameters, you can select a scaling resistor to make the meter display 
just about anything you want for a full scale reading.

Having said that, and NOT having looked at the Tesla Tuner schematic in a 
while, IIRC, the tuner feeds AC to the LEDs, and relies on the human eye to 
integrate the pulses of light to balance the two LEDs' output.
If that is correct, a DC meter won't help you.  You will have to build a 
simple precision rectifier (otherwise known as an absolute value circuit) 
with an op amp, a pair of diodes and a few resistors.  Circuits for these 
are easily found with a Google search.
BTW, there's a really neat shareware program out there called MeterScale or 
something to that effect, that makes very nice custom meter faceplates with 
your choice of legend and graduations.  Definitely worth the $10.00 
registration fee, and makes a very professional looking replacement meter 
face of nearly any conceivable size, scale,and configuration..  I can 
forward you a copy or a link if you are interested.

HTH,
Dave Speck

> I would like to use one of these to indicate the output level of the Tesla 
> Coil Tuner, in addition to or in place of the LED. Any help will, of 
> course, be greatly appreciated. I will even accept well intentioned 
> criticism about what a stupid idea this is!! Regards.
> Paul
> Think Positive