Re: Ammeters and Voltmeters

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: David Speck <Dave@xxxxxxxxxxxxxxxx>

Gerry,

If you are talking about op amps and diodes, then you should look up 
the circuit for a precision rectifier, or absolute value 
circuit.  These circuits use a pair of diodes ingeniously connected 
with an op amp in such a way that the op amp completely compensates 
for the diode's forward drop.  Only problem with these circuits is 
isolating them from the high potentials used in TC work, but you can 
use a 1:1, or different rating scaling transformer to provide 
isolation between your measured circuit and the op amp stuff.  Since 
very little current is drawn, the transformers maintain pretty linear 
operation.
For commercial power monitoring circuits, the standard way of 
measuring AC currents is the use of a current transformer.  The usual 
standard is to scale what ever your max desired power current will be 
to 5 amps.
Most commercial switchboard meter movements have a full scale 
deflection for 5 amps AC, and the faceplate is labeled for the scaled 
current.
Thus, if you want to measure a circuit with a 100 amp max current, 
you buy a 100:5 amp current transformer, and connect it to a panel 
meter with a 5 amp full scale deflection, but a scale that reads from 
o 0 100 amps.  Such meters and current transformers are readily 
available on eBay.
You can also buy prepackaged current and voltage transducers that 
very nicely take a defined range of AC volts or currents and convert 
them to a pretty DC current or voltage ready to feed to your analog 
panel meter.  They are usually not too expensive if you are patient 
on eBay, and save having to reinvent the wheel.
One transducer variant fits an old standard, the 4 - 20 mA current 
loop, where 4 mA represents "0", and 20 mA represents full 
scale.  Less than 4 mA indicates a failure of the signal 
system.  Many digital process control meters, and even a few analog 
moving coil meters are built for the 4-20 mA standard.  Nice thing 
about this standard is that the reading is independent of the wire 
length or resistance between the transducer and your display meter, 
and there is a fail safe indication of your circuit is open -- the 
meter is displaying something way less than zero.  MPJA.com has some 
nice current transducers for $12 each that have switch selectable 
input ranges of 5, 10, or 50 amps and convert that to a 4 - 20 mA 
current loop.  They are even split core units, so you can place them 
around the measured circuit without having to open the wire to pass 
it though the measuring core.
Even though digital panel meters look sexy and op amp circuits look 
neat in the lab setting, the electrical environment of a running TC 
is so full of electrical noise that most of the TCers I know take the 
easy way out and stick with good old fashioned moving coil analog 
meters, and avoid the fancy electronics.  Diodes can rectify all 
sorts of spurious RF signals, and even a well shielded enclosure will 
not guarantee that your solid state circuits will give reliable 
indications when the TC is firing.  Therefore, IMHO, I'd suggest 
looking at hamfests or on eBay for analog meters scaled for the 
measurements you want to make.

HTH,
Dave

Tesla list wrote:
> Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Ed,
>
> What you say sounds like a problem.  One would need to drop more 
> voltage than 50mv to account for the diode drop (that I forgot 
> about) like you say. I suppose one could put an opamp in the meter 
> to boost the voltage taken from the sensing resister before 
> rectifying to make the diode drop less significant.  But the diode 
> drop would still cause an offset error.
>
> Gerry R.