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.