Terry, John, ALL
The original wattmeter which I used was found in EDN magazine; October
14, 1994 edition, "Ideas for Design" segment titled Optical Isolator
Computes Watts. I immeditely recognized that this circuit could help
stop the banter about my KVA is bigger than your kVA BS and cut to the
chase with a parameter which is a primary measurement standard; POWER.
If you examine the circuit, the meter is originally designed for 120V,
1ph, 3W circuit (H,N,G). The current monitoring shunt is in the neutral
side of the circuit. This is fine for table top coils, but us folks
using pole-pigs or potential transformers are in some difficulty. To
make this circuit work on a 240V, 1ph feeder typical for powering our
higher power transformers, the shunt ends up on one of the power lines
120V above ground but up to 240V from the voltage sense input. With
0.100 inch between pins; and 0.300 inch between opposing pins of a 16
pin DIP package IC is too close with 240VAC and EMI additive on top of
powerline. The basic circuit as shown I built and made a home-brew
single sided PCB with large ground plane. The circuit has worked
flawlessly and has NO failures in 4 years service. The only design
modification was I originally substituted a 741 opamp for Analog Devices
OP27 originally spec'd. Meter still worked but with reduced accuracy.
I then changed to a Maxim MAX427CPA instument grade op-amp, which
significantly improved lower range accuracy and linearity.
The circuits mentioned by several folks in the 1997 or 1998 EDN I have
also seen, I'm more inclined to "smoke" a PCB with $40 (1995) parts,
then a PIC or PC with an A/D, etc. etc. I'm a stickler for details, and
I don't feel comfortable providing sub-assemblies that are being
overstressed anyway in a TC environment, and have someone say that the
product was defective or caused an injury, fire; or was miswired in the
field and blewup etc. The circuits are in the public domain and are
probably available by this tool (the interthingy). All components are
available at MCM or Digikey.
DAVE SHARPE, TCBOR