I'm playing around with some ideas for RMS to DC conversion and
thought I'd bounce ideas off the list (and yes, the DC output IS to drive
a PIC!).
        The typical RMS to DC converter is pretty much an analog
computer, computing the square, the average, the square root, etc.
        Another way to determine RMS is to go back to the "heating value"
idea of RMS.  This is used in the old thermocouple RF ammeters (such as
those from Simpson).  The thermocouple seems like an interesting
approach, since it is indeed a differential temperature sensor.  Since a
thermal resistance will "drop" so many degrees per watt, it seems that we
can directly determine the power dissipated in a heater by using the
differential temperature measured by the thermocouple and the
characteristics of the "insulator" or "thermal resistor" developing the
temperature drop.  This SHOULD be independent of ambient temperature,
since it's temperature rise (or drop) we're measuring, not absolute
temperature.
        Linear Technology uses a similar idea in the LT1088.  Here,
however, instead of measuring temperature drop, they have a pair of
heaters and a pair of temperature sensors.  An op amp is used to drive
one heater so its temperature matches the other heater.  It's then easy
to figure out the power dissipated in the op-amp driven heater, which
matches the power driving the "unknown" heater.  It seems that there
might be a difficulty due to temperature coefficient of resistance of the
heaters, but as long as they match, I'd guess that the errors cancel.
        The LT1088 is an interesting part.  It accurately measures RMS up
to 100 MHz.  However, the "unknown" heater terminals must be within 40
volts of ground.  I'm interested in measuring RMS of an isolated circuit
that is up to several hundred volts from ground.
        So...  how about measuring the LIGHT emitted by the heater,
instead of the temperature of the heater?  I recall years ago seeing
optocouplers (I think they were called Sigma Datacells) that included an
incandescent lamp driving a CDS photocell.  It SEEMS that this could be
used to measure the RMS voltage applied to the incandescent lamp.  There
may be repeatability problems and nonlinearity problems, but it MIGHT
work.
        CDS cells suffer from "memory" in that they have a "light
adapted" and "dark adapted" resistance that are different.  It seems that
perhaps a photodiode or a phototransistor would be a better sensor for
this purpose.  I think we need to stay with the incandescent lamp to get
the RMS conversion.  An LED is just too fast.  The output would just
follow the input.  The lamp filament design could determine the time over
which the RMS value is determined.
        So... comments?


Thanks!

Harold





Harold Hallikainen
harold@hallikainen.com
Hallikainen & Friends, Inc.
See the FCC Rules at http://hallikainen.com/FccRules and comments filed
in LPFM proceeding at http://hallikainen.com/lpfm

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