Valhalla 2300 Operation Manual - page 76
IC8 and its associated components form an
RMS-to-DC converter. Its output is a DC
voltage which is equal in value to the RMS
AC voltage input. If the RMS converter or
multiplier are operated outside of their range
of input levels, inaccurate readings may result.
To prevent this source of error, peak
comparator IC7 monitors the output of IC4. If
the peaks exceed the limits set by R33 to R36,
opto-isolator IC6 is activated, which in turn
drives the front panel peak overload LED.
10-3-3.
Power Multiplier
The schematic for this section may be found
on 2300-070. IC9 and its associated
components form the power multiplier. The
output of the multiplier is one tenth the
product of its inputs. With the inputs from the
current and voltage amplifiers at 5 volts the
multiplier output will be 2.5 volts. R39 and
C18 smooth the multiplier output.
10-3-4.
Isolators
The schematic for this section may be found
on 2300-070. The DC analog levels of
voltage, current and power are transmitted
across the floating earthy barrier by a voltage-
to-time converter. The isolators produce a
constant frequency, variable duty cycle wave
to drive the opto-isolators to bridge the
barrier.
The DC voltage to be isolated is compared to
a very linear triangle wave by IC13. The
output of IC13 is a variable duty cycle pulse.
The pulse width is equal to the period of time
that the triangle wave is above the DC
voltage. If the voltage increases, the pulses
become narrower. If the voltage drops, the
pulses become wider. The duty cycle will be
50% for zero DC voltage.
IC's 11, 12 and their associated components
form a precision triangle generator. The
triangle's frequency is about 5KHz. The
actual frequency does not matter. The triangle
amplitude is set by zener diodes CR4 and
CR5. IC12 is configured as an integrator and
IC11 as a comparator. Positive feedback
around the integrator-comparator combination
produces oscillations generating a precise
triangle at the output of IC6.
10-3-5.
Control Isolators
The schematic for this section may be found
on 2300-070. The gains of the current and
voltage amplifiers along with the current
shunt selection are controlled by the front
panel keyboard. The control signals are
isolated by opto-isolators IC17 and 18.
Transistors TR1-5 buffer the opto-isolator
outputs to drive the gain selection relays K1-
K5.
10-3-6.
Receivers
The schematic of this section may be found on
sheets 1 and 2 of 2300-071. The receivers
convert the variable duty cycle isolator
outputs into DC voltages. The opto-isolator
outputs are squared by comparator IC103.
The outputs of IC103 are clamped to precision
levels by diode pairs CR116-CR115, CR114-
CR113, CR112-CR111. The clamp levels are
established by zener diodes CR17 and CR18
and their associated components. The outputs
of the clamps are applied to second order low
pass filters IC104-IC106 and their associated
components. The outputs of the filters are DC
voltages proportional to watts, volts, and
amps. Relays K1-K9 select which isolator
output is applied to the summing amplifier
IC4 and the ranging circuits.
IC4 sums the three watts outputs to provide
total watts displays for 3-phase 3-wire and 3-
phase 4-wire measurements. Relay K10
selects the gain of IC4 for optimum display
resolution.
The outputs of the voltage and current
comparators are also applied to overload
detection circuits. These detectors
monitor the presence of the variable
duty cycle pulses. If the isolators are