DESCRIPTION
1.1 Introduction
Electronic Wattmeter
CHAPTER 1
The Feedback EW604 Electronic Wattmeter is based
on
an
analogue multiplying circuit using the logarithmic voltage-to-
current transistor characteristic. The inputs to this multiplying
circuit are currents that are derived via range-scaling resistors
from the input voltage terminals and the input current terminals.
The output is displayed
on
the front panel moving-coil meter
calibrated
in
watts.
The wattmeter terminals are arranged as two pairs marked
'SUPPLY' and
'LOAD' to facilitate correct connection.
Separate warning lights are provided
on
the voltage input
and
current input to indicate when
an
overload might affect the
reading accuracy. Additional precautions are taken to prevent
damage to the instrument
in
the event of gross overload of
current or voltage.
Pushbuttons enable the meter deflection to
be
reversed to
measure reverse power flows and also to increase the meter
sensitivity
by
x2
to improve readability of small deflections.
The wattmeter contains its
own
partly-regulated power supply
for operation
on
normal line voltages.
The value of the wattage is displayed
on
the front panel meter
which responds to the true wattage or potential heating ability of
the current in the associated load.
In
a resistive load this
corresponds to:
2
2
(V
rmsl
. .
(I
rmsl
R or R
1rrespect1ve
of waveform*
and to:
(Irmsl
(Vrmsl
cos
<jl,
in
a reactive circuit with sinusoidal
excitation
and
in
general to:
T
Limit T
1
J v i dt
T->=
0
*High crest values applied to EW604 will illuminate the
0/L
lamp(s) if they are of such a value as to cause limiting
in
the
circuit
and
thus errors.
5
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