71M6521DE TERIDIAN [Teridian Semiconductor Corporation], 71M6521DE Datasheet - Page 47

• Manufacturer Part Number: 71M6521DE
• Description: Energy Meter IC
• Manufacturer: TERIDIAN [Teridian Semiconductor Corporation]

FUNCTIONAL DESCRIPTION
The energy delivered by a power source into a load can be expressed as:
Assuming phase angles are constant, the following formulae apply:
For a practical meter, not only voltage and current amplitudes, but also phase angles and harmonic content may change
constantly. Thus, simple RMS measurements are inherently inaccurate. A modern solid-state electricity meter IC such as the
TERIDIAN 71M6521DE/FE functions by emulating the integral operation above, i.e. it processes current and voltage samples
through an ADC at a constant frequency. As long as the ADC resolution is high enough and the sample frequency is beyond
the harmonic range of interest, the current and voltage samples, multiplied with the time period of sampling will yield an
accurate quantity for the momentary energy. Summing up the momentary energy quantities over time will result in accumulated
energy.
Figure 15 shows the shapes of V(t), I(t), the momentary power and the accumulated power, resulting from 50 samples of the
voltage and current signals over a period of 20ms. The application of 240VAC and 100A results in an accumulation of 480Ws
(= 0.133Wh) over the 20ms period, as indicated by the Accumulated Power curve.
The described sampling method works reliably, even in the presence of dynamic phase shift and harmonic distortion.
v1.0
P = Real Energy [Wh] = V * A * cos φ* t
Q = Reactive Energy [VARh] = V * A * sin φ * t
S = Apparent Energy [VAh] =
-100
-200
-300
-400
-500
500
400
300
200
100
0
0
Figure 15: Voltage. Current, Momentary and Accumulated Energy
Current [A]
Voltage [V]
Energy per Interval [Ws]
Accumulated Energy [Ws]
© 2005-2008 TERIDIAN Semiconductor Corporation
P +
5
2
Theory of Operation
Q
E
2
=
∫
0
t
V
10
) (
t
I
) (
71M6521DE/71M6521FE
t
dt
15
Energy Meter IC
20
DATASHEET
JANUARY 2008
Page: 47 of 101