Example, Master clock – Rainbow Electronics AT73C502 User Manual
Page 23
AT73C500
23
where ELSB is the energy value of one LSB in the energy
register, 0.4Wh in default conditions. When the meter is
operated in non-standard conditions, the energy LSB may
be recalculated as:
where f is the clock frequency used, and U
FS
and I
FS
are
the full-scale values of voltage and current.
In case the meter is used with a non-default voltage divider
or current sensor, MCC factor is a convenient way to read-
just the impulse rate.
Example
The meter is to be configured for use in 120V networks,
with a maximum line voltage of 140V. The display pulse
rate is required to remain at 100imp/kWh. To start off, the
front end of the meter must be configured for the new line
voltage. The voltage dividers must be configured to pro-
duce an input signal of 0.707V at the input of the ADC at
maximum line voltage. At nominal meter settings, the volt-
age divider ratio is 270V:0.707V, in this case it must be
140V:0.707V.
Note that adjusting the line voltage of the meter will render
the formatting of most calculation registers to alternative
settings. For example, the meter constant pulse rate will
change as follows:
In our case of a meter for 120V networks, the new meter
constant pulse rate would be:
To make the meter constant pulse rate to an even number,
for example 2500, we may choose to either re-scale the
line voltage or scale the maximum line current. 2500
impulses per kilowatt hour is gained by either setting the
maximum line voltage to:
or by retaining the line voltage at 140V and scaling the
maximum line current to:
Regardless of which parameter (or both) is chosen, the
scaling process is a simple matter of gain calibration. If, for
example, the line voltage is chosen to be rescaled to 135V,
this is realized with a resistor divider of half the nominal,
and finetuning using the voltage gain coefficients. Also, all
values resulting from voltage calculation, such as the data
transferred via energy registers, should be normalized with
respect to the new voltage setting.
Going back to the calibration of the display pulse rate, the
new LSB value of energy registers is:
To maintain the display pulse rate at 100, the MCC calibra-
tion coefficient must be programmed as:
The energy value of each display counter impulse is there-
after:
I n m o d e 7 , t h e d e f a u l t d i s p l a y p u l s e r a t e i s 1 0
impulses/kWh(kVArh) instead of 100 impulses/kWh. This is
convenient for meters where only one decimal digit wants
to be shown. This default rate can also calibrated and the
calibration formulas are:
and
Master Clock
The master clock of AT73C500 is generated by a crystal
oscillator with crystal connected between pins XI and XO of
AT73C501/AT73C502. Master clock can also be fed to the
XI input from a separate clock source. The system clock
r a t e o f A T 7 3 C 5 0 0 i s t h e s a m e a s t h e c l o c k o f
AT73C501/AT73C502 and is fed to the CLK input of the
device from the CLK output of AT73C501/AT73C502.
E
LSB
3.2768MHz
f
------------------------------
U
FS
I
FS
×
270V
80A
×
-------------------------------
0.4Wh
Ч
Ч
=
MC
270V
80A
×
U
FS
I
FS
×
-------------------------------
f
3.2768MHz
------------------------------
1250
imp
kWh
------------
Ч
Ч
=
MC
270V
140V
--------------
1250
imp
kWh
------------
×
2410.714
…
imp
kWh
------------
=
=
U
FS
270V
2500
imp
kWh
------------
-------------------------
1250
imp
kWh
------------
×
135V
=
=
I
FS
270V
80A
×
140V
2500
imp
kWh
------------
×
----------------------------------------------
1250
imp
kWh
------------
×
77.143
…
A
=
=
E
LSB
140V
270V
--------------
0.4Wh
×
0.20741
…
Wh
=
=
MCC
1000
PR
E
LSB
×
------------------------------
25
–
1000
100
imp
kWh
------------
0.20741Wh
×
-----------------------------------------------------------
25
–
23.216
…
23
≈
=
=
=
IMP
(25
23)
1
imp
---------
140V
270V
--------------
0.4Wh
10.0
Wh
imp
---------
≈
Ч
Ч
+
=
IMP
(250
MCC)
E
LSB
×
+
=
PR
1000
(250
MCC)
E
LSB
Ч
+
-----------------------------------------------------
=