Maxim Integrated 71M6533-DB - Manual (Page 21)

71M6533- DB Demo Board User’s Manual
Page: 21 of 75 ` REV 3
1.8.2 USING THE DEMO BOARD FOR ENERGY MEASUREMENTS
The 71M6533-DB Demo Board was designed for use with current transformers (CT).
The Demo Board may immediately be used with current transformers having 2,000:1 winding ratio and is
programmed for a Kh factor of 3.2 and (see Section 1.8.4 for adjusting the Demo Board for transformers with
different turns ratio).
Once, voltage is applied and load current is flowing, the red LED D5 will flash each time an energy sum of 3.2
Wh is collected. The LCD display will show the accumulated energy in Wh when set to display mode 3
(command
>M3 via the serial interface).
Similarly, the red LED D6 will flash each time an energy sum of 3.2 VARh is collected. The LCD display will
show the accumulated energy in VARh when set to display mode 5 (command
>M5 via the serial interface).
1.8.3 ADJUSTING THE KH FACTOR FOR THE DEMO BOARD
The 71M6533-DB Demo Board is shipped with a pre-programmed scaling factor Kh of 3.2, i.e. 3.2Wh per pulse.
In order to be used with a calibrated load or a meter calibration system, the board should be connected to the
AC power source using the spade terminals on the bottom of the board. The current transformers should be
connected to the dual-pin headers on the bottom of the board.
The Kh value can be derived by reading the values for IMAX and VMAX (i.e. the RMS current and voltage
values that correspond to the 250mV maximum input signal to the IC), and inserting them in the following
equation for Kh:
Kh = IMAX * VMAX * 66.1782 / (In_8 * WRATE * N
ACC
* X) = 3.19902 Wh/pulse.
The small deviation between the adjusted Kh of 3.19902 and the ideal Kh of 3.2 is covered by calibration. The
default values used for the 71M6533-DB Demo Board are:
WRATE: 683
IMAX: 208
VMAX: 600
In_8: 1 (controlled by IA_SHUNT = 0)
N
ACC
: 2520
X: 6
Explanation of factors used in the Kh calculation:
WRATE: The factor input by the user to determine Kh
IMAX: The current input scaling factor, i.e. the input current generating 177mVrms at the IA/IB/IC
input pins of the 71M6533. 177mV rms is equivalent to 250mV peak.
VMAX: The voltage input scaling factor, i.e. the voltage generating 177mVrms at the VA/VB/VC input
pins of the 71M6533
In_8: The setting for the additional ADC gain (8 or 1) determined by the CE register IA_SHUNT
N
ACC
: The number of samples per accumulation interval, i.e. PRE_SAMPS *SUM_CYCLES
X: The pulse rate control factor determined by the CE registers PULSE_SLOW and PULSE_FAST
Almost any desired Kh factor can be selected for the Demo Board by resolving the formula for WRATE:
WRATE = (IMAX * VMAX * 66.1782) / (Kh * In_8 * N
ACC
* X)
For the Kh of 3.2Wh, the value 171 (decimal) should be entered for WRATE at location 21 (using the CLI
command
>]21=+171 ).
1.8.4 ADJUSTING THE DEMO BOARDS TO DIFFERENT CURRENT TRANS-
FORMERS
The Demo Board is prepared for use with 2000:1 current transformers (CTs). This means that for the
unmodified Demo Board, 208A on the primary side at 2000:1 ratio result in 104mA on the secondary side,
causing 177mV at the 1.7

resistor pairs R24/R25, R36/R37, R56/R57 (2 x 3.4

in parallel).