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71M6515H Demo Board User’s Manual
1.8.7
ADJUSTING THE DEMO BOARDS TO DIFFERENT CURRENT TRANSFORMERS AND VOLTAGE
DIVIDERS
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).
In general, when IMAX is applied to the primary side of the CT, the voltage V in at the IA or IB input of the
71M6515H IC is determined by the following formula:
V in = R ? I =
R ? IMAX
N
where N = transformer winding ratio, R = resistor on the secondary side
If, for example, IMAX = 208A are applied to a CT with a 2500:1 ratio, only 83.2mA will be generated on the
secondary side, causing only 141mV The steps required to adapt a 71M6515H Demo Board to a trans-
former with a winding ratio of 2500:1 are outlined below:
177 mV
R x = IMAX
1)
The formula
N
is applied to calculate the new resistor R x . We calculate Rx to
2.115 ?
2)
3)
Changing the resistors R24/R25, R106/R107 to a combined resistance of 2.115 ? (for each
pair) will cause the desired voltage drop of 177mV appearing at the IA, or IB inputs of the
71M6515H IC.
WRATE should be adjusted to achieve the desired Kh factor, as described in section 1.8.6.
Simply scaling IMAX is not recommended, since peak voltages at the 71M6515H inputs should always be in
the range of 0 through ±250mV (equivalent to 177mV rms). If a CT with a much lower winding ratio than
1:2,000 is used, higher secondary currents will result, causing excessive voltages at the 71M6515H inputs.
Conversely, CTs with much higher ratio will tend to decrease the useable signal voltage range at the
71M6515H inputs and may thus decrease resolution.
The 71M6515H Demo Board comes equipped with its own network of resistor dividers for voltage
measurement mounted on the PCB. The resistor values (for the 4-layer Demo Board) are 2.5477M ? (R15-
R21, R26-R31 combined) and 750 ? (R32), resulting in a ratio of 1:3,393.933. This means that VMAX equals
176.78mV*3,393.933 = 600V. A large value for VMAX has been selected in order to have headroom for
over-voltages. This choice need not be of concern, since the ADC in the 71M6515H has enough resolution,
even when operating at 120Vrms or 240Vrms.
If a different set of voltage dividers or an external voltage transformer is to be used, scaling techniques
similar to those applied for the current transformer should be used.
In the following example we assume that the line voltage is not applied to the resistor divider for VA formed
by R15-R21, R26-R31, and R32, but to a voltage transformer with a ratio N of 20:1, followed by a simple
resistor divider. We also assume that we want to maintain the value for VMAX at 600V to provide headroom
for large voltage excursions.
When applying VMAX at the primary side of the transformer, the secondary voltage V s is:
V s = VMAX / N
V s is scaled by the resistor divider ratio R R . When the input voltage to the voltage channel of the 71M6515H
is the desired 177mV, V s is then given by:
V s = R R * 177mV
Resolving for R R , we get:
R R = (VMAX / N) / 177mV = (600V / 30) / 177mV = 170.45
This divider ratio can be implemented, for example, with a combination of one 16.95k ? and one 100 ?
resistor.
Revision 2.0
? Copyright 2005 TERIDIAN Semiconductor Corporation
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