Maxim Integrated 71M6533-DB - Manual (Page 22)

71M6533- DB Demo Board User’s Manual
Page: 22 of 75 ` REV 3
In general, when IMAX is applied to the primary side of the CT, the voltage V
in
at the IA, IB, or IC input of the
71M6533 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 se-
condary side, causing only 141mV. The steps required to adapt a 71M6533-DB Demo Board to a transformer
with a winding ratio of 2500:1 are outlined below:
1) The formula R
x
= 177mV/(IMAX/N) is applied to calculate the new resistor R
x
. We calculate Rx to 2.115

2) 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, IB, or IC inputs of the 71M6533 IC.
3) WRATE should be adjusted to achieve the desired Kh factor, as described in 1.8.3.
Simply scaling IMAX is not recommended, since peak voltages at the 71M6533 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 71M6533 inputs. Conversely, CTs
with much higher ratio will tend to decrease the useable signal voltage range at the 71M6533 inputs and may
thus decrease resolution.
1.8.5 ADJUSTING THE DEMO BOARDS TO DIFFERENT VOLTAGE DIVIDERS
The 71M6533-DB Demo Board comes equipped with its own network of resistor dividers for voltage
measurement mounted on the PCB. The resistor values (for the 71M6533-DB 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 71M6533 has enough resolution, even
when operating at 120Vrms or 240Vrms.
If a different set of voltage dividers or an external voltage transformer (potential 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 71M6533 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.
If potential transformers (PTs) are used instead of resistor dividers, phase shifts will be introduced that will re-
quire negative phase angle compensation. Standard Demo Code accepts negative calibration factors for phase.