AN_6612_007
78M6612 Hardware Design Guidelines
Rev 2
9
3.3 Voltage Transformers
The pseudo-differential voltage measurement circuit of VB-VA shown in Figure 6 can be replaced with a
voltage transformer (VT). The VT replaces the two 2 M
Ω
resistor divider networks and requires use of
only V. Use of a CT with a VT provides complete galvanic isolation from the plant wiring. A good quality
VT provides accurate linear measurements from 100VAC to 240VAC.
The basic connection of a VT to the 78M6612 is similar to Figure 5. The VT’s secondary output voltage
range in conjunction with the manufacturer’s recommended burden resistor value must meet the
78M6612’s signal input range of ±176.78 mVrms (±250 mVpk). Higher output VT secondary voltages can
be accommodated using a resistor divider between the VT and the 78M6612.
Use only good quality VTs which introduce minimal phase shift between the primary to secondary
winding. The CT phase calibration routine can be used to compensate for any VT phase delay.
Figure 7 shows a basic connection diagram of a CT and VT based system.
NEUTRAL
LINE
VA
VB
IA
IB
LOAD
78 M6612
V3P3
0.1µF
RBurden
750
V3P3A
750
0.1µF
0.1µF
1000pF
1000pF
750
0.1µF
1000pF
CT
750
VT
RBurden
Figure 7: Basic Connection Diagram of a CT and VT Based System
