AN_6612_007
78M6612 Hardware Design Guidelines
Rev 2
7
3 Isolated Configuration
Alternatively, the 78M6612 can sense the load current using a current transformer (CT) for an isolated
configuration. In this configuration, the 3.3 VDC supply rail (V3P3) for the 78M6612 is not directly tied to
AC mains. The analog inputs to the 78M6612 are used as follows:
•
The A0 and A2 inputs are used to measure the line voltage differentially.
•
The A1-A3 inputs are used to measure the load currents.
•
The IB input is not used and is tied to V3P3. It can be used to measure the load current to a second
outlet and in this case would use similar topology as the IA input.
3.1 Current Transformers
The selection of a current transformer with respect for the desired measurement accuracy includes
factors such as line frequency, measured current range and the CT’s turns ratio. Also, subjecting a
current transformer to load currents above the manufacturer’s rated current specification may saturate the
CT and cause winding failures due to excessive temperature rise. On the other hand, a current
transformer that is rated much higher than the target load current might be restrictively too large and
expensive for its purpose.
Figure 5: Current Transformer (CT) Basic Connections
Usually, current transformers have turns ratios ranging from 10:1 to 2500:1. The higher the turns ratio
(TurnRatio = Nsecondary/Nprimary), the higher the resolution of the current measurement. A too high
turns ratio increases distributed capacitance and leakage inductance. These characteristics may
decrease the CT’s accuracy and capability to operate at higher frequencies. However, if the number of
turns is too low, the output signal may distort or “droop” (for positively sloped unipolar input signals). Such
distortion may cause measurement inaccuracies. We recommend a minimum turns ratio of 1000:1.
The next step towards selecting a current transformer is the calculation of the burden resistor’s value
(RBurden). The 78M6612 signal input range is 176.78 mVrms (250 mVpk). Therefore, the CT’s secondary
output voltage (Vout) must operate within this range. Assuming the maximum load current is 20 A rms
(28.284A pk), a 1000:1 ratio current transformer will produce a secondary current of 20 mA rms (28.284
mA pk). Per Figure 5, the burden resistor’s value is calculated as follows:
Using the values in the above example, the value of the burden resistor is:
A standard value 8.2
Ω
resistor is recommended.
