Design Guidelines Instruction Manual

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78M6610+PSU Hardware Design Guidelines AN_6610_107

8 Rev 0

3.1 Current Transformer Selection

The consideration for selecting a current transformer should include measurement accuracy, line

frequency (and harmonics), current range and the CT’s turns ratio. The load current range should also be

considered. 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.

I Load

I Secondary

AIN

AIP

750

3P3A

750

100n

1000p

RBurden

78M6610+PSU

10K

Figure 4: 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 78M6610+PSU 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 Arms (28.284A pk), a 1000:1 ratio current transformer will produce a secondary current of 20 mA rms

(28.284 mA pk). Per Figure 4, the burden resistor’s value is calculated as follows:

 =





Using the values in the above example, the value of the burden resistor is:

 =

0.250

0.028284

= 8.85 

A standard value 8.2 Ω resistor is recommended. Always consult the CT’s manufacturer for proper usage

of their CT and burden resistor recommendations.

The use of a CT allows for the 78M6610+PSU’s V

3P3

to be isolated from the plant NEUTRAL wiring. This

topology eliminates the safety issues stated earlier regarding shunt-based current sensing.

The V

3P3

reference point critical to multi-shunt measurement performance is not an issue with CTs. The

output currents generated by the CT’s secondary winding is small enough that the sheet resistance of the

1 oz. copper plating does not present measurement errors from adjacent CTs. Shield the CTs secondary

pins, burden resistor and filter components with top and bottom printed circuit board layer V

3P3

plane

surfaces. Insert multiple V

3P3

vias to interconnect the top and bottom V

3P3

structures for a low impedance

shield.