UM_6631_077

78M6631 Evaluation Board User Manual

Rev 1

11

2.5 Burden Resistor Selection

The 78M6613 Evaluation Board does not include surface-mount current transformers (CTs). The CTs

included with the kit are externally attached to the evaluation board which has the correct value of burden

resistor on board..If using different CTs with the evaluation board, it is important to either disable the on-

board burden resistor or change the burden resistor value for optimum signal magnitude in order to utilize

The 78M6631 Evaluation Board provides a 4.99Ω burden resistor that can be inserted or excluded using

jumpers J2, J3, J6, J8, J11, and J12. Use the equation below to determine if the onboard burden resistor

can be used with the selected CT or if another value should be used.

At maximum load current (IMAX), the voltage built up across the burden resistor should not exceed 176

various systems considerations, it may be necessary to select a burden resistor value lower than the

manufacturer’s recommended value. For most CTs, this will not pose a problem.

Example:

A CT with a turn ratio of 2000:1 yields a secondary current of 1/2000 of the primary current. Following

Ohm’s law, the resulting voltage V across the burden resistor R is:

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where N is the turns ratio, ILoad is the primary load current and RBurden is the value of the burden

resistor.

The proper IMAX value should then be set in the relevant register and the CT must be calibrated for

optimal performance. The calibration includes gain adjustment and CT phase error adjustment. Refer to

the appropriate Firmware Description Document for register addresses, calibration commands, and other

information.