Maxim Integrated 78M6631 Evaluation Board - Manual (Page 11)

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 full range of the ADC inputs (±250mV
P-P
).
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
mVrms (±250mV
P-P
). It is useful to provide a voltage margin to accommodate transients above IMAX. For
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:
𝑉𝑜𝑢𝑡 =
𝐼𝐿𝑜𝑎𝑑
𝑁 ∗ 𝑅𝐵𝑢𝑟𝑑𝑒𝑛
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.