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PM9109BFA

MONOCHIP SINGLE PHASE BIDIRECTIONAL KILOWATT HOUR METERING MODULE

South African Micro Electronic Systems 

PM9109BF

The SA9109B data sheet uses 80A and 230V for rated current and rated voltage

when explaining functionality of the device. This is just for example and any other

rated values may be used. All AC voltages and currents are RMS values unless

otherwise specified.

Let us assume that we want to measure the energy for current range 0 ... 200A

and voltage range 0 ... 120V. In this case ILR=200A is a rated line current and

VLR=120V is a rated line voltage. Rated line power is 200A * 120V = 24kW.

Three simple steps must now be followed:

STEP 1

Select the shunt resistor RSH value and adjust the values of resistors R1, R2 in

such a way that current flowing into the device’s current sense input (pins 9 and

10) is 16µA for rated line current (200A). This should be simple and straightforward

task because current sense input of the device (pins 9 and 10) behaves as a

virtual short. For this example a shunt resistor with a value of 625µΩ is selected.

This will result in the current sense input resistors having a value of R1=R2=1.6kΩ.

It is important to ensure that the voltage drop across the shunt resistor is not too

low as the precision may suffer.

STEP 2

Choose resistors R3, R6 and R4 such that current flowing into the voltage sense

input (pin 8) is 14µA for rated line voltage (120V). R3 and R4 forms a voltage

divider from line voltage to approx. 10-20V and R4 is a serial resistor for setting

the required voltage sense current of 14µA. Input to the device again behaves as

a virtual short (with respect to GND - pin 11).

The value of capacitor C5 introduces a phase shift which can be used to

compensate for current transformers, it is chosen to ensure that it forms a short

with respect to R4 for typical mains frequency.

For this example R3 = 106K, R6 = 14K and R4 = 1M.

STEP 3

To calculate how much energy is represented by one pulse on output SDO (pin

39) and one Display increment, the following is now performed:

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