PM4104APDA datasheet(3/14 Pages) SAMES

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PRELIMINARY

PM4104APDA

Using a Current Transformer

Figure 3 shows the circuit diagram of the current sense

input network when using a current transformer (CT) to

sense the line current as it is implemented on the

PM4104APDA. The CT is connected externally.

At maximum rated mains current (IMAX) the current input

network should be designed to supply an input current of

16μARMS to the current sense inputs (IIP and IIN) of the

SA4104A/B. The current sense inputs saturate at an input

current of ±17.6μARMS (±25μAPEAK), so this allows about

10% headroom until saturation occurs. Referring to Figure

3, the resistors R1 and R2 form the current transformers

termination resistor. The reference level is connected in the

centre of the termination resistor to achieve purely

differential input currents. The voltage drop across the

current transformer termination resistors at maximum rated

mains current should be in the order of 100mVRMS. The

value of the termination resistors R1 and R2 is therefore

MAX

100

where NCT is the current transformer ratio and IMAX is the

maximum rated mains current.

The resistors R3 to R6 define the current flowing into the

energy metering device. The optimum input network is

achieved by setting the input resistors equal, i.e. setting R3

= R4 = R5 = R6. Noting that the energy metering device

creates a virtual short circuit between the differential current

sense inputs (IIP and IIN) the value for the input resistors

can be calculated as follows:

MAX

If, for example, NCT = 2500 and IMAX = 40A then RB » 3W and

therefore RC = 1.5kW.

To reconfigure the PM4104APDA for a different IMAX or a

different CT turns ratio typically only the current transformer

burden resistors need to be changed. This will then not

affect any other characteristics of the current input networks.

The new value for the burden resistors can be calculated

using

MAX

The values of the burden resistors for some typical values of

IMAX when using current transformers with a turns ratio of

1:2500 (NCT = 2500) and assuming RC = 1.5kW are shown in

Table 1, rounded to the nearest available resistor value.

Table 1: Current transformer burden resistors required for

some common values of IMAX when using current

transformers with a turns ratio of 1:2500 and RC = 1.5kW

IMAX (A)

Value of Burden Resistors (

6.2

4.7

3.9

2.4

1.5

100

1.2

120

SA4104A

SA4104B

IIN

IIP

GND

CT1

Current In

16μA

RMS

Current Out

GND

JS1 “P”

JS1 “N”

SA4104A

SA4104B

IIN

IIP

GND

CT1

Current In

16μA

RMS

Current Out

GND

JS1 “P”

JS1 “N”

Figure 3: Current input network on the PM4104APDA when using a current transformer as current sensing element

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