TYPICAL APPLICATION

The analog (metering) interface described in this section is

designed for measuring 230V/60A with precision better than

Class 1.

The most important external components for the SA2005M

integrated circuit are the current sense resistors, the voltage

sense resistors and the bias setting resistor. The resistors

used in the metering section should be of the same type so

temperature effects are minimized.

Current Input IIN1, IIP1, IIN2, IIP2

Two current transformers are used to measure the current in

the live and neutral phases. The output of the current

transformer is terminated with a low impedance resistor. The

voltage drop across the termination resistor is converted to a

current that is fed to the differential current inputs of the

SA2005M.

CT Termination Resistor

The voltage drop across the CT termination resistor at rated

current should be at least 20mV. The CTs have low phase shift

and a ratio of 1:2500. The CT is terminated with a 3.6W resistor

giving a voltage drop of 86.4mV across the termination resistor

at rated conditions (I

for the meter).

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Current Sensor Input Resistors

The resistors R1 and R2 define the current level into the

current sense inputs of phase one of the SA2005M. The

resistor values are selected for an input current of 16µA on the

current inputs of the SA2005M at rated conditions. For a 60A

meter at 2500:1 CT the resistor values are calculated as

follows:

R1 = R2

= ( I / 16µA ) x R / 2

LSH

= 60A / 2500 / 16µA x 3.6W / 2

= 2.7kW

I = Line current

L

RSH = CT Termination resistor

2500 = CT ratio

The three current channels are identical so R1 = R2 = R3 =

R4=R5=R6.

Voltage Input IVP

The voltage input of the SA2005M (IVP) is driven with a current

of 14µA at nominal mains voltage. This voltage input saturates

at approximately 17µA. At a nominal voltage current of 14µA

allows for 20% overdriving. The mains voltage is divided with a

voltage divider to 14V that is fed to the voltage input pins via a

1MW resistor.

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SA2005M

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Voltage Divider

The voltage divider for phase one is calculated for a voltage

drop of 14V. Equations for the voltage divider in figure 4 are:

RB = R20 + R23 + R26

RB = R12 || (R17 + P1)

A 5k trimpot will be used in the voltage channel for meter

calibration. The center position on the pot is used in the

calibration P1 = 2.5kW.

Combining the two equations gives:

(RA + RB ) / 230V = RB / 14V

Values for resistors R17 = 22kW, P1 = 2.5kW and R12 = 1M W

is chosen.

Substituting the values result in:

RB = 23.914kW

RA = RB x ( 230V / 14V – 1 )

RA = 368.96kW.

Standard resistor values for R24, R25 and R26 are chosen to

be 130kW.

Standard resistor values for R18, R19, R20, R21, R22 and

R23 are chosen to be 120kW.

The capacitor C1 is used to compensate for phase shift

between the voltage sense inputs and the current sense inputs

of the device, in cases where CTs with phase errors are used.

The phase shift caused by the CT may be corrected by

inserting a capacitor in the voltage divider circuit. To

compensate for a phase shift of 0.18 degrees the capacitor

value is calculated as follows:

C = 1 / (2 x p x Mains frequency x R12 x tan (Phase shift angle))

C = 1 / ( 2 x p x 50 x 1MW x tan (0.18 degrees ))

C = 1.013µF

Reference Voltage Bias resistor

R7 defines all on chip and reference currents. With R7 = 24kW

optimum conditions are set. Device calibration is done on the

voltage input of the device.

Device setup

The resistor values calculated above is for a 60A rated meter

so RA must be set to VDD, according to the description under

Input Signals. The RE and IM inputs of the device must be set

to the appropriate states for the meter to function correctly.