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PRELIMINARY

RM4104ASEB

The output voltage of the first regulator will be close to 5.6V

and that of the second stage 3.1V. The 3.3V zener diode

should be biased at approximately 3mA so the bias

resistors R4 and R5 should be (5.6V-3.1V)/3mA=820

Ω.

The half-wave rectifier output at nominal load current is

typically in the order of 12V. The 5.6V zener diode does not

require a very high bias current given that its line regulation

is not critical, so a bias current between 2mA and 3mA is

sufficient. The bias resistors R2 and R3 should therefore

be set at (12V-5.6V)/ 5.5mA

≈ 1k

Ω.

The final consideration is the temperature stability of the

supply. A 3.3V zener diode typically has a temperature

exhibits a similar temperature coefficient, leading to a total

temperature coefficient of between -4mV/ºC and -5mV/ºC.

This will cause significant voltage drop over the meter's

standard operating range and could lead to accuracy

problems on the SA4014A. The 5.6V zener diode on the

other hand has a positive temperature coefficient, typically

in the order of +2mV/ºC. As the temperature increases the

current through R4 and R5 will therefore increase and will

cause the output voltage of the 3.3V zener regulator to

increase due to the low bias current as well as the high on

resistance of the 3.3V zener diode. This configuration

almost cancels the temperature coefficient of the output

voltage of the regulator.

The series capacitor and resistor (C2 and R1) limit the

current drawn from the mains supply and determine the

power consumption (VA rating) of the supply. A 390nF

capacitor is sufficient to ensure a power supply that

operates over a range of 220V±20%. In this state the

supply consumes about 6VA. If a meter is designed for a

lower nominal mains voltage (i.e. 110V) the value of this

capacitor has to be scaled accordingly to about 820nF.

Some filtering is required on the output of the supply in the

form of two ceramic capacitors (C6 and C7) to ensure that

high frequency signals cannot reach the SA4014A through

the supply lines. These capacitors should be placed close

to the transistors to ensure that noise is filtered at its

source. A 10

μF electrolytic capacitor is required to filter out

any low frequency noise spikes in the power supply

voltage.

IMMUNITY TO ELECTROMAGNETIC

DISTURBANCE

The typical electromagnetic disturbances that can cause

the meter to fail or incorrectly record energy are:

n

Electrostatic discharge and surges

n

Electromagnetic HF (high frequency) fields

n

Fast transient burst (FTB)

Typically these disturbances will either cause complete and

permanent failure by damaging the sensitive CMOS

circuitry of the SA4104A through high voltage levels or

cause

incorrect

registration

of

energy

through

high

frequency signals that are aliased into the operating

bandwidth of the device, thereby disrupting the energy

calculation algorithm. The immunity of a class 1 watt-hour

meter to these types of disturbances is prescribed by the

IEC61036 specification.

Generally two protection mechanisms are required. The

first is to ensure that dangerously high voltage levels are

sufficiently attenuated or clamped so that no permanent

damage can occur to the SA4104A and other sensitive

components. To ensure this the complete meter is

protected with an MOV S20K275 (Metal Oxide Varistor) to

clamp any high voltage potentials. A 10nF capacitor (C1) is

placed in parallel with the MOV to ensure that any parasitic

inductance that can increase the amplitude and duration of

dangerous voltage spikes, is cancelled. The LIVE IN and

NEUTRAL inputs are connected directly across the MOV

with the lowest possible impedance. With the MOV and C1

in place all extremely high voltage levels are clamped.

Typically a larger MOV can absorb more energy and will

therefore be more effective at clamping high voltage levels

to protect the SA4104A. The subsequent filtering and

attenuation of the input networks will also ensure that no

damage can occur to the SA4104A by further reducing the

Voltage levels that can reach the device.

An MOV alone will not be sufficient to ensure that the

meter's performance is not affected by the presence of

electromagnetic disturbances during operation. All high

frequency signal components need adequate filtering by

the filters in the current sensing network, voltage sensing

network and the power supply. This should ensure that the

meters performance is not affected by the presence of high

frequency signals that are directly applied (FTB test) or

induced (HF immunity test). The immunity to these types of

interference can be improved by reducing the cut-off

frequency of the filter networks. This is however only

practical to a certain point, before matching between the

filters on the voltage and current channel cause linearity

issues at non-unity power factor. Alternatively ferrite beads

could be used to enhance the filtering, by using them