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PRELIMINARY

RM4104ASEB

The variables DF_LED and DF_MO are defined by setting

FMS, R0, R1,R2 and R3 according to Table 2.

FMS

R3

R2

R1

R0

DF_LED

DF_MO

0

0

0

0

0

220

256

0

0

0

0

1

440

128

0

0

0

1

0

880

64

0

0

0

1

1

1760

32

0

0

1

0

0

220

128

0

0

1

0

1

440

64

0

0

1

1

0

880

32

0

0

1

1

1

1760

16

0

1

0

0

0

220

64

0

1

0

0

1

440

32

0

1

0

1

0

880

16

0

1

0

1

1

1760

8

0

1

1

0

0

220

32

0

1

1

0

1

440

16

0

1

1

1

0

880

8

0

1

1

1

1

1760

4

1

0

0

0

0

1464

64

1

0

0

0

1

2928

32

1

0

0

1

0

352

64

1

0

0

1

1

704

32

1

0

1

0

0

584

32

1

0

1

0

1

1168

16

1

0

1

1

0

352

32

1

0

1

1

1

704

16

1

1

0

0

0

584

16

1

1

0

0

1

1168

8

1

1

0

1

0

352

16

1

1

0

1

1

704

8

1

1

1

0

0

292

16

1

1

1

0

1

584

8

1

1

1

1

0

234

16

1

1

1

1

1

468

8

Table 2: Division factors available on the SA4104A

‘0' indicates that the input is connected to VSS

'1' indicates that the input is connected to VDD

POWER SUPPLY

The RM4104ASEB meter uses a capacitive supply with two

zener diode based regulators with series transistors to

generate the required +2.5V and -2.5V power supply. The

complete power supply circuit is shown in Figure 4. The

main consideration behind this type of supply is low cost.

A typical simple single stage zener diode regulator does not

have sufficient stability across temperature variations, load

current variations and input mains voltage variations to

supply the SA4104A with an adequate quality power supply.

Furthermore, a typical 2.5V zener diode has a very large

series resistance which causes large voltage variations as

the load current varies. The series transistor output stage

ensures that the zener diode can be biased at almost

constant current for varying load current, thereby improving

the load regulation of the supply. This is especially important

in this application, where the load current can peak when

now in series with the load so a 3.3V zener diode is chosen.

If this diode is biased at a fairly low current (approximately

3mA) the breakdown voltage is approximately 3.1V. Now

voltage of the regulator is 2.5V. The transistors can be any

low-cost switching transistors.

The output series transistor configuration improves the load

regulation

but

cannot

improve

the

line

regulation

characteristic of the regulator. Due to the high on-resistance

of a low voltage zener diode the line regulation of the supply

is quite poor. Ensuring good line regulation is important

because mains voltage variations and the high load current

surges can change the input voltage to the rectifier

significantly. If the line regulation is poor, the performance of

the SA4104A will be affected. To improve the line regulation

a second zener diode regulation stage is used. This stage is

based on a 5.6V zener that basically creates a fairly stable

input voltage for the second regulator stage.

C2

390nF

250VAC

1N4007

+

+

+2.5V

VDD

VSS

-2.5V

+

LIVE IN

NEUTRAL

R1

470

Ω

D2

D3

1N4007

C3

220µF

C4

220µF

R2

1k

Ω

R3

1k

Ω

D4

5V6

D5

5V6

820

Ω

R5

820

Ω

D6

3V3

D7

3V3

R4

Q1

BC846B

Q2

BC856B

C5

10µF

C6

680nF

C7

680nF

C2

390nF

250VAC

1N4007

+

+

+2.5V

VDD

VSS

-2.5V

+

LIVE IN

NEUTRAL

R1

470

Ω

D2

D3

1N4007

C3

220µF

C4

220µF

R2

1k

Ω

R3

1k

Ω

D4

5V6

D5

5V6

820

Ω

R5

820

Ω

D6

3V3

D7

3V3

R4

Q1

BC846B

Q2

BC856B

C5

10µF

C6

680nF

C7

680nF

Figure 4: Circuit diagram of the power supply circuit