DS1231/S

022698 5/9

APPLICATION – MODE PIN CONNECTED TO

GROUND

When the Mode pin is connected to ground, pin 1 is a

current source of 30

held below the trip point by a switching device like an

opto-isolator as shown in Figure 6. Determination of the

sense point has the same criteria as discussed in the

previous application. However, determining component

values is significantly different. In this mode, the maxi-

mum dynamic range of the sense point versus desired

trip voltage is primarily determined by the selection of a

zener diode. As an example, if the maximum voltage at

the sense point is 200V and the desired trip point is

150V, then a zener diode of 150V will approximately set

the trip point. This is particularly true if power consump-

tion on the high voltage side of the opto-isolator is not an

issue. However, if power consumption is a concern,

then it is desirable to make the value of R1 high. As the

value of R1 increases, the effect of the LED current in

the opto-isolator starts to affect the IN trip point. This can

be seen from the equation shown in Figure 6. R1 must

also be low enough to allow the opto-isolator to sink the

30

µA of collector current required by pin 1 and still have

enough resistance to keep the maximum current

through the opto-isolator’s LED within data sheet limits.

Figure 7 illustrates how the DS1231 can be interfaced to

the AC power line when the mode pin is grounded.

AC VOLTAGE MONITOR WITH TRANSFORMER ISOLATION Figure 5

IN

MODE

TOL

GND

NMI

RST

RST

TO

µ P

DS1231

VOLTAGE SENSE POINT

NOTE: RST requires a pull–up resister.