49
I
OFF
I
leak
Refer to the information on input and output leakage current shown below.
The value of I
OFF
depends on the Unit used. For Input Units that have an unspe-
cified OFF current, however, the value of I
OFF
is always 1.8 mA.
V
CC
: Power supply voltage V
R
: Sensor output residual voltage
V
ON
: PC ON voltage I
OUT
: Sensor control output (load current)
I
ON
: PC OFF current I
leak
: Sensor leakage current
I
OFF
: PC OFF current R: Bleeder resistance
R
IN
: PC input impedance
Two-wire sensor
DC Input Unit
V
R
R
V
CC
R
IN
When two-wire sensors, such as photoelectric sensors, proximity sensors, or
limit switches with LEDs, are connected to the PC as input devices, the input bit
may be turned ON erroneously by leakage current. In order to prevent this, con-
nect a bleeder resistor across the input as shown below.
Sensor
Input
power
supply
Bleeder
resistor
R
PC
If the leakage current is less than 1.3 mA, there should be no problem. If the leak-
age current is greater than 1.3 mA, determine the value and rating for the
bleeder resistor using the following formulas.
7.2
2.4 x I – 3
––––––
R =
kW max.
W =
2.3
R
–––
W min.
I = leakage current in mA
If there is a possibility of leakage current causing a transistor or triac to malfunc-
tion, connect a bleeder resistor across the output as shown below.
PC
Load power supply
OUT
COM
Bleeder resistor
L
R
Determine the value and rating for the bleeder resistor using the following for-
mula.
E
on
I
–––
R
Input Leakage Current
Output Leakage Current
I/O Connections Section 4-4
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