73
3. Relation between current when the PC is OFF and sensor leakage current:
I
OFF
y
I
leak
Refer to
Input Leakage Current
later in this section
for details.
The I
OFF
values differ for each Unit, but is always 1.3 mA for Input Units
whose OFF current specifications are not given.
Two-wire sensor
DC Input Unit
V
CC
: Power voltage
V
ON
: PC ON voltage
I
ON
:
PC ON current
I
OFF
: PC OFF current
R
IN
: PC input impedance
V
R
:
Sensor output residual current
I
OUT
: Sensor control current (load current)
I
leak
: Sensor leakage current
R:
Bleeder resistance
V
R
R
V
CC
R
IN
Output Circuits
If a load connected to the output terminals is short-circuited, output elements
and printed boards may be damaged. To guard against this, incorporate a
fuse in the external circuit.
When connecting TTL circuits to transistor Output Units, it is necessary to con-
nect a pull-up resistor and a CMOS IC between the two. This is because of the
residual voltage left on the transistor output after the output turns OFF.
Output Leakage Current
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.
V
ON
I
–––
R
Where
V
ON
= ON voltage of the load in V
I = leakage current in mA
R = bleeder resistance in k
Ω
Output Short-circuit
Protection
Transistor Output Residual
Voltage
Wiring
Section 3-2
