Omron NX-ID - Page 122

4 Installation and Wiring
4 - 46
NX-series Digital I/O Unit User’s Manual (W521)
(c) Relation between OFF current of the DC Input Unit and sensor leakage current
The DC Input Unit cannot detect sensor output OFF unless the following conditions are satisfied:
I
OFF
≥ I
leak
When I
leak
is greater than I
OFF
, connect a bleeder resistor R.
Use the following equation to calculate the bleeder resistance constant of the resistance input.
R ≤ R
IN
× V
OFF
/ (I
leak
× R
IN
- V
OFF
)
Use the following equation to calculate the bleeder resistance constant of the constant current input.
R ≤ (V
OFF
/ I
OFF
) × V
OFF
/ (I
leak
× (V
OFF
/ I
OFF
) - V
OFF
)
For both the resistance input and constant current input, use the following equation to calculate the
rated power of bleeder resistor.
Rated power W of bleeder resistor ≥ (V
CC
- V
R
)
2
/ R × 4 [allowable margin]
The voltages and currents related to the conditions for NPN type sensors are shown in the figure
below.
IN
IOG
IOV
IOG
IOV
V
R
V
CC
R
I
OFF
I
leak
R
IN
V
OFF
Two-wire sensor
DC Input Unit
Communications Coupler Unit or
Additional I/O Power Supply Unit