Cutler-Hammer D320 PLC, User Manual

Page: 48 / 258

36
D320 PLC User's Manual
The power rating W required for the bridge resistor R can be calculated as follows:
W = (Power Voltage)
2
/R
When specifying the resistor, set it within 3 to 5 times of this value.
LED Limit Switch
When using a limit switch with internal LED On/Off indication, the input may not be turned off due to
the effect of leakage current, or the LED may be incorrectly illuminated. Connecting the bridge
resistance as shown in the figure below may help solve these problems.
Example: D320DIM1624D—12-24 VDC type input module (Off voltage 2.5V, input impedance 3k
Ω
)
For many sensors, the manufacturer will provide the value of the internal resistance r, in which case
the leakage current I can be directly calculated in the following equation (the Off voltage of the input is
2.5 V, and power voltage is 24 V):
I = (24 – 2.5)/r
Alternatively, the value of I can be experimentally calculated by simply measuring the current draw of
the sensor during use. Once I is calculated, the bridging resistor specification can again be calculated
using the following equations:
R
≤
7.5/(3I – 2.5)(k
Ω
)
W = (Power Voltage)
2
/R
×
(3 to 5 times)
LED Reed Switch
When using a reed switch with an LED On/Off indication, the voltage going into the input terminal
should not exceed the On voltage under normal Off conditions. No type of bridging resistor is required.
r = Internal resistance (k
Ω
) of limit switch
R = Bridge resistance value (k Ω )
Bridge
Resistance
R
r
LED Limit Switch
DC Input Module
Input
Terminal
COM
Terminal
Over 10 V
LED
Contact
LED Reed Switch
DC Input Module
Input
Terminal
COM
Terminal
12 to 24 V