XW-210 WiFi™ Users Manual
2.5 Digital Input Connections
The XW-210 has an optically-isolated digital input which can be used to control the relay and/or a remote
relay (over the network), or simply to monitor the state of a device. To use this input, connect a DC
control voltage directly to the input, and customize the input function using the XW-210 configuration
pages.
2.5.1 DC Input
The digital input is connected internally through a current-limiting resistor directly to a photo-coupler
circuit. No external resistor is necessary as long as the input signal is within the proper range
(See
Appendix A: Specifications)
. A DC voltage can be reduced with an external resistor of the appropriate
value and power rating to reduce the input current.
The digital input has an internal 3K ohm resistor. The forward voltage drop of the photo-coupler is
approximately 1.2V and works well with an input current of 9mA. Use the following formula to determine
the resistor value needed for other voltage ranges:
R =((Vin-1.2)/0.009)-3000
Where:
•
R = External resistor value required
•
Vin = Desired input voltage
•
1.2V = forward voltage drop of the LED in the photo coupler
•
.009A = workable LED current
•
3000 ohm = Internal resistor
For example:
To connect a 48VDC signal voltage to the XW-210:
R = ((48-1.2)/0.009)-3000 = 2200 ohms
Check the power dissipated by the resistor:
P = I x I x R, The resistor must be at least .009 x .009 x 2200 = 0.18 Watts, so use a 1/2
Watt resistor.
Xytronix Research & Design, Inc.
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