WebRelay Users Manual
2.4.2 DC Input Connections
The digital input is connected internally through a current-limiting resistor directly to a photo-coupler
circuit. The photo coupler inputs isolated from the power supply circuits. No external resistor is
necessary as long as the input signal is within the proper range
(See Appendix D: 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 WebRelay:
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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