Dc Input Connections - ControlByWeb WebRelay X-WR-1R12-1I5-5 User Manual

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:
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For example:
To connect a 48VDC signal voltage to the WebRelay:
Check the power dissipated by the resistor:
Xytronix Research & Design, Inc.
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
R = ((48-1.2)/0.009)-3000 = 2200 ohms
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.
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