ICP DAS USA PCI-P16C16 - Manual (Page 19)

Although the normal input voltage range is 5 to 24V AC or DC, the input
can still be changed to a larger range by choosing suitable external resistors.
The following figure shows how to connect to a larger input. Please note that
the input current should be limited between 2mA to 20mA; too large an input
current will burn down the internal resistor Ri, while too low of an input current
will not active the photo-coupler isolator. Calculate input voltage and current,
then replace resister Ri.
PC - 814
R
i
= 1.2K Ω /1W
I
f 20mA
Vin
On board circuits External Circuit
Figure 2-4. Isolated Digital Input
I
f = 2mA ~ 20mA
A rough estimate:
if Vin = 120V and we ignore photo-coupler turn-on voltage.
We’ll get:
Vin / I
f =
Ri
Vin = 120(V) , I
f
=10(mA) , Ri = Vin/ I
f
120(V) / 0.01 (A) = 12000 (Ω)
If you replace 1.2KΩ as resistor Ri, we can calculate the power consumption of
Ri as follows:
P = I
2
R
ex
= (10mA)
2
* 1.2KΩ = 1.2W
The power consumption is 1.2 watts, but choosing 1.5 or 2 watts is better.
Thus, we can choose a 1.2K / 2W resistor to replace the resistor Ri.
PCI-P8R8/P16R16/P16C16/P16POR16 User’s Manual (Ver.2.2, 2005/5/5) … 19