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application. Therefore, a voltage high enough to cause the LED to turn on will be read by the application as
a logic high level.
The input impedance of each isolated input is approximately 560 ohms (factory default). The opto-isolator
requires approximately 3mA to turn on. The maximum input current is 50mA. There are two things to
consider when selecting the input resistor. The first is turn on voltage for the circuit to sense, and second
is the maximum input voltage. Maximum input voltage must not provide too much power to the input
resistor and must also not overdrive the opto-isolator input current specification. The following formulas
apply:
Turn on Voltage = diode drop + (turn on current) x (resistance) [Ex: 1.1 + (.003) x R]
Input Current = ((input voltage)-1.1V) / (resistor value)
Maximum voltage = 1.1 + square root of (.25(resistor value))
The following table shows common input resistors and the ranges associated with each.
Input Resistor
220Ω
560Ω
1KΩ
2.2KΩ
3.3KΩ
4.7KΩ
The turn-off voltage for all resistors is less than 1V. The opto-couplers require a minimum voltage
to turn on and this voltage is dependent on the input resistor shown on the table above.
Increasing the input resistor accordingly can increase the maximum input voltage. Because socketed DIP
resistors are utilized, they can easily be replaced with a different value. Sealevel can do this, if necessary.
The input circuits are not intended for monitoring 120-volt AC circuits. In addition to being too high
a voltage for the circuits, it is dangerous to have that high a voltage on the card.
© Sealevel Systems, Inc.
8004e Manual | SL9207 12/2022
Turn-On
Input Range
1.8V
1.8 – 7.0V
2.8V
2.8 – 10.6V
4.1V
4.1 – 13.8V
7.7V
7.7 – 20.0V
10.0V
10.0 – 24.0V
15.2V
15.2 – 28.0V
Max Input
Max Current
8.5V
27mA
12.9V
20mA
16.9V
15mA
24.5V
10mA
30.0V
9mA
35.0V
7mA
12
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