12
8 INPUT/8 OUTPUT PCI CARD
When selecting the input resistor, consider:
1. Power-on voltage for the circuit to sense.
2. The maximum input voltage. Maximum input
voltage must not overload the input resistor,
and
must not overdrive the opto-isolator input-current
specification. The following formulas apply:
• Power-on current: 3 mA
• Isolator diode drop: 1.1 V
• Resistor power max: 0.25 W
Power-on voltage = diode drop + power-on current x
resistance
Or:
1.1 + (.003 A) x R
Maximum voltage = square root of (0.25 [resistor
value])
The table on the next page shows four common input
resistors and the ranges associated with each.
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