Chapter3 MakingSignal Connections
©NationalInstruments Corporation 3-9 6527User Manual
Reducing the Forward Current for High Voltages
Asinput voltage increasesabove 5 V, th e input current drawn by the 6527
(forward current If) also rises. At 24 V, for example, current per line is
found by the following equation:
Ifyou wish to reduce the current and power the 6527 draws—to reduce the
impacton a circuit you aremonitoring, for example—you can add another
resistorin series with the 3 kΩcurrent -limiting resistor on the 6527, as
showninFigure3-4.
Figure3-4. Reducing Input Current for High-Voltage Signals
Itis recommended you choose a resistance value allowing at least 1 mA to
flowthrough the LED. Assume a maximum drop across the LED of 1.5 V.
Forexample, for 24 V inputs you could use a m aximum resistance for Rs
asfoundbythefollowingequation:
24V 1.5V–()
3k
Ω
--------------------------------7.5mA=
0.25W
6527
Isolation
3k
Load
DIG+
DIG–
Supply
IsolatedGround
Rs
If
+
24V 1.5V–()
1mA
--------------------------------3k
Ω
–20 k
Ω
=