National Instruments 6527 - Manual (Page 25)

Chapter 3 Making Signal Connections
© National Instruments Corporation 3-9 6527 User Manual
Reducing the Forward Current for High Voltages
As input voltage increases above 5 V, the input current drawn by the 6527
(forward current I
f
) also rises. At 24 V, for example, current per line is
found by the following equation:
If you wish to reduce the current and power the 6527 draws—to reduce the
impact on a circuit you are monitoring, for example—you can add another
resistor in series with the 3 k
Ω current-limiting resistor on the 6527, as
shown in Figure 3-4.
Figure 3-4. Reducing Input Current for High-Voltage Signals
It is recommended you choose a resistance value allowing at least 1 mA to
flow through the LED. Assume a maximum drop across the LED of 1.5 V.
For example, for 24 V inputs you could use a maximum resistance for R
s
as found by the following equation:
24V 1.5V –
( )
3 k
Ω
-------------------------------- 7.5 mA =
0.25 W
6527
Isolation
3 k
Load
DIG+
DIG–
Supply
Isolated Ground
R
s
I
f
+
24V 1.5V –
( )
1 mA
-------------------------------- 3 k
Ω
– 20 k
Ω
=