Chapter 3
Making Signal Connections
©
National Instruments Corporation
3-9
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
=
