Reducing The Forward Current For High Voltages - National Instruments 6527 User Manual

Reducing the Forward Current for High Voltages

© National Instruments Corporation
As input voltage increases above 5 V, the input current drawn by the 6527
(forward current I ) also rises. At 24 V, for example, current per line is
f
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 kΩ current-limiting resistor on the 6527, as
shown in Figure 3-4.
Isolation
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
as found by the following equation:
(
------------------------------- - 3 k Ω
Chapter 3
( )
24V 1.5V –
------------------------------- - = 7.5 mA
3 k Ω
6527
3 k
0.25 W
DIG+
I
f
DIG–
)
24V 1.5V –
– =
1 mA
3-9
Making Signal Connections
R
s
+
Supply
Load
Isolated Ground
20 k Ω
6527 User Manual
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