FP-DO-400
4
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Discrete Output Circuit
The discrete outputs of the FP-DO-400 consist of optically-
isolated sourcing outputs. In the ON state, a transistor is turned on
between the positive external supply voltage (the V and V
sup
terminals) and the output (V
out
) terminal. In the OFF state, this
transistor is turned off, allowing only a small leakage current to
flow. Choose the impedance of the loads driven by the output
channels so that the current supplied by any one channel in the ON
state is no more than 2 A, and the total current supplied by all
channels at any one time is no more than 9 A
2
.
Use the following formula to ensure the total current per module is
not exceeded. Square the current of each channel and add them all
together. The result must be equal to or less than 9 A
2
. For example,
two channels with 2 A and one channel with 1 A
≤ 9 Α
2
would be
as follows:
(2 A)
2
+ (2 A)
2
+ (1 A)
2
≤ 9 Α
2
The outputs must not be short circuited to the potential of the C or
COM terminals (the negative voltage of the external supply). Such
a short circuit would cause many amps of current to flow through
an output and would damage the output. Check all wiring carefully
before applying power.
In the ON state, there is an effective resistance of 0.3 Ohms
between the output (V
out
) and the supply voltage (the V and V
sup
terminals). This resistance causes a voltage drop between the
external supply voltage and the output voltage. For example, if the
external supply voltage is 24 V and the output current is 1 A, the
output voltage is 23.7 V: [24 V – (1 A x 0.3
Ω
) = 23.7 V].
Note
Using a 5 V external supply increases the output
impedence to 2 Ohms. This increase limits the amount of
current each channel can supply.
