Overcurrent Protection - National Instruments FieldPoint FP-PWM-520 Operating Instructions Manual

Pulse-Width Modulated Output Circuit
The FP-PWM-520 PWM output channels have optically isolated
sourcing outputs with overcurrent protection circuitry. In the ON
state of the output period, a transistor is turned on between the
positive external supply voltage (the V and V
output (V ) terminal. In the OFF state, this transistor is turned off,
out
allowing only a small leakage current to flow. Select 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 1 A.
The output channels need an external power supply of 5 VDC
or between 10 and 30 VDC connected to the C and V terminals
of the module.
In the ON state, the effective resistance between the output and the
external supply voltage is 0.3 Ω, which causes a voltage drop.
For example, if the external supply voltage is 5 V and the output
current is 1 A, the output voltage is 4.7 V:
Figure 3 shows a diagram of the discrete output circuit of one
channel.

Overcurrent Protection

Overcurrent protection circuitry on each output deactivates the
output if the current it sources to the V
level. This level is always at least 1 A. A slightly higher inrush
current that exists for a short time will not trigger the protection
circuitry. If the protection circuitry disables an output that would
otherwise be in the ON state, the status indicator for that channel is
still lit, but the output transistor is turned off.
FP-PWM-520
[5 V – (1 A x 0.3 Ω) = 4.7 V].
V
Optical
Isolation
C
Figure 3. Discrete Output Circuit
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V
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