© National Instruments Corp. 5 cFP-RLY-421
Connecting Loads to the cFP-RLY-421
Wire an external power supply to the load and IC terminal of the
individual channel as shown in Figure2. This method of wiring
keeps the electrical connections of the cFP-RLY-421 within safe
current limits.
Figure 2. Connecting a Load
The cFP-RLY-421 has eight SPST (Single-Pole-Single-Throw)
electromechanical relays. The power-up state is off (open) to
ensure safe installation. In the ON state, the NO and IC contacts
connect to form a short circuit. In the ON state, there is an effective
resistance of 100 mΩ between the NO and IC terminals, which
causes a voltage drop. For example, if the current is 1.5 A, the
voltage drop across the NO andIC terminals is 0.15 V.
The amount of current the relay can switch depends on the voltage,
the type of load, and ambient temperature. Refer to the
Specifications section for more information.
Protecting Contacts for Inductive Loads
When inductive loads are connected to the relays, a large
counter-electromotive force may occur at relay switching time
because of the energy stored in the inductive load. These flyback
voltages can severely damage the relay contacts and greatly
shorten the life of the relay.
It is best to limit flyback voltages by installing a flyback diode
across an inductive DC load or a metal oxide varistor (MOV)
across an inductive AC load. Refer to the Guidelines for Selecting
Contact Protection Circuits section for more information.
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To next channel
cFP-RLY-421
IC
NO
V C
VSUP
COM
or
AC
Sourcing
Load