National Instruments FieldPoint cFP-RLY-425, Operating Instructions Manual

© 

National Instruments Corp.

9

cFP-RLY-425

The cFP-RLY-425 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 CH

x

 and COM

x

 relay 

contacts connect to form a short circuit. In the ON state, there is an 
effective resistance of up to 50 m

Ω

 between the CH

x

 and COM

x

terminals, which causes a voltage drop.

1

 For example, if the current 

is 5 A, the voltage drop across the CH

x

 and COM

x

 terminals can 

be as high as 0.25 V.

The amount of current the relay can switch depends on the voltage, 
the type of load, and the 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.

In addition, the cFP-RLY-425 has internal protection MOVs to 
prevent excessively high voltage from being applied across the 
contacts. The MOVs are located between the CH

x

 and COM

x

contacts of each relay. However, National Instruments 
recommends the use of a protection circuit across each inductive 
load. The flyback protection causes a small leakage current, which 
is detailed in the 

Specifications

 section.

Refer to the 

NI Switches Help

 for information about selecting and 

installing contact protection circuits. Go to 

ni.com/manuals

 and 

select 

Current Manual Revisions»Switches»Software»

NI Switches Help

.

Status Indicators

After you install the cFP-RLY-425 onto a backplane and apply 
power to the network module, the green 

POWER

 indicator lights 

and the cFP-RLY-425 informs the network module of its presence. 
When the network module recognizes the cFP-RLY-425, it sends 

1  

At the end of relay life, the path resistance rises rapidly above 1 

Ω 

.