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National Instruments Corporation
15
NI 6520/6521 User Guide
Mechanical Relays
Relay Forms
Table 4 describes the relay forms available on the NI 6520/6521.
Non-Latching Relays
The relays on the NI 6520/6521 are non-latching. A non-latching relay has
an initial position of normally closed (NC). This position is maintained by
the force of a spring or permanent magnet while no current flows. The
normally open (NO) contact is closed by the force of a magnetic field while
current flows through the coil. When the current stops, the relay reverts
back to its initial NC position.
Non-latching armature relays are useful in control applications when the
switch must return to a known state if power is lost.
Figure 8.
Non-Latching Relay States
Relay Power Ratings
Caution
The maximum switching current is limited by the maximum switching power, the
maximum voltage, and must not exceed 60 W, 60 VA.
(NI PCI-6520 Only)
The maximum power rating for the NI PCI-6520 relays is
60 W or 60 VA, with a maximum current of 2 A for all channels from 0 to
55 °C, external PC ambient.
Table 4.
NI 6520/6521 Relay Forms
Form
Symbol
Description
Form A
Single-pole single-throw (SPST) relays with a default state of
normally open.
Form C
Single-pole double-throw (SPDT) relays that break the
connection with one throw before making contact with the other
(break-before-make).
NC
NO
NC Position
NC
NO
COM
NO Position
NC
NO
COM