National Instruments USB-6525 - Output Operation and Load Protection; Solid-State Relay (SSR) Outputs; Protecting Inductive Loads

© National Instruments Corporation 11 USB-6525 User Guide and Specifications
closed, current flows through both the load and the USB-6525 optocoupler,
and the USB-6525 registers a logic high for the channel.
Solid-State Relay (SSR) Outputs
You can connect loads to the USB-6525. Connect the load to one of the
leads of the power source. Connect either the P0.xA or the P0.xB terminal
to the load and the other terminal to the other lead of the AC or DC power
source. Figure 7 shows a possible configuration where the load is
connected to the P0.xB terminal and the DC or AC power source.
Figure 7. Connecting a Load to the USB-6525
Caution
Use twisted-pair field wiring to reduce EMC noise.
Power-On and Power-Off Conditions
The default power-on state of the solid-state relays is open. By default, the
solid-state relays remain open when the chassis and the USB-6525 device
are powered off.
Protecting Inductive Loads
When inductive loads are connected to the USB-6525 SSR outputs, a large
counter-electromotive force may occur at switching time because of the
energy stored in the inductive load. These flyback voltages can damage the
SSR outputs and/or the external power supply.
Limit flyback voltages at your inductive load by installing one of the
following:
• For DC loads—Install a flyback diode within 18 in. of the load.
• For AC loads—Install a metal oxide varistor (MOV) rated for 30 V
rms
or slightly higher.
+
_
or
AC
USB-6525
P0.x A
P0.x B
Load
Twisted-Pair
Wiring