National Instruments CFP-RLY-421, EIGHT CHANNEL SPST RELAY MODULE Guidelines for Selecting Contact Protection Circuits

cFP-RLY-421 6 ni.com

In addition, the cFP-RLY-421 has internal protection MOVs to

prevent excessively high voltage from being applied across the

contacts. The MOVs are located between the NO and IC contacts

of each relay. However, National Instruments still recommends the

use of a protection circuit across an inductive load. The flyback

protection causes a small leakage current, which is detailed in the

Specifications section.

Guidelines for Selecting Contact

Protection Circuits1

Proper selection is critical, as the use of a contact-protection device

can extend contact life. When mounting the protection device,

always locate it near the immediate area of the load or contact.

Typically, you should mount a protective device within 18 in. of the

load or contact.

Typically, contact-protection circuits are provided for an overview,

but you should thoroughly examine the circuit you are planning

touse.

1 This section has been reprinted with permission from American Zettler, Inc.

Diagram Notes

Use in DC applications only.

Use when diode circuit causes too long

release time.

Use zener diode with zener voltage about

equal to power supply voltage.

Load