[TowerTalk] Relay lightening protection

[Jim Lux]

-----Original Message-----
>From: Kipton Moravec <kip at kdream.com>
>Sent: Jan 28, 2009 2:29 PM
>
>I can answer the diode question.
>
>It is generally good practice to put a diode across the relay because of
>the inductive spike when turning it off. I have seen the spike go 3-4X
>the input voltage. If you may have a very long line to the relay the
>spike may be more. 
>

Actually, the relay manufacturers (e.g. Tyco, P&B, etc.) recommend NOT using the diode across the coil, because it reduces relay life and changes actuation speed.  Depending on whether you're suppressing across the coil or load, the approaches are different.

http://relays.tycoelectronics.com/appnotes/app_pdfs/13c3311.pdf

as they say:
"Many engineers use a rectifier diode alone to provide the transient suppression for relay coils. While this is cost effective and fully eliminates the transient voltage, its impact on relay performance can be devastating."

Jim, W6RMK