[TowerTalk] Relay lightening protection

[Roger (K8RI)]

Jim Lux wrote:
> -----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."
>   
Yah, but...they also recommend the diode AND 
resistor/zenner/varistor/MOV/ reverse bias..., or some combination.
IOW they too recommend the diode, just not the diode by itself.  Nearly 
all DC relays and particularly sensitive ones keyed by a transistor need 
some sort of clamping to protect the driver from the reverse EMF.

If you take a small DC supply sufficient to pull in one of those relays 
at say 12 or 24 volts, hang onto the wires, and make the connection, 
watch the contacts close, then pull off one of the wires.  That sucker 
will bite! <:-)) Larger coils have teeth in them.

73

Roger (K8RI)
> Jim, W6RMK
>
>
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