[TowerTalk] Relay lightening protection

[Kipton Moravec]

I had never heard of that before. The ULN2003 is a seven darlington
switch and has the diode built in. 

Also MOSFETS usually have the diode built into the switch for the diode
in parallel with the switch case they show in second part of Figure 1.

They recommend the silicon transient suppressor diode to optimize for
normally open contacts.


But the diode is effective for a normally closed switch. 

"Now that we have provided suggested suppression techniques based on
normally-open contact performance, we must add a qualifying comment
concerning the normally-closed contacts. When the primary load is on the
normally-closed contacts (and a small load or none on the
normally-open),
it may be desirable to use a rectifier diode alone as the relay
suppression
(or perhaps a rectifier diode and a lower value of series resistor). The
retarded armature motion that adversely impacts normally-open contact
performance will typically improve normally-closed contact performance.
The improvement results from less contact bounce during closure of the
normally-closed contacts. This results from the lower impact velocity
created by the retarded armature motion and has been utilized in the
past
improve normally-closed contact performance on certain relays."


So of course that begs the question, what to do with double throw
switches?

It seems protection for one side harms the other.

Kip

KipOn Wed, 2009-01-28 at 14:55 -0800, 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."
> 
> Jim, W6RMK
> 
> 
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-- 
Kipton Moravec AE5IB
"Always do right; this will gratify some people and astonish the rest."
--Mark Twain