[TowerTalk] Fwd: Relay lightening protection

[HansLG at aol.com]

The diodes in ULN2003 are there to protect the switchers in the IC from too  
high voltage when connected to relay coils etc. It works well provided the  
"other end" of the diodes are terminated correctly (usually to Vcc). 
 
You usually don't need any protection diodes if you use mechanical  switches 
and don't switch frequently, although protection diodes extend the  lifetime 
of a mechanical switch.
 
I think you are better off if you put the protecting diode next to the  
switch as you then take care of the inductance in the wires to the relay as  well.
 
The diode in a MOSFET (the so called body-diode) can not (or should not) be  
used for this purpose as it is not in the correct position.
 
Normally you don't have to worry about the influence the  diode has on the 
timing of the relay. If you need the faster release time I  suggest you connect 
a resistor in series with the diode. Choose a resistor of  the same value as 
the DC resistance of the relay coil. That's a good start. You  can be 
sophisticated by adding capacitors etc. to a network around the relay but  I don't 
think you gain anything substantial in this application.
 
73 de N2JFS, Hans
 
In a message dated 1/28/2009 8:35:37 P.M. Eastern Standard Time,  
kip at kdream.com writes:

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


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