Rich Graziano wrote:

>I apologize if I missed some basic part of the problem, but is the transient 
>the result of the fast rise or collapse of flux of the relay coil, which is 
>what generates the transient spike?  Or is the transient from something 
>other than the relay coil?  If the transient is not generated from switching 
>the relay coil, can you please explain where the transient is being 
>generated.  Would this circuit still have the transient spike and the diode 
>problem if there was no relay coil?  What is the diode for if there is no 
>relay coil?    I thought that the coil would appear as a dead short for a 
>very brief time before the flux density increased, and an inrush of current 
>would rapidly expand the field. But perhaps I am wrong. I am curious, now. I 
>would like to learn something.  :-)
>
>  
>
Er, well, the recommendation was made that a 1N4148 would suffice as 
transient protection
to a 2n3904 transistor.

I challenged that- my experience was that while problem-solving on a 
troubleshooting contract, I found a high-
volume customer using 1N4148 across a relay that were failing by 
punch-through (shorting). This took out
the power supply fuse the next time our transistor tried to pull in the 
relay. Our switching IC was much stouter
than a 2N3904 (I think it was a SN75150? or something like that)..

I studied the literature, and replaced it with a unipolar TVS with a 
voltage rating just over 12V (15 I think).
I assumed at the time that TVS's were now the accepted present state of 
the art.

We speculated that transient spiking above the 100V max reverse voltage 
rating must have damaged
the junction, but there was no formal inspection (there are companies 
that examine faults like this for
a fee). It was not a lot failure as these diodes were from several 
different manufacturers.

The relay used had a very high duty cycle, as it was used for on-hook 
and pulse-dial..

This was a solid fix in my view, and no later failures with the TVS's 
were ever passed to me.



On the PIClist, I seemed to have been corrected, as other folks still 
use them in volume and never have
a failure. In fact, using a 2N3904 (a very slow switch) might prevent or 
reduce transients because of
slow flux changes. I never use a 2N3904 for anything; I use 2N2222A or a 
MOSFET instead.

Perhaps something else was occurring, but if so, installing the TVS 
covered that over as well... that
happens sometimes in electronics.

I still believe the fix was the right thing to do... the failure rate 
was too high  for my client to tolerate
under his circumstances. In my own designs, I have used TVS, Varistors, 
etc all over, and I've never
had any failures whatever. I do not use diodes as protective devices, 
mostly because  TVS has  a
much faster clamping speed (< 1 pS vs 4nS or slower for diodes)  which 
helps EMC compliance.

Then other folks offered a few other solutions, like RC snubbers, etc

That's the story.

--Bob

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