Alan B. Pearce wrote:

>>The relay used had a very high duty cycle, as it
>>was used for on-hook and pulse-dial..
>>    
>>
>
>I suspect this was the real reason for the failure, effectively the pulse
>dialling exceeded the repetitive surge current rating, causing the diode to
>fail. The only ones that failed were probably close to the min spec limit,
>with most devices having enough margin that they do not fail, although on
>the basis of the failure rate, one would possibly need to look at long term
>reliability.
>  
>

agreed.

>  
>
>>This was a solid fix in my view, and no later
>>failures with the TVS's were ever passed to me.
>>    
>>
>
>And I do not think anyone can fault this fix.
>  
>

thanks.

>I suspect that most of the discussion has resulted from TVS devices being
>perceived (probably without foundation) as harder to find, and potentially
>more expensive.
>
When I did this fix, TVS _were_ fairly new and more costly. Today, the 
costs are low
(but not as low as a 1N4148 diode).

> In this instance I feel the extra expense (if any) could be
>justified, and certainly the time taken to source the device is justified,
>and would certainly justify changing new build to fit the TVS instead of a
>diode.
>  
>
Later layouts put the TVS in permanently.

--Bob

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