"Thyractors" on the DEC LT33 modified ASR33 Teletype

Dave Brown tractorb at ihug.co.nz
Thu Mar 24 02:45:47 CST 2005

----- Original Message ----- 
From: "Wai-Sun Chia" <waisun.chia at gmail.com>
To: "General Discussion: On-Topic and Off-Topic Posts" 
<cctalk at classiccmp.org>
Sent: Thursday, March 24, 2005 7:41 PM
Subject: Re: "Thyractors" on the DEC LT33 modified ASR33 Teletype

> On Wed, 23 Mar 2005 12:54:35 -0500, Paul Koning 
> <pkoning at equallogic.com> wrote:
>> If it's a voltage clamp, then either back to back zeners, or a MOV
>> should work.  MOV might be the better choice since they are 
>> explicitly
>> designed as protective devices.  Zeners would be ok in the larger
>> sizes, but the small ones may be fragile enough that they don't do
>> well in this application.
> I agree. MOVs are designed to be dirt-easy and robust to boot. If I
> were to do it, I'd parallel a couple of 5W zeners BEHIND a suitably
> sized MOV just to be on the safe side.
> /wai-sun

Seems to be not well known that MOVs die - slowly but inevitably - 
with use. Every time they shunt a surge, they shorten their useful 
life. The harder they work, the shorter their life.  If they are known 
to be earning their keep, they should be routinely replaced to 
maintain the required/designed protection level.
For a low power application like this I'd be inclined to use 
back-to-back (in series opposing) zeners with suitable ratings.  Make 
'em big enuff and they'll last forever.
DaveB, NZ

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