24 Mar
2005
24 Mar
'05
12:45 a.m.
----- 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:
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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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