On 10/5/12 4:38 AM, Julius Fazekas wrote:
This topic is interesting and I have played with a few different HB
options. Which is the source of my question, what is the best method
(how?) of determining the value of MOVs, TVSs, flash tubes or other
options? Or is it any value is better than nothing, which seems to be
the case in most ham related writings (which seem to be few and far
between) on the topic.
IN theory, you choose the highest voltage that won't damage the
downstream equipment. What you don't want is "normal" events hitting
the MOV repeatedly. This is one of the causes of fires in "surge
suppressor plug strips".... Advertising a low clamp voltage, which
actually isn't needed, and periodic "swells" in line voltage hit the
threshold and dissipate power in the MOV. (and killing the MOV a bit
the back to back zener schemes (TransZorb, for instance) are better than
MOVs (but more expensive), because they don't have the incremental life
consumption of MOVs.
A study some 10-20 years ago by, I think, a Canadian organization,
looked at the response of consumer electronics to line transients
(Canada has lots of above ground distribution AND thunderstorms) and
found that almost all modern equipment can tolerate 2000V spikes on the
power line (a wall wart, for instance, will block that with no sweat,
either common mode or differential, as will most anything with a power
supply either linear or switching.) There's very little sold these days
that operates directly off-line in the All-American 5 tube radio scheme
or older TV designs.
What kills things today is big differentials between "chassis" and
"signal"; and they're getting tougher. For instance, the parallel
printer port is notorious: the connector pins went right to the 74LS244
or similar, and these days, right to some ASIC. It doesn't take much to
kill that. Cheap serial ports (that don't use RS232 line drivers, which
can easily take 30V spikes and have current limiting on the signal
leads) get killed too. USB is much tougher, of course.
If you want to know what to really do, and to get good explanations..
get Ronald Standler's book "Transient Protection of Electronic
Circuits".. it's something like $17 in paperback from DOver and has both
theory and cookbook "what do you do in situation X". ANyone who is
*designing* equipment should read it, because doing the "right thing" is
no more difficult than the "wrong thing" (and he has lots of examples of
poor design from a transient damage standpoint... some of which are
straight out of *old* editions of the ARRL handbook..)
As the cover blurb says: "This text presents practical rules and
strategies for circuits designed to protect electronic systems from
damage by transient overvoltages"
I see that Dover now charges $27...
Still worth every penny.
Standler's website has useful information too, but the book is better.
So far I've been lucky with only a few popped diodes or transistors
and two fried UPS units, but would prefer fewer incidents.
Tennessee Contest Group
Tennessee QSO Party
Elecraft K2 #4455
Elecraft K3/100 #366
Elecraft K3/100 #4461
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