[Premium-Rx] Power Line Surge Protection for PremRX Receivers

[George Georgevits]

Hi Ahmet and avid Premium Rx followers,

Thanks for pointing me to a most interesting article. As a little
background, I have been involved with surge protection for many years now as
an engineer, and I confess to having written a number of articles on the
subject of surges and surge protection for a local cabling magazine here in
Australia. I also regularly seem to get involved with lightning protection
and the repair of commercial electronics, such as test equipment and
industrial controllers etc. from time to time.

So, turning to the article at hand, I must say I am always suspicious of
someone who discovers some new phenomenon which has been with us,
undetected, for a long time, and at the same time just "happens" to have
developed a miracle cure, whilst at the same time having a pecuniary
interest in the said "cure". I am not sure what is happening in other
countries, but here in Australia there has recently been many articles in
the press about pharmaceutical companies trying to do exactly this with new,
exotic diseases which no-one has ever heard of but by sheer coincidence many
people seem to have the symptoms. Hating to go to work on cold, wet Monday
mornings is one which I suffer from badly! Fortunately, a nice hot cup of
coffee before starting off is a direct cure.

Anyway, back to the matter at hand. Whilst I agree with much of what the
author has to say, in my view there are a couple of flaws in his logic.

As I understand it, in-rush current occurs when a mains powered device is
switched on at a moment when the mains voltage happens to be instantaneously
high. The in-rush current occurs when the capacitors within the equipment
are first charged. It has nothing to do with lightning-induced or other
surges which may appear on the mains from time to time. Mains voltage sags
are really only a lesser case of what happens whenever the device is
switched on.

Also as I understand it, the problems associated with in-rush current really
only affect the reliability of switched mode power supply (SMPS) based
equipment these days. The in-rush current at switch-on usually has to pass
through the power semiconductors within the SMPS of the equipment in
question, because the SMPS has to initially charge some components internal
to the equipment. This initial charging current places a peak load stress on
the SMPS semiconductors in question.

Because power switching semiconductors are not so good at handling such
transient loads (certainly the older types were not), they tended to fail
after a number of such start-up operations. In the past, computer monitors
and TV sets were commonly seen to be afflicted with this ailment. I lost
count of how many failed computer monitors I have seen.

I also make the following comments on this subject:

.	As I understand it, the damage to the SMPS power semiconductors occurs
progressively over time, after many operations involving in-rush currents. I
think I read somewhere that small parts of the chip within the power
semiconductor are progressively destroyed with each excessive current surge,
until the device finally fails. So explaining the random failure incidence
is not really such a "mystery" as the article suggests. Given the failure
mechanism, it is also never clear whether any adjacent equipment has
suffered damage, because partially damaged semiconductors will keep working,
so that symptoms of the damage are not always evident.

.	Modern SMPS semiconductors are designed better these days, and seem to
last longer because they are better able to withstand the repetitive peak
loads.

.	Many premium receivers do not use SMPS technology (fortunately!), so the
article is not really applicable as such. If you are worried about surges,
then transient voltage protection across the mains is all that would be
required. In 30 years I don't think I have ever seen the input circuit
components to a linear power supply fail due to in-rush current.

.	For those receivers that do use an SMPS, one would hope that they are
better designed that the average old (and cheap) CRT based computer monitors
or TVs.

.	The power requirements for a modern (well, solid state at least)
communications receiver should be less that for a TV or computer monitor,
and the power demand is more steady, so the power supply should overall be
less stressed, if designed properly (some of course were not!).

.	Limiting incoming mains transients can be achieved simply and effectively
by using a mains input filter which contains significant inductance as well
as capacitance. Such filters are designed to be low pass at around the mains
frequency, and hence effectively block voltage spikes AND smooth out brief
voltage sags. I once recovered such a filter from an old IBM mainframe
computer at a scrap yard. It provides excellent protection against all forms
of transients.

.	The article states that brief voltage sags typically occur 30 to 100 times
a year. How many times do you turn your computer or premium Rx on and off in
a year?

.	Lastly, and most importantly, an in-rush current, like all other currents
(hopefully), obeys ohm's law. You can't just have an in-rush current
appearing by itself, without the accompanying low impedance load and a high
driving voltage. In-rush currents don't just suddenly appear on the mains
like voltage transients. And like I mentioned above, voltage sags are really
no different to what happens every time at switch-on.

So, have I missed something here? Looking forward to some interesting
comments.


Regards,
George Georgevits
VK2KGG