[Lowfer] TX output protection.

Tue, 02 Sep 2003 13:31:27 -0400

An MOV is not just a large capacitor though they do have a significant
capacitance due to the ZnO dielectric constant.  I have taken the liberty of
pasting here a post by Steve, WB2WIK, on the "Towertalk" reflector his comments
about MOVs.

snip:

"MOVs used doped grains of zinc (or other) oxide in series to build up
breakdown voltage (since a single doped grain has a BV of only about 2.2V),
and as surges are applied, the grains not only avalanche as they should, but
begin to break down permanently as conductors.  Obviously, each grain that
fails (and the failure mode is low-resistance, initially) reduces the
breakdown voltage of the string by about 2.2V, and this is a measurable
parameter, using a curve tracer or any number of instruments.

If you measure the leakage current of a typical 120Vac line MOV device (or
MOV-protected device), it will be very low initially, in the microamps.  As
grains break down permanently, leakage current will continue to increase,
because the remaining grains are approaching, or into, avalanche and
conducting more current.  Just before the device fails altogether, leakage
current can be very high, like 10mA or so, so now the MOV is dissipating
serious power even without any transient (surge) events.

Then, when it gets so hot that the lead bonds melt, the device will "open
circuit," and revert back to no leakage current -- and no protection -- at
all.  However, in most situations, prior to this catastrophic failure, the
leakage current becomes so high that a fuse or circuit breaker will pop,
indicating a problem!  At least, it theory that's what's "supposed" to
happen.  In reality, it often doesn't.

Since testing MOVs is a chore and a nuisance, I simply avoid using any, and
only buy the "unprotected" outlet strips!

-WB2WIK/6"

Switching times are, however, typically slow compared to gas tubes (like, e.g.,
PolyPhaser).

If this has already been answered furthur down in the mail, I apologize - still
working my way down thru several days of mail.

73,

Tom - WA2BPE

Bill Ashlock wrote:

> Kurt,
>
> I don't buy the idea that an MOV is just a large capacitor. It's true that
> they have a large amount of capacity, and sometimes it's too large to use
> them to protect a certain circuit, but they are very fast in sensing an
> overvoltage and are capable of absorbing a large amount of energy without
> damage to themselves (something a capacitor cannot do).
>
> Two years ago I was responsible for re-qualifying our complete product line
> of controllers, pressure sensors, and chillers to a new power line
> resilience specification EN60126 required for CE sales (virtually all of the
> non-USA world). The main provision of this spec was to assure that any
> equipment inside a industrial building would withstand a lightening hit to
> the building. The EN60126 test requirement was something like a spike of
> 1500v at 50A with a duration of ~50 microseconds. The product had to
> withstand this spike applied to power AND signal inputs. The final testing
> was performed by a EMI/ESD test house but I had to, first, design the
> changes needed to make the circuits pass. I used a 2000v, 5uf capacitor with
> a series 30 ohm resistor to simulate the $20,000 equipment used at the test
> house and found the test results were similar.
>
> It turned out that the MOV (along with a small series resistor) was by far
> the best way to eat up a pulse of high energy without passing it on to other
> components. Check out the specifications for MOVs (also called varristers,
> ZNRs, and surge absorbers) on the Web. BTW, Digi-Key has a good selection of
> these.
>
> If the MOV capacity (1000pf for the ones I was using) was too large for the
> application (such as the input and output of a Lowfer final) I used a 1N4004
> to ground and Vcc, and then a MOV from Vcc to ground. Also I had to assure
> sufficient series resistance so the current resulting from the conduction of
> the diode did not damage any other component. I later adapted this approach
> to my Lowfer final and I've taken a number of indirect hits without damage.
> Since the Lowfer setup has become a lightening 'test bench', of sorts, I
> never disconnect anything when a storm approaches.
>
> Bill A
>
> >From: "KD7JYK" <[email protected]>
> >Reply-To: [email protected]
> >To: <[email protected]>
> >Subject: [Lowfer] TX output protection.
> >Date: Sat, 30 Aug 2003 22:06:44 -0700
> >
> >I hear the horror stories of zapped finals due to lightning and ESD, has
> >anyone considered MOVs on the output?  I have seen them available from just
> >a few volts to several hundred.  If not these, what is used?
> >
> >Kurt
> >
> >---
> >Ross Technologies Signals Intelligence Division
> >Rosetta Proving Grounds
> >
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