[Lowfer] TX output protection.

Wed, 03 Sep 2003 00:24:33 -0400

Tom,

After reviewing my copy of the Harris Transient Suppression Devices handbook 
I find a number of discrepancies with the information below. The most 
important one is that minimal increase in leakage current occurs with the 
number of applied pulses, up to a reasonable number, *when the MOV is 
operated within it's specifications*  It doesn't appear that they are 
designed to handle 1000s of current spikes, but for most applications that's 
not a problem. MOVs can be very fast, a few nanoseconds, in fact.

I'm not sure how many consumer products you have opened lately, but MOVs are 
used extensively in the ones I've looked at. The only failed outlet strips 
using MOVs I have seen, out of 10,000s of hours of use, were some here at 
work. We had a loss of a neutral line on the 3-phase distribution in the 
office area. This caused the breakdown voltage rating of the MOVs inside the 
strips to be exceeded.

Frankly, I feel a lot safer with MOVs in use when the thunderstorms roll in.

Any other comment from the group?

Bill A

>From: WA2BPE <[email protected]>
>Reply-To: [email protected]
>To: [email protected]
>Subject: Re: [Lowfer] TX output protection.
>Date: 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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