[Amps] MOVs

[Roger (K8RI)]

On 1/24/2014 12:49 PM, Saandy wrote:
> Guys , think! Nothing gets conducted to a point not connected anywhere!
> A MOV is intended to a supposedly safe limit a voltage differential between
> 2 points. Protection between the line and something ground connected is
> achieved with a common mode choke.
> It's more important to remember that MOV's have another problem: they have a
> finite life time. They can be zapped a given number of times only; lattice
> crystal inside get destroyed every surge and after a while you end up with
> no protection, even though you think you're safe.
> Use them sparingly and with circumspection!

I think it would be more correct to say an MOV can absorb a given amount 
of energy in joules rather than a number of hits.

We don't know how many hits it can take because we do not know the 
energy in each hit. Take a 100 joule MOV and hit it with a single spike 
containing 100 joules. It's all done.

I quit work to go to college in 87. and the lastime I worked with MOVs 
was well before that.

I do know an MOV that has taken a number of hits and is still surviving 
will handle no where near its original rating.  I don't remember if it's 
linear or not.  Will a 100 joule MOV which will be done after one 100 
joule his be capable of taking 100 1 joule hits?

But Alex is correct. MOVs have a finite life.  I'd add that their raring 
are reduced with every hit, so one would do well to design with extra 
capacity in mind.

The MOV is like a voltage variable resistor, but in reverse. That and 
the conduction trace has a knee.
They do not have a specific voltage where they turn on. They have a 
voltage where they start to conduct, but at a high resistance. As the 
voltage goes up, the resistance goes down.  They reach a point where the 
resistance goes down rapidly, but this changes with the amount of energy 
the MOV has absorbed. With each successive hit the turn on voltage is 
reduced and that knee also occurs at a lower voltage.  I don't remember 
for sure, but I believe the knee is softened, or spread out.  As I said, 
it's probably been in the neighborhood of 30 years since I worked with 
these.

In general terms, it still breaks down to the MOV has a finite life and 
with use, its ratings are reduced.

73

Roger (K8RI)

> Alex	4Z5KS
>
> -----Original Message-----
> From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of Jim Brown
> Sent: Friday, January 24, 2014 8:32 AM
> To: amps at contesting.com
> Subject: Re: [Amps] MOVs
>
> On 1/23/2014 7:18 AM, Paul Baldock wrote:
>> If the amp is running off 240V where in the USA this is symmetrical
>> about ground and then the MOV is placed between the two phases, it
>> seems to me that there would not be a surge current to ground.
>
> Right.  The problem comes when you take the MOV to the ground.
>
> 73, Jim K9YC
>
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