Eric - VE3GSI wrote:
> Hi Jim,
>
> I'll check your info out first chance I get. But..... in my years of
> servicing I have seen MOV built into many AC circuits of home electronic
> equipment. In fact years ago, I remember one company (Zenith) requesting
> them to be installed after repeated failure of one model of their TV that
> failed even if it seen lightning on the horizon, the MOV fixed that problem.
> Are MOVs not common place with just about all quality switching supplies
> these days?
>
MOVs are the equivalent of a clipping diode and protect from spikes.
They do little if anything for filter out noise and I've never head of
them being used for that purpose. They are often used to protect
inexpensive power supplies or those sensitive to power line spikes. I
would expect them to become noise generators rather than suppressors.
The Metal Oxide Varistor (MOV) is a whole batch of metal oxide diodes,
physically compacted to form a disk. There is a threshold voltage at
which conduction begins, but unlike a Zener diode the threshold is just
that, a place where conduction begins and increases as voltage increases
to a point where the device basically becomes a short circuit. Each
time an MOV conducts it loses a few of those little diodes so the
performance and threshold degrade over time with use. They may fail
shorted or open depending on conditions. A large spike can literally
leave nothing but a pair of leads. Lots of little spikes will
eventually cause the diode to short. If the circuit has insufficient
protection it too will end up as a pair of leads.
MOVs are generally rated at threshold conduction in volts and jouls of
capacity. When used in an AC circuit you have to keep the threshold
*above* the peak voltage. IOW, multiply RMS by 1.414 and then add a bit
of head room or the device will have a very short life.
The larger MOVs with capacities of 50,000 jouls or more can fail
spectacularly. It's even more spectacular if you are only a few feet away.
Yes, IIRC nearly all the computer power supplies I've taken apart had
MOVs across the AC line for protection.
73
Roger (K8RI)
> But with the fellow that has the noisy neighbour these little filters might
> be just what he needs for the charger in question. I have seen reduced noise
> with the Belkin units with RFI ratings (not the generic models). On one of
> my own Dynamax chargers I have seen noise from it reduced to a liveable
> level, until I had a chance to replace it with a better quality one.
>
> Besides I can't see Belkin setting out to blow up everybody's home theatre
> system and computer.
>
> I will check your article,
> Eric - VE3GSI
>
>
>
>
>> -----Original Message-----
>> From: Jim Brown
>> Sent: April-11-09 6:06 PM
>> To: rfi@contesting.com
>> Subject: Re: [RFI] update <was> 40m (and other) noise
>>
>> On Sat, 11 Apr 2009 17:44:22 -0400, Eric - VE3GSI wrote:
>>
>>
>>> The ones I use are by Belkin
>>> called and 'Surge Master' available at Home Depot here in Canada.
>>>
>> Note that I do NOT recommend MOV surge suppression products, which
>> is what these devices are. MOVs can be really bad news on branch
>> circuits, because they can actually CAUSE destructive failures of
>> interconnected equipment. The only good application for them is as
>> part of a "whole house" suppression system installed at the service
>> entrance. For a technical discussion of this, see the Power and
>> Groudning tutorial or the RFI tutorial on my website.
>>
>> 73,
>>
>> Jim Brown K9YC
>>
>>
>
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