Help With a MOV (Varistor?)

replacement

The little penny sized MOVs are too small, the sub should be more like the size of a quarter. Radio Smack used to sell the GE or whatever MOVs that were the 'right' size, and the number was the same or similar to yours, above.

However that being said, if you listen to "w_tom", you will _never_ever trust a surge suppressor power strip to protect your equipment. I agree with him on many points, but the guy's an opinionated hardcore fanatic, IMHO. I say just cut out the MOVs and use the power strip without them. If the power supplies in the equipment _really_ needed the MOVs, they would have put them in the power supply. If you really want to protect your equipment, use an isolation transformer between it and the AC line.

Well, I believe that the MOV are a good way to see is surges are present. They will prove that the power line is at fault, and in my case that is important, as I have been saying that to the site owners where I connect the equipment. I realize that the MOV won't give great protection for the equipment, but I like 'em to be in there anyways.

Thanks, for the nfo tho, I will check Radio Crap next time by, may be cheaper than paying shipping from Digikey.

- Tim -

First, to be properly sized, the power strip should contain about 3000 joules. Notice how few joules were inside that ineffective power strip.

Second, safety issues must be considered. Better is to replace that MOV with TMOVs from littelfuse.com (or something equivalent). This simplifies safety considerations.

Third, if the protector is intended to detect destructive transients, then it should be plugged into a receptacle on breaker box - with 6' power cord shortened as much as possible. To have a grossly undersized protector detect destructive transients, the protector attached to breaker box receptacle makes short connection from AC wire to earth ground. Furthermore, tranzorbs may perform detection better. Use a smallest wattage tranzorb or transil or equivalent.

Fourth, protecti> Well, I believe that the MOV are a good way to see is surges are

case

They were RS cat no. 276-568 and were two bucks in their 2002 catalog. But they've stopped selling so many parts at the stores that I seldom go there anymore.

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The V130LA20A would be suitable imo. As you suspected 130V is the design breakover voltage. The "20" means that it has a nominal energy dissipation of 20 Joules.

Full specs are; Max input volts (RMS) = 130 Max recurrent peak volts = 212 Max DC input volts = 330 Max energy = 20Joules Max average power dissipation = 0.85W Max peak current for t < 6uS = 2000A Varistor peak voltage @ 1mA AC (peak) = 184V (min) - 254V (max) Typical capacitance = 1500pF Max thermal resistance body to air = 37 degC/W Calmp ratio @ 10A = 2.0

The VxxxLAxx nomenclature was originally used by GE back in the 1970's for their GE-MOV VARISTORS and Harris continued with it when they took over this product line.

Tim, This reply is from some research done a few years ago on surge protection. I am recalling what I found.

The problem with MOVs is the reaction time. By the time they begin to conduct the spike has already pentrated your equipment. A spike can be 10KV with a duration of a small fraction of a second.

The more expensive spike filters use either a transient zener or a MOV in conjunction with a large ceramic capacitor. The capacitor has the fastest reaction time, but can't take a lot of juice. The cap will keep it under control until the MOV turns on.

As far as I know they still don't put spike protection in televisions. I'm sure this is great for the service industry. A cheap power strip is a waste. As of a few years ago a good one was about 75-100 bucks. They write the reaction time on the sockets.

Use an a.c. rated ceramic cap. An "across the line" ceramic will have an a.c. rating of 130vac or better. About .02uf should do it. A standard ceramic with d.c. rating has about 10% of that rating for a.c. There are also many ATL plastic caps, but I don't know how fast they react. Hope this helps you out. Mark

The reaction time even of slower protection devices such as Gas Discharge Tubes (GDTs) has been more than fast enough. However if discussing reaction time, then wire impedance even in those two inch MOV leads must be considered. Also other delays cause by sharp wire bends must be included. In short, reaction time is irrelevant.

A transient is discussed as if it were a wave crashing on the beach. It is electricity. That means the same current is flowing through everything in that circuit as current increases. IOW of course "the spike has already penetrated your equipment". That is how electricity works. First electricity flows through everything in a circuit. Then as the current increases, something in that circuit fails.

MOVs don't stop, block, or absorb surges to keep them out of equipment. MOVs shunt. All electronics contain effective protection. Protection that may be overwhelmed if the transient is not shunted - diverted - before it can enter the building. Shunting the transient to earth is the function of an MOV.

The problem with MOVs is not their reaction time. The problem with MOVs is that many humans do not understand what MOVs do. MOVs will only be as effective as the earth ground. Quality of that earthing connection, distance to earthing, how that connection is routed, and even that the earthing point is the only earthing point. IOW if the spike is 10KV, then the MOV is mislocated or not properly earthed.

Again you use the term 'spike filter' which has nothing to do with what an MOV does. Filters would be series mode protection. An MOV is part of a shunt mode protection 'system'. And yes, the MOV is only one part of a 'system'. Some shunt mode protection 'systems' don't even need MOVs. But all shunt mode protection 'systems' have one essential and required component - the single point earth ground. SPG is the one missing component that so often makes MOVs ineffective. MOV will only be as effective as its earth ground.

Meanwhile if spending $75-100 on a power strip protector, then it is probably one of the most ineffective protectors such as Monster Cable. For example, a minimally sized power strip would be 3000 joules. How many joules in that $100 protector? Notice so much money and so few joules.

Money says nothing about the quality of a protector. Quality is defined by the earthing connection. No earth ground means no effective protection. And so we have myths about reaction time - so that you will not learn about earthing. Learn about earthing and you will never waste good money on a power strip that costs tens of times more money per protected appliance. Or in the case of that $100 protector -

Meanwhile, putting a capacitor across the line rated at only

The effective MOV protector is a 'whole house' protector with a short c> Tim, This reply is from some research done a few years ago on surge

Tom: I'm short of time right now, but perhaps your concept of a spike through the house wiring is incomplete. This is a very complex subject when one gets deep into it.

An MOV doesn't have to be hooked to the house ground, it simply bypasses voltages that excess its turn-on voltage.

They can be used on the secondary of a transformer to absorb the turn-off overvoltage.

Across the line caps are rated at 130Vac.

The initial transient spike must be intercepted and the reaction time is important.

This is what I recall to be the case.

You better have more than one MOV book. A book that is better than the old GE MOV Application Book. You better have sufficient EE training to comprehend the underlying concepts. MOV databooks do not discuss the entire circuit and assume the reader understands the many types of transients. To understand protection, a circuit must include everything from the cloud to earthborne charges. What an MOV does is both trivial and little different from wire used by Ben Franklin in

Yes, concepts of house wiring during a transient get complex. Many IEEE papers discuss this. Remember, even that concrete floor is an electrical conductor. Did your MOV book discuss the floor as part of a circuit? If a plug-in protector attempts to earth a transient via the wall receptacle, then a transient is induced on adjacent household wires and other appliances. Did your MOV book discuss induced transients? Just another way that plug-in protectors cause problems for adjacent household appliances.

Demonstrated is why plug-in protectors typically are undersized; have so few joules. Why spend more money for more joules when the protector does not even claim to protect from the typically destructive transient? Did your book discuss the electrical characteristics various transients or what joules define? Why not?

Household wiring is so complex that plug-in protector becomes ineffective AND can even contribute to damage of the adjacent electronics. Again, we simplify the complex circuit inside a house by keeping the transient outside. That 'whole house' protector is only as effective as its earth ground. And not just any ground. A single point earth ground. Did the MOV book discuss equipotential and other complexities of earthing?

A protector is only as effective as its earth ground. Rather than teach why, industry professionals are cited. One full day worth of technical reading is hyperlinked in alt.comp.periphs.mainboard.asus on 30 Mar 2005 entitled "UPS unit needed for the P4C800E-Deluxe" at

Again you claimed an MOV is:

MOVs absorb energy as copper wire absorbs energy. MOVs do not **"stop, block, filter, or absorb"** transients. If an MOV protects by absorbing energy, then so does copper wire. Numbers in those datasheets should make that blunt obvious. If not true, then post those numbers. Show me.

The reaction time even for slower GDTs was more than sufficient. Where are numbers provided by your source for reaction time? No numbers means junk science reasoning. Discover how that 'one days worth of reading' does provide plenty of numbers. No numbers is a symptom of junk science reasoning.

An MOV is > Tom: I'm short of time right now, but perhaps your concept of a

I will post on top.

First, I was trying to help the original poster provide some cheap and effective spike protection.

I was not trying to give you a reason and opportunity to grind your egotistical axe.

The MOV turns on and has a constant voltage drop that generates heat. It must absorb energy to survive, but it basically "clamps" the energy source and diverts the spike to the system ground.

You are predicating your arguement on lightning strikes. Although this is a valid concern, the real and erosive danger to equipment is the constant bombardment by line noise. For example, when the neighbor's refrigerator turns off and send a spike through the wiring.

MOV specs are from advertizing brochures that push the optimism to the limit. I was simply stating that if he wants to be sure he is protected then he should strap a line-rated cap (the plastic ones are probably as good as the ceramics) across the MOV. MCM sells some 250Vac Siemens caps at a good price, about 50 cents.

The initial spike is mostly voltage and has an erosive effect. Like ESD into semiconductors, these line spikes continually erode semiconductor junctions in solid state devices, thus leading to early failure.

An MOV is a package of series-parallel semiconductor junctions all of which have a voltage drop and capacitance. The article that I read years ago pointed out that the MOV is not all that it is supposed to be. Using the conventional capacitor (which is always turned on) is the best way to be sure that the initial spike is contained until the MOV responds.

There are also inductive filters like the CORCOM (spelling may be wrong) that have been used for decades, but they cost a lot more.

Also, putting just one MOV on the house won't suffice. Turn-off transients within the home react with wiring impedance on a localized basis. Having the protective device as close to the equipment is best.

Time to go, too much to do, Mark

Clearly, your intentions were never in doubt. Either was the doctor who bled a patient to death to cure the fever. To cure a patient, at minimum, understand basic electrical principles such as the many types of transient.

Meanwhile, even a MOV manufacturer does not claim to absorb energy of a surge. For same reason that a wire gets warm, so does the MOV. It shunts .. and to earth ground.

Meanwhile, electronics already contain effective internal protection as even demanded by industry standards of 30 years ago. Items such as the Corcom or equivalent are already inside appliances; even due to FCC requirements. Even that capacitor is often inside the appliance - but with higher voltage ratings for human safety reasons. IOW any protection that might work on a power cord is already inside the appliance. Plug-in protectors hope you never learn that; so they publish brochures chock full of half truths.

Once Apple installed MOVs inside their products. Why does Apple no longer install those $0.10 parts? Because MOVs inside the computer or on a power cord are not effective. Again (and to appreciate why, one must understand some first year EE concepts), the effective MOV earths a typically destructive transient. That means it must be located short (less than 10 feet) to a single point earth ground.

Does a refrigerator generate transients seen by MOV? If so, then the MOV will degrade to ineffective in but weeks or months. A graph provided in MOV datasheets makes this obvious. MOVs are installed for transients that occur typically once every eight years. Any trivial transients such as noise from the refrigerator - the appliance must already have protection to make such transients irrelevant.

If a refrigerator generates such destructive transients, then what protects your dimmer switches, GFCIs, smoke detectors, and furnace controls? We don't visit the hardware store weekly to replace these because, well, first learn the numbers. Those transients are made irrelevant by protection already inside appliances. Those hyping ineffective plug-in protectors again hope you don't learn this.

You may wish to take a short break. Already posted are significant technical facts that require grasp of basic EE principles.

What does an MOV do when located on the appliance power cord? It shunts a transient from one wire to all others. Shunts - not stop, block, filter, or absorb. It does what Ben Franklin demonstrated in 1752. The typically destructive transient seeks earth ground. A transient on the black (hot) wire has been shunted to white (neutral) and green (safety ground) wire. IOW the transient now has three paths to find a path to earth ground, destructively, via the appliance. Where is the protection? As posted earlier, an adjacent protector can even contribute to damage of the adjacent appliance.

No wonder plug-in protectors are so undersized (MOVs of too few joules). But again - how many joules on that protector you think is being protected? What are the numbers? Did that brochure note why joules must be sufficient?

Again, I am only repeating what is provided in that 'days worth of reading':

But again, what do all those industry professionals discuss obnoxiously in that 'days worth of reading'? Earth ground. What do you want? Something obnoxiously repeating what plug-in manufacturer propaganda ignores? Or a sugar coated and grossly overpriced solution sold in Staple, Circuit City, Sears, and Kmart?

No earth ground means no effective protection. The MOV is only as effective as its earth ground. What do ineffective plug-in protector hope you never learn about? Earthing.

To truly help the OP, first read that 'days worth of reading'. Appreciate how widespread the plug-in protector propaganda is. Effective MOVs are installed, in greater numbers, within well earthed 'whole house' protectors. The protector being only as effective as the quality of and connection to that earth ground. By tens of times, the cheapest and most effective spike protection is properly earthed 'whole house' protectors.

Meanwhile, I believe the > ...

Nonsense. You either presume that everything is a series circuit, which is totally unrealistic, or you don't understand how current flows in a parallel circuit.

Ed

MOVs absorb the surge energy they are exposed to at their terminals. E=I*R applies. The MOV absorbs electrical energy and converts it to heat energy, as long as it is working. They absorb whatever amount of the total surge energy they are exposed to across their terminals, again, only as long as they are working. Energy arriving at their terminals beyond their specs will burn them out, which, of course, demonstrates (sometimes dramatically) that they absorb energy.

Ed

MDC = Maida - they're on the web...

Cheers, Brian

Cheap and effective are mutually exclusive when it comes to lightning storms.

If you have a right to express your opinion, why are you trying to deny others that same right?

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PC switching power supplies already have both 'X2' and 'Y' rated capacitors across the line and to ground. They also have a transformer, usually toroid, with dual windings that filter the AC as it passes thru.

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The PC switching power supply makes its own noise - so much so that the PS has to have good filtering on the AC power to keep it from disturbing other appliances. But it still does - listen to your AM radio when you turn on yout PC.

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