Surge Supressor Clamps in 85 Picoseconds

All we want are the facts, ma'am

Sounds like Mission Improbable to me.

Electricity power quality products and services are a dime a dozen (in that there are many). However, from exactly how you have explained the claims the company in question is a fraud and the claims a scam.

If I read between the lines (a lot) the claims could just be misinterpreted as a hopeless misunderstanding and marketing by power quality and protection salespeople.

Do you have a website link, this is quite different from most other power scams I have come across (if what you say is true). Perhaps you (or the company) meant to say 20% savings through increased equipment life (reduced equipment failure) due to clamping of voltage transients at the main switchboards.

Can you please provide a link to the company's website as we are genuinely interested in more information and clarification regarding these claims (well I certainly am).

http://www.utiliguard.com/ is the company website for Diversified Facility Solutions, LLC here is the page with their guarantee of energy savings http://www.utiliguard.com/documents/guarantee.pdf and finally here are approx 300 plus companies' websites that provide their service for them across the country: http://www.google.com/search?q=link%3Autiliguard.com&ie=utf-8&oe=utf-8&aq=t&client=firefox-a&rlz=1R1GGLL_en___US366#sclient=psy&hl=en&client=firefox-a&hs=Wke&rlz=1R1GGLL_en___US366&source=hp&q=link:+utiliguard.com&aq=f&aqi=&aql=&oq=&gs_rfai=&pbx=1&fp=64371607d2fffabb

Cheers!

So what?

I am interested to know whether these MOVs etc at the main input side really help protect MOSFETS in electronic circuits down stream.

For example let us say we have a electronic gadget operating from 240V AC, 50 Hz. What should be the rating of MOV to be used at the input. I have seen may using MOVs marked 275 or 305 being used and these have breakover voltage of over 600 V or more and then if we use MOSFETs rated 500V, how can it protect? So the voltage marking and breakover voltage rating are too far apart & confusing!!! Manufacturers of MOV like devices need to explain.

What about failure of MOV itself - also means replacement costs. So does anyone use further MOV protection components upstream before MOV on mains side? Any experience advice on this?

I'm sorry no one has responded to your questions, they are good ones, and certainly relevant to the topic. You are correct about the elevated clamp voltages of MOVs that are defending against rare incoming high-voltage incoming spikes. But consider, a transient spike's energy is proportional to V², and by clamping a say 3kV spike at 600V, the MOV has absorbed 96% of the spike's peak energy (0.6² / 3² = 0.04). That's a rather significant piece of protection.

As for a 500V offline MOSFET, presented with an attenuated spike, it has it's own protective measures during a breakdown event. First, the mosfet isn't generally sitting right across the AC line. Being in a switching supply, it's on the other side of a common-mode choke, which has a series leakage inductance. Furthermore, the AC-line wiring has series inductance, e.g., dI/dt = V / L. Small mosfets also have a fairly high internal breakdown resistance. All these serve to limit the maximum current that can flow during an over-voltage event.

Let's think about spikes. There are two "standard" spikes, the most intense pulse has a 8us risetime and lasts 20us to the 50% decay point, the other pulse type has a 10us risetime and lasts for 1000us. So we're talking about more or less strong 25us and weaker 1.5ms single-event clamped current pulses into our mosfet. How's a poor little mosfet going to deal with these over-voltage pulses?

On the left we have a portion of a typical mosfet datasheet. This one is fairly-wimpy part, a Fairchild FQP4N50, rated at 500V and only 70 watts. Mouser sells this little mosfet for $0.34 qty 250.

OK, if you can read it, check out the spec called E_AS (fifth from the top). That's the part's single-pulse avalanche-energy rating, and the value shown is 260mJ. Crudely-speaking, as long as we keep the total energy below E_AS during the avalanche-breakdown event, our mosfet will be fine. Its semiconductor junction will heat up, yes, but it won't fail. This spec means that the part can absorb a power level of P = E/t during the pulse. That works out to 10kW of power-handling capability for 25us, and173W for 1.5ms of breakdown duration. Keep in mind the mosfet only has to handle the left-over 4% of the pulse energy that the MOV failed to absorb itself.

This has not been a detailed analysis, but it should serve to show that our mosfet isn't going to immediately fail if a nearby protective MOV fails to clamp the transient-event voltage to under 500V. In fact, the 600V clamping is what will save our 500V mosfet.

What's not often known is that MOV's have a finite life. Check the manufacturers specs.

They only dissipate a certain amount of energy then they're useless. That's OK if they're replaced, but we all know that rarely happens. A MOV mounted on a circuit board is a bit of a con job (fisher & paykel are you listening). Ffej

While you cannot visually tell if a MOV has gone open, you can definitely tell if it fails catastrophically.A fuse in series with the MOV provides protection from a melt down, and a lamp across the MOV indicates an open(normal)condition.If the lamp is off, this indicates a catastrophic failure has occurred, and the MOV should be replaced.

This is not foolproof, as the MOV can be destroyed without blowing the fuse,but it is better than no monitoring of the condition.

This implementation is used frequently on main panels for sensitive equipment, with the indicator lamp (usually Neon) mounted on the outside of the panel.

When I repair circuit boards (a lost art nowadays) that show signs of spike damage, I replace any MOV's present, and add appropriate ones when possible for future protection.They do work,and have saved me many service calls, as well as building my company's reputation for reliable repairs.

The telephone companies use spark gap protection,where a small air gap is created between signal and ground. An overvoltage condition will arc to ground. Usually 300 volts at building, 600 volts at cable service box.They also use a carbon-arc air gap device at the building at the entry point.These are being phased out in favor of gas type.

Perhaps as technology matures, there will be MOV's with intrinsic indicators,such as (color change) to indicate their condition.

Insofar as saving money on utility bills? Deja Moo! (I've heard that bull before).

Transient voltages on the power line are only one source of voltage spikes on electronic components.EMP from nearby lightning strikes are another,and induce voltage almost instantaneously as the magnetic field passes thru a conductor,and is such a high frequency and voltage that it rides the skin of the conductor.MOV's have proven very effective at suppressing this type of surge also, as well as static electricity discharge from improperly grounded circuity or equipment.

An old TV repairman showed me his method of surge suppression,back before MOV's were available: He tied several knots in the 2 conductor cord of the TV,In effect, creating a choke,and he swore by it.