Sense Power Line Cord, Turn-on Fan

Hello Bob,

They do age with every strike above a certain threshold. Here is a brief explanation:

"Other surge suppressors rely on "sacrificial components," Metal Oxide Varistors (MOVs) or gas discharge devices that shunt or divert power surges from the hot line to the neutral or ground line. Sensitive digital equipment systems depend on a clean and uncontaminated ground.

SurgeX® power filters offer an all-new, revolutionary Series Mode technology, which captures surge energy and slowly releases it onto the neutral line. Surge energy will not contaminate ground lines or wreak havoc with your system peripherals. With every surge above a modest level that hits your system, the MOVs in ordinary surge protectors degrade until they eventually fail. Unfortunately, it is impossible to tell when a MOV has become ineffective, leaving you exposed to potential equipment destruction."

Anyway, they are indeed sacrificial components. The detailed mechanism should be explained in mfgs app notes. I have seen it but don't recall where, since I rarely use MOVs.

Well, usually a current transformer is used for current sensing and not to convert energy. There are certainly exceptions but then you'd rather need a "real" transformer.

Regards, Joerg

[snip]

Looks like it should have been a PTC type to get the circuit past turn-on surges on the control load, with sensitivity settling to that of R2.

What kind of 115VDC coil is that that stays on throughout a half cycle without current? My guess was that it was an AC coil, large inductive reactance and a shading turn on the coil to time smooth the flux intensity.

No arguments there. We've sort of gotten off on a tangent.

My original reason for citing an MOV was to clamp any HV spike that might be present in the secondary of a current transformer while it was not connected. That possibility might exist during a switching transient in the monitored line, or a sudden short in the monitored load. Current transformers, by their nature, usually have high turns ratios, so it could happen . . .

Given that the transformer cores I'm using have around 0.15 square inches of iron in them and any HV transient that might occur would be of very short duration and probably far less than a joule an MOV would probably last far longer than any equipment or personnel it protected.

I don't have to be sold on the limited protection MOV's offer. Seen too many of them fail for that. But the applications where they do tend to fail are when connected across the power line or line to neutral, with lightning strikes or big inductive loads.

One place I worked had some "Reliance" DC drives. 460 volt three phase input with 10-100 HP motors on them. If the mains fuses blew, the grossly underrated MOV's would vaporize. There'd be a piece of lead soldered to a ring terminal where the MOV's used to be. Never found any shrapnel either, just some carbon and copper flashed on to the chassis.

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Here's a link to a paper that is related to this, but does not directly address the question, which boils down to: if an MOV operates to supress a surge, is it or is it not always degraded?

The link:

Here's another link, that discusses MOV operation/structure at the atomic level, and is more closely related to the question:

Watch out for line wrap in the links.

Now addressing the question directly: It is my understanding that while it is theoretically possible for an MOV to operate (ie switch from very high resistance to very low resistance) without damage, for all practical purposes, an MOV is degraded any time its threshhold is exceeded.

I'll describe a way to think of an MOV to illustrate puncture within it. This is not an accurate description - only an easy way to visualize what goes on. For the real description, read the second link. In what follows, references are made to the paper found at that site.

Picture an MOV as a huge number of parallel paths, consisting of 2 conductors separated by an insulator. For a variety of reasons, these paths are not 100% identical, despite manufacturing efforts to make them so. This leads to a phenomenom the paper in the 2nd link refers to as "local Joule heating" in the presence of a surge. That means that a pulse of energy will heat some paths more than others, causing a puncture in the ZincOxide material. The paper states: "...puncture failure is a progressive form of irreversible degradation..." Thus, one (or more) of the paths can be thought of as having a hole torn in the insulator (with resultant heat) by the surge energy, allowing the two conductors to short out, thus producing more heat due to I^2R. The result is that the path is destroyed - the irreversible degradation referenced above - by thermal runaway. But there are an enormous number of paths, so if only one is gone, the MOV is damaged, not totally destroyed. Of course, with a long enough surge, or enough surges of shorter duration, all of the paths will go.

There is another material failure mode within the MOV - fracture of the crystaline microstructure - discussed in the paper. It occurs with larger amplitude shorter duration surges, as exhibited by the graph in fig 11 in the paper. The MOV material simply cannot expand fast enough as it heats due to conduction.

Ed

Newsgroups

I don't think that PTC is necessary because the load is always on in this case. The main thing is the ratio of standby of run current. I like the AC relay idea, may require a little experimentation, but it should go with a trigger not far from 90o point, maybe the 60o point. So Rs is selected to trip the SCR at that point with safe margin at running load current. The circuit drops out in standby. It's a Radio Shack project: View in a fixed-width font such as Courier.

. .. +---------------------o/ o-------+-----+ .. | | | .. | 1A | | | .. | 400V _____ | | .. | SCR 1N4007 ----- | | .. +-----|>|---||----+ | | .. | | 1N4007 | | -|-- N .. | | | | | - | E .. |1N4007| >10xRs | | | o| O .. +-|>|--+----/\\/\\--+ | | |--- | N .. | | | | -|-- .. ___ | | | | | .. | \\ | Rs | | | | .. L ---| |--+-----/\\/\\--------+ | | | .. | | | | | | .. N ---| |--+-----------------|----------|---|-----+ .. ---/ | | | | .. | | | | .. | -------- | | | -------- .. | | | | | | | | .. | | __ | | | | | __ | .. | | |------+ | +-----| | | .. | | -- | | | -- | .. | | ____ | | | ____ | .. +------| |----------------+ +----| | | .. | | ---- | | | ---- | .. | | | | | | .. | | | | | | .. | -------- | -------- .. | TRIP | AUX .. | LOAD | LOAD .. | SKT | SKT .. +--------------------------------| .

Put it in a plastic project box and if you want adjustablility , include a little header with a jumper to select different Rs- all available from RS. If your running load is say 50W then Iac is maybe 500mA so that the trip can be set at 500mA (45o) - then you want 20mA*10*Rs + 0.7V=

500mA*Rs or Rs=2.2 ohms @ 1W. The circuit is completely dead when Ipk*2.2

Would help to get the SCR in right: View in a fixed-width font such as Courier.

. .. +---------------------o/ o-------+-----+ .. | | | .. | 1A | | | .. | 400V _____ | | .. | SCR 1N4007 ----- | | .. +-----|10xRs | | | o| O .. +-|>|--+----/\\/\\--+ | | |--- | N .. | | | | -|-- .. ___ | | | | | .. | \\ | Rs | | | | .. L ---| |--+-----/\\/\\--------+ | | | .. | | | | | | .. N ---| |--+-----------------|----------|---|-----+ .. ---/ | | | | .. | | | | .. | -------- | | | -------- .. | | | | | | | | .. | | __ | | | | | __ | .. | | |------+ | +-----| | | .. | | -- | | | -- | .. | | ____ | | | ____ | .. +------| |----------------+ +----| | | .. | | ---- | | | ---- | .. | | | | | | .. | | | | | | .. | -------- | -------- .. | TRIP | AUX .. | LOAD | LOAD .. | SKT | SKT .. +--------------------------------| .

Shaking the Heathkit idea, a TRIAC should be much better on the conducted EMI with coil inductance doing the basic filtering. View in a fixed-width font such as Courier.

. .. . .. .. +---------------------o/ o-------+-----+ .. | | | | .. | 0.001 100 | | .. +---||--/\\/\\------+ | | | .. | | | | .. | 400V | | | | .. | TRIAC | _____ | | .. | MT1 |>| | ----- | | .. +-----|10xRs | | -|-- N .. +------+----/\\/\\--+ | | | - | E .. | | | | | o| O .. | | | | |--- | N .. +-----LVTVS-------+ | | -|-- .. ___ | | | | | .. | \\ | Rs | | | | .. L ---| |--+-----/\\/\\--------+ | | | .. | | | | | | .. N ---| |--+-----------------|----------|---|-----+ .. ---/ | | | | .. | | | | .. | -------- | | | -------- .. | | | | | | | | .. | | __ | | | | | __ | .. | | |------+ | +-----| | | .. | | -- | | | -- | .. | | ____ | | | ____ | .. +------| |----------------+ +----| | | .. | | ---- | | | ---- | .. | | | | | | .. | | | | | | .. | -------- | -------- .. | TRIP | AUX .. | LOAD | LOAD .. | SKT | SKT .. +--------------------------------| .

View in a fixed-width font such as Courier.

. .. . .. .. +---------------------o/ o-------+-----+ .. | | | | .. | 0.001 100 | | .. +---||--/\\/\\------+ | | | .. | | | | .. | 400V | | | | .. | TRIAC | _____ | | .. | MT1 |>| | ----- | | .. +-----|10xRs | | -|-- N .. | +----/\\/\\--+ | | | - | E .. | | | | | o| O .. | | | | |--- | N .. +-----LVTVS-------+ | | -|-- .. ___ | | | | | .. | \\ | Rs | | | | .. L ---| |--+-----/\\/\\--------+ | | | .. | | | | | | .. N ---| |--+-----------------|----------|---|-----+ .. ---/ | | | | .. | | | | .. | -------- | | | -------- .. | | | | | | | | .. | | __ | | | | | __ | .. | | |------+ | +-----| | | .. | | -- | | | -- | .. | | ____ | | | ____ | .. +------| |----------------+ +----| | | .. | | ---- | | | ---- | .. | | | | | | .. | | | | | | .. | -------- | -------- .. | TRIP | AUX .. | LOAD | LOAD .. | SKT | SKT .. +--------------------------------| .

Hello Bob,

In my domain (med electronics) the situation is similar. Everything must be isolated and often up to defibrillator safey level, meaning it must withstand a 5kV jolt without flinching. Lots of agency involvement and a new agency to deal with in every market, US, EU, Asia plus some "non-connected" countries who run their own regulatory scenarios. This is where transformers and mostly of the ferrite type play a major role. The actual defibrillator test usually impresses the regulatory folks. You can see their faces become a bit pale and the chairs slowly rolling backwards when I get ready to press that big button.

IIRC they are also used in GFCI outlets that you find in bathrooms and kitchens. I haven't seen small ones used for power conversion yet, just the larger versions, >1" or so.

Regards, Joerg

Hello Bob,

Old rule: Never let a current transformer secondary run open circuit. Not even for a brief moment of switching.

Regards, Joerg

Absolutely. If there's no burden in the design, go to jail, go directly to jail, do not pass go, do not collect $200.00. An MOV doesn't replace the burden R.

Ed

[snip]

Got this back from the vendor...

The adjustment range is limited on the LCG1/2 models & recommended for standard computers only. (use LCG3/4/5 or SCG4/5 for Apple, IBM & Dell computers).

The LCG3/4/5 & SCG4/5 can be adjusted to turn on with any load 7 watts to 1000 watts. Turn off is always set to 10% lower then turn on.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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I love to cook with wine.      Sometimes I even put it in the food.

Great to know that info!!! Thanks for asking the question to the vendor... I had wondered about that spec, but never enough to follow up on it. Thanks for posting it.

--
Dave M
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the address)

Never take a laxative and a sleeping pill at the same time!!

150:1 range? May be hard to set and with that 10% hysteresis. That's a dying niche market for you, now that everything is green.

[current xfmr, windings, etc]

Years ago, when I traveled with tools, I built a universal NiCad charger. It had a bridge rectifier & cap on the input, and a fixed voltage regulator in a TO-220, I think. A switch set the current limit to oh, 2, 20, 200 etc ma.

I could connect it across any local LV supply up to 30v. Once I used a Motoboatarola HT-220 charger.

A collection of clip cords helped you connect to the drill, screwdriver, IsoTip iron, whatever battery needed charging. (One of those 300 ohm TV "clothespin" connectors was esp. useful.)

Two issues soon emerged. One was you were never sure you had good contact. I put 3-4 scavanged 1N4001's in series with the output; and a diode, series R and green LED across the 4001 stack. Bingo.

The other issue was.. well it was a diecast ~~2"x1"x0.75" box...and it got hot... The knob for the current switch soon melted. I learned to hang the box in mid air, usually by the power supply fan exhaust, and got a Bakelite knob...

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