DC fuses and arcs

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I cant have been drawing *much* more than 10 amps. The PSU was a metal caged monster in one corner of the room good for something like 250A Individual outlets had IIRC 2 strands of 5A fuse wire in the fuse holder for protection. We *might* have put a third one in, I cant remember.

The choke lived on a trolly and got pushed around the lab to where it was needed. Looked like it was a hand wound core of what I now know was soft iron wire with a black wooden terminal board, black tarry fabric insulation, ceramic insulating washers and brass terminals and a little engraved plaque that was probably ivory giving its ratings, some instrument maker's name and a date sometime back in the 1890's. It was basically square, about three foot across the case and over a foot deep with wooden top and base boards joined by some sort of cast brackets at the corners and open sides you could see two monster bobbins through with a bound wire core. If its still in existence 30 years later, it belongs in a museum, in fact it belonged there at the time . . .

The contact on the wire was near the tip of the pencil. If you have hefty supply carefully carve a notch down to the lead about an inch back from the tip and wrap a good few turns of fusewire round the half exposed lead and the wire you are connecting it to. That reduces the burnup problem some. Use a 5B pencil nicked from the Art department if you can, the purer thicker graphite worked *much* better ;-)

IIRC we sooted the lenses of some safety glasses we'd liberated. I would have warned you if I thought you were curious enough to try it!

As you may have gathered, this experiment was strictly sub rosa and never appeared on the curriculum.

Watch out, that choke would deliver a kick like mule if you broke the the arc and were'nt *bloody* careful what you were touching. *DONT* get yourself in circuit!

--
Ian Malcolm.   London, ENGLAND.  (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
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Nah. Lurk awhile - you'll see MUCH worse. ;-)

You can arc weld with a car battery, for a little while. ;-) (you get a little more stable arc with 24-36V.)

An arc is a negative resistance, you know.

Cheers! Rich

It's probably the paint. The wood smoke is not a problem, but I wouldn't want to inhale burned paint.

Yes - the UV from the arc can sunburn your corneas, and the IR can, and will, literally burn a hole in your retina, which doesn't heal. (neurons don't reproduce.)

Don't ever look at an arc again, please!

And if you're only protecting your eyes, the rest of your face can get a serious sunburn.

Go lurk at sci.engr.joining.welding for a while - it's fascinating. :-)

Good Luck! Rich

At the shop where I sit, they do MONGO welding - 200-300 Amps, with arc voltages in the low 30's.

Cheers! Rich

I did that with Mom's iron in series; I bypassed the iron's thermostat because it kept turning the iron off (opening the circuit) and Mom was irate when her iron melted!

Cheers! Rich

On Fri, 04 Dec 2009 10:42:42 +0000, Adrian Tuddenham wrote: ...

I think this one is AC:

Cheers! Rich

Heh, I used 10uF paper capacitor in series with 220V. Made an arc from two pieces of glass. Glass is good conductor when it is heated, and glass burns away less then carbon. The light from arc damaged my eyes, still remember how badly it hurt for few days.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

As a kid, my first experiment was with a sharpened rod from a D-sized battery with a fluorescent light choke in series with 220 Vac.

Since the current was quite low (less than 0.5 A) it was quite hard to move the electrodes far enough before bypassing the choke without triggering the 2.5 A automatic fuse that I had.

However, using a 2 kW heater (9 A) as a series load, it was easy to move the electrodes far enough and then bypass the series resistor, without tripping the fuse.

Definitely - lots of hum and no bangs or whistles.

--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk

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Thanks Ian, Sounds like quite a choke! I was using a 36V 3A DC supply... I sometimes saw the current peak up to 3 A's, but mostly around 2. Given the 12 ohms in my pencil (standard #2 HB contacted at the opposite end from the point with an alligator clip.) I must have been getting a little boost from the inductor. I could try contacting the pencil closer to the tip. But, I've had my fun for the day and won't play any more till I bring some welding goggles in.

George H.

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"> It's probably the paint. The wood smoke is not a problem, but I wouldn't

No, it was the wood. Right where the wood thinned down from the pencil sharpener. (I was trying to be funny.)

And don't worry too much about my eyes. Once I saw the arc, I put a book in front of it and just looked at the light reflecting off the wall behind my lab bench. I've done a tiny bit of welding and know that arcs are bad!

I'll check out the welding discussion.

George H.

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Yeah I figured all you 'life long' electronics guys did all this fooling around stuff in your youth. I had a sideways path into electronics and never played with any of it as a youngster....

George H.

Did it all the time with car batteries and the electrodes from D cells! So works fine at

Strange that glass is used for insulators in high-voltage AC power distribution then.

Chris

Is the glass used in high-voltage AC power distribution heated?

Sylvia.

"christofire" "Vladimir Vassilevsky"

** Vlad failed to mention that he heated the glass with ( presumably ) a gas flame until it softened before significant current would flow. Then the arc supplied the heat.

BTW

the glass and ceramic (ie porcelain) insulators used on high voltage power lines have melting points well over 1000C.

..... Phil

I've often wondered how much steam is raised by HV insulators when it's raining but when I've read course notes for installers/maintainers I've not found mention of this as a significant cause of power loss. Perhaps the lengths of the insulator strings, and their construction (e.g. number of ribs), are sufficient to make steaming insignificant. I didn't know that, as PA has clarified, glass conducts when it is molten.

Chris

General purpose glass gets noticeably conductive at mere ~300C, long before it loses mechanical strength. It is actually an interesting experiment to see for yourself.

Vladimir Vassilevsky DSP and Mixed Signal Design Consultant

I tried burning the carbon rod arcs inside a glass tube in order to reduce the UV radiation and operate the experiment without sun glasses. No doubt the electrodes might have touched the hot glass at times especially when the glass became soft. Even then, the conductivity of the glass must have been far less than the conductivity of the arc, since the arc current did not change significantly.