Topics Topics Edit Profile Profile Help/Instructions Help Member List Member List  
Search Last 1|3|7 Days Search Search Tree View Tree View  

A good laugh

:: EPE Chat Zone ­:: ­Radio Bygones Message Board :: » EPE Forum Archives 2007-2009 » Archive through 13 February, 2007 » A good laugh « Previous Next »

  Thread Last Poster Posts Pages Last Post
  ClosedClosed: New threads not accepted on this page        

Author Message
Top of pagePrevious messageNext messageBottom of page Link to this message

Obiwan
Frequent Contributor
Username: Obiwan

Post Number: 1410
Registered: 12-2005

Rating: N/A
Votes: 0 (Vote!)

Posted on Tuesday, 23 January, 2007 - 05:50 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

But we're doomed of course.

This was posted on a forum dealing with a show called "Myth Busters". They find "myths" and then check them out to see if they could be true or not, like is it possible to raise a sunken ship with ping pong balls. That sort of thing.

The forum is a place where the average person can check in and suggest a "myth" to test out.

Man, we're doomed. The incredible stupidity of the people posting there is just unbelieveable! And never ending!!!

Here's my favorite, among many, many of them.

swedishcancerboi
Junior Member
Registered: 06-08-04

Posted 06-09-04 12:30 AM Edit or Delete MessageReport This Post
I found a few topics on this, but nothing conclusive. I have been debating with a friend, computer guy, about protecting computers from lightning strikes. I am an electrician and am also a little dumbfounded without actually trying it out. His idea is to make a loop in the surge protector cord, then tie it in a knot. a knotted loop. The idea is that with the high voltage, the lightning will travel once through the loop, but will not travel back through the knot. Since electricity takes the path of least resistance, it would be less resistant for it to travel back where it came from to ground than it would to go through the knotted loop, blowing out the cord. The surge protector SHOULD protect the computer from any electrical bleed over that might make it through the loop since not all of the power would obviously not jump through the cord. The principal of electricity jumping through insulation is also seen in high voltage, long distant powerlines. Most electric lines are not insulated b/c the electricity will simply jump through and destroy the insulation. 500,000 volt lines, for example, can arch over 11 feet. Insulation to protect these lines would have to be massively thick. Any Thoughts? I would like to see this tried out.
buck19
Member
Registered: 06-08-04

Posted 06-09-04 02:34 AM Edit or Delete MessageReport This Post
only one problem. i learned this in a little fire and rescue training. tying power cords in knots will create enough heat, possibly due to more resistance like you stated, to melt away the insulation and catch surrounding objects on fire.

theres a myth that they should bust. knot catching on fire.
Do Not Hit The Fly That Lands On The Tigers Head.
Top of pagePrevious messageNext messageBottom of page Link to this message

Terrym
Frequent Contributor
Username: Terrym

Post Number: 348
Registered: 05-2005

Rating: N/A
Votes: 0 (Vote!)

Posted on Tuesday, 23 January, 2007 - 07:18 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Stop the World, I want to get off.
Top of pagePrevious messageNext messageBottom of page Link to this message

Grab
Frequent Contributor
Username: Grab

Post Number: 383
Registered: 05-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Tuesday, 23 January, 2007 - 09:23 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I just pray this "electrician" doesn't ever come near my house!
Top of pagePrevious messageNext messageBottom of page Link to this message

Magnum4
Frequent Contributor
Username: Magnum4

Post Number: 380
Registered: 04-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Tuesday, 23 January, 2007 - 11:57 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Just goes to show how someone can misunderstand the term "Path of least resistance", forgetting the copper conductor in the knot.

On the knot in the cord thing, It can cause a problem, esp with heavy load. The knot will act like an coil without an iron core, and heat to melting point.

The number of people who use the likes of a heater with an extension lead, but leave it coiled up, would open your eyes.

Melted cable and spool
Regards,
Jim
Top of pagePrevious messageNext messageBottom of page Link to this message

Epithumia
Frequent Contributor
Username: Epithumia

Post Number: 252
Registered: 06-2006

Rating: N/A
Votes: 0 (Vote!)

Posted on Tuesday, 23 January, 2007 - 01:07 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Isn't the heating thing just resistance? A coil stores energy, rather than dissipating it.

Rob

(Message edited by epithumia on 23 January, 2007)
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
Top of pagePrevious messageNext messageBottom of page Link to this message

Magnum4
Frequent Contributor
Username: Magnum4

Post Number: 381
Registered: 04-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 25 January, 2007 - 07:57 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Im not sure Rob?

We were told that by an old guy,during training, no explanation, but we took it as true?

I always thought it was the impedence, as being a power cord, we were dealing with AC.?

I have seen hundreds of extension cables melted because they were used coiled up with even a moderate load.

Maybe someone can shead some light on it?
Regards,
Jim
Top of pagePrevious messageNext messageBottom of page Link to this message

Epithumia
Frequent Contributor
Username: Epithumia

Post Number: 259
Registered: 06-2006

Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 25 January, 2007 - 08:29 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I've learned here that there's always more to learn!

But I don't think inductance makes sense. I just measured an extension lead: inductance of one conductor- 84uH, inductance of a live and neutral with a short at the far end: 8uH (which I suppose means that the return current cancels out the magnetic field of the forward current).

At 50Hz an few uH is negligible, and in any case inductors don't dissipate heat. That's why they are used as filters in switch mode power supplies.*

What we're seeing here is resistive heating. Because the cable is coiled, there's nowhere for the heat to go. It's a physical effect of the coil, not an electrical one.

Rob

* I once saw a website where someone had build an 'efficient' switching regulator but omitted the inductor. The only thing limiting the current in his design was the resistance of each individual circuit element. I don't think his circuit did what he thought it did...



(Message edited by epithumia on 25 January, 2007)
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
Top of pagePrevious messageNext messageBottom of page Link to this message

Epithumia
Frequent Contributor
Username: Epithumia

Post Number: 260
Registered: 06-2006

Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 25 January, 2007 - 08:42 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I think this is the circuit I saw:

http://www.rowan.sensation.net.au/electronics/stepdown2.html

He even observes that there must be something wrong as the transistor is getting hot! He hasn't spotted that the only thing limiting the current is the internal resistance of the transistor, the diode and the capacitor!

Does this circuit make any sense at all?

Rob
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
Top of pagePrevious messageNext messageBottom of page Link to this message

Magnum4
Frequent Contributor
Username: Magnum4

Post Number: 382
Registered: 04-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 25 January, 2007 - 08:43 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Rob ,
Thanks for that.
Maybe it was a case of get the glass hammer

And it stuck too Just shows how something like that can stay with you :-) Well it did teach us to keep power tool leads tidy

Maybe you can explain why a coil from the likes of a solenoid will burn very quickly, if it is removed from the plunger?

Appreciate the reply
Regards,
Jim
Top of pagePrevious messageNext messageBottom of page Link to this message

Epithumia
Frequent Contributor
Username: Epithumia

Post Number: 261
Registered: 06-2006

Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 25 January, 2007 - 09:13 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

> Maybe you can explain why a coil from the likes of a solenoid will burn very quickly, if it is removed from the plunger?

I didn't know that, but here's a guess. I'm assuming it's AC. Unlike the extension lead, the coil has many turns and much inductance. The inductance limits the current under normal operating conditions. The inductive reactance is higher than the DC resistance. Much like the inductance of a mains transformer limits the current to a sensible value, far less than the DC resistance of the coil would suggest.

The plunger is the core of the inductor. Remove it and the inductance goes down so the coil current goes up. At this point the resistance of the coil comes into play and it heats up.

Rob
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
Top of pagePrevious messageNext messageBottom of page Link to this message

Magnum4
Frequent Contributor
Username: Magnum4

Post Number: 385
Registered: 04-2005


Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 25 January, 2007 - 09:17 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks again Rob

More research needed
I hate not knowing.

The coil will burn either AC or Dc.

I have done both

Thanks.
Regards,
Jim
Top of pagePrevious messageNext messageBottom of page Link to this message

Epithumia
Frequent Contributor
Username: Epithumia

Post Number: 263
Registered: 06-2006

Rating: N/A
Votes: 0 (Vote!)

Posted on Thursday, 25 January, 2007 - 09:34 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hmm, I don't know why it would happen at DC, unless the plunger is acting as a heatsink.
If you need me, Neil and me will be hanging out with the Dream King. - Tori Amos
Top of pagePrevious messageNext messageBottom of page Link to this message

Gordon
Frequent Contributor
Username: Gordon

Post Number: 171
Registered: 05-2005

Rating: N/A
Votes: 0 (Vote!)

Posted on Sunday, 28 January, 2007 - 03:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I think the inductance of the coil also has an effect on the DC current flowing for a short period of time, especially for large values of inductance.

Some solenoids are not driven by steady DC current, they are pulsed with higher current to increase the pull-in force of the plunger/core.

Depending on how many times the solenoid coil is pulsed at DC, without the core, higher currents than intended could flow to cause the coil to overheat.

I can relate to the effect with AC. When I was younger I dismantled a transformer, and connected it to the mains without the core. The smell of melted bobbin and overheated wire stays with you for a while!
Top of pagePrevious messageNext messageBottom of page Link to this message

Poriet
Frequent Contributor
Username: Poriet

Post Number: 130
Registered: 06-2005

Rating: N/A
Votes: 0 (Vote!)

Posted on Tuesday, 06 February, 2007 - 07:21 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Gordon,
I'm really glad to hear about the transformer which burned up when you removed the core. In fact, although I've never done it, I guessed that this is what would happen. There was a discussion some weeks ago when I said as much, but some guy in the USA flamed me ( told me I was too stupid to understand electromagnetism )and I couldn't be to argue with him.
Some years ago a friend of mine was doing some house DIY and he was using a drill a lot. It was on an extension cable, but the cable was only partly unwound. After some weeks of this the fuse blew. When he tried to unwind the extension cable, he found the insulation had got so hot inside the coiled-up cable that it had melted and stuck successive layers together until , finally, there was a short. I dont believe inductance plays any meaningfull role in this. Any inductance will be piffling and the impedance at 50Hz so low as to be unmeasurable, I'll bet. It's simply that the wire has a finite resistance( low, but finite ) and at heavy currents it gets warm. One story I heard was some guy found his cat always lay on the same spot on the carpet. Exactly underneath that spot was a coiled-up mains cable!
Regards
P.
Top of pagePrevious messageNext messageBottom of page Link to this message

Obiwan
Frequent Contributor
Username: Obiwan

Post Number: 1444
Registered: 12-2005

Rating: N/A
Votes: 0 (Vote!)

Posted on Tuesday, 06 February, 2007 - 11:05 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Apparently you just proved my point, as I never "flamed you". But since you think so, I'll let it stand.

I'm sorry you don't seem to understand some very basic point of electricity. Not my place to educate the unwilling.

But frankly, I don't care. I've said it before. I'm not here to prove anything to anybody. I don't carry a grudge either. And I'm certainly not going to sit here and argue basic electricity with somebody that doesn't even know when he's been "flamed".

Believe me, if I had flamed you, you'd certainly know about it.
Do Not Hit The Fly That Lands On The Tigers Head.

Administration Administration Log Out Log Out   Previous Page Previous Page Next Page Next Page