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Old 14th Apr 2019, 7:59 pm   #21
broadgage
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Default Re: Resistive Line Cord ... would it be legal today?

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Originally Posted by crackle View Post
Yes, subject to choosing the correct type from the many variations that are offered.
Most heating tapes consist of a large number of short and parallel connected heating zones. Not ideal for line dropper cord.
However the above company also offer SERIES heating cable that consists of a single high resistance conductor from one end to the other.
Calculate the series resistance needed to drop say 120 volts.
Select a length of series heating cable with this resistance.
Check the resultant dissipation in watts per meter to make certain that the cable wont become too for comfort if handled.

Example, load current of 0.15 amps, desired voltage drop of 120 volts.
Watts to be dissipated=18 watts.
Suggested minimum length of about 3 meters so as to limit dissipation to 6 watts per meter and ensure it becomes only slightly warm in use.
Suggested maximum length of about 6 meters for reasons of cost and convenience.

Fit an RCD plug in case of a fault part way along the dropper cord.
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Old 14th Apr 2019, 8:57 pm   #22
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Default Re: Resistive Line Cord ... would it be legal today?

The lightest grade of cable you are likely to use in practice is 0.5mm² (16 strands, 0.2mm.)
Per Wikipedia, the resistivity of copper ρ =1.68e-8 Ωm.
0.5mm² mains cable has a cross-sectional area A = 0.5e-6 m².
This means it has a resistance per unit length of 0.0336Ω/m.

We want 867Ω, so that requires a length of 25803m. But since there are two conductors in our cable, we actually need only half that amount of cable; so the total required length is 12.9km. Or 42330 feet, or 8 miles and 1.5 chains. 129 hundred-metre drums, anyway.

The absolute minimum you could get away with would be 7/0.2 equipment wire ("hookup wire"). This will have 7/16 of the cross-sectional area, therefore 16/7 the above resistance, and so we will only need 7/16 of the length. But this is single-core, so we will require twice as much, or 7/8 of the original length; that is 11.3km. Or 23 500m. drums.

That's ..... well, actually, that's not so altogether beyond the bounds of feasibility that nobody would be prepared to try it for real .....
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Old 14th Apr 2019, 9:15 pm   #23
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Default Re: Resistive Line Cord ... would it be legal today?

How about a mains transformer from a broken tuner/amp with a full wave voltage doubler?
They give about 45 volts across the outer taps and are fully isolated.
I have got such a set that works fine from a 60V bench PSU in series with another 30V one.
I have a 40V plus 40V dot matrix printer transformer waiting to be cased up for it.
It will not need a doubler.
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Old 14th Apr 2019, 9:34 pm   #24
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Default Re: Resistive Line Cord ... would it be legal today?

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I seem to remember a friend bringing home from France an inspection lamp based on a fluorescent tube and not much else... And the lead became warm in use so was presumably a resistive ballast. (Dunno how the start-up sequence functioned). This was in the early 1990's, and was new then!
Those were sold at least until electronic ballasts took over. They used a normal starter and presumaby relied on the low starting voltage of the tube and if needed the stray inductance of the resistive mains lead.
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Old 16th Apr 2019, 10:29 pm   #25
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Default Re: Resistive Line Cord ... would it be legal today?

Hello, some years ago 1990s The company I worked for had wire wrapping tools that were 120 volt with a plastic line cord which was quite durable,
on 240 volts As is usual the cord became damaged and was replaced with three core cable, you know the answer lasted 5 seconds. might be worth looking in USA websites It might still be around?
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Old 17th Apr 2019, 10:05 am   #26
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Default Re: Resistive Line Cord ... would it be legal today?

We used drums of the stuff in the '50s largely due to the influx of small 110v radios from the states and replacing it was quite a common job.

I think the problem now would be the asbestos content if it was still made the same.

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Old 17th Apr 2019, 12:38 pm   #27
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Default Re: Resistive Line Cord ... would it be legal today?

Asbestos would be prohibited.
Alternatives exist, but new manufacture of line dropper cord seems most unlikely for reasons already given.
Use of an existing product such as trace heating wire seems a distinct possibility for restoring an existing radio to largely original condition.
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Old 18th Apr 2019, 5:04 pm   #28
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Default Re: Resistive Line Cord ... would it be legal today?

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Originally Posted by BK0scillation View Post
Hello, some years ago 1990s The company I worked for had wire wrapping tools that were 120 volt with a plastic line cord which was quite durable,
on 240 volts As is usual the cord became damaged and was replaced with three core cable, you know the answer lasted 5 seconds. might be worth looking in USA websites It might still be around?
There hasn't been resistance line cords made in the U.S. for at least 50 or more years. The inspection agencies such as U/L never approved of them.
I've managed to find space in even the smallest midget sets for a dropper cap. We can get away with a lower voltage dropper cap because of our lower mains voltage.
All the NOS resistance line cords that I looked at in the last many years were already starting to crumble from age.
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Old 18th Apr 2019, 5:44 pm   #29
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Default Re: Resistive Line Cord ... would it be legal today?

For cautious experimentation: would it be feasible to coil non-resistive mains flex, in low-load applications, as a means of voltage dropping? Perhaps around a solid core at some point.
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Old 18th Apr 2019, 6:17 pm   #30
Leon Crampin
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Default Re: Resistive Line Cord ... would it be legal today?

Copper wire makes a poor substitute for nichrome in a ballast because of its high positive coefficient of resistance with respect to temperature. You would end up with an unacceptably high heater current with a cold ballast. Electric blankets are wound with nichrome....

Leon.
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Old 18th Apr 2019, 6:55 pm   #31
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Default Re: Resistive Line Cord ... would it be legal today?

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Quote:
Originally Posted by BK0scillation View Post
Hello, some years ago 1990s The company I worked for had wire wrapping tools that were 120 volt with a plastic line cord which was quite durable,
on 240 volts As is usual the cord became damaged and was replaced with three core cable, you know the answer lasted 5 seconds. might be worth looking in USA websites It might still be around?
There hasn't been resistance line cords made in the U.S. for at least 50 or more years. The inspection agencies such as U/L never approved of them.
I've managed to find space in even the smallest midget sets for a dropper cap. We can get away with a lower voltage dropper cap because of our lower mains voltage.
All the NOS resistance line cords that I looked at in the last many years were already starting to crumble from age.
Weren't they called curtain burners? Presumably for a good reason.

I don't think the more modern (European) variant used in inspection lights, contains asbestos.
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Old 18th Apr 2019, 7:02 pm   #32
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Default Re: Resistive Line Cord ... would it be legal today?

Not that I have any experience whatsoever in these matters, but it struck me, why not experiment with a (lowish) wattage lamp limiter in series?
Hope I am not shot down in flames!! Still, better than the curtains...!
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Old 18th Apr 2019, 8:11 pm   #33
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Default Re: Resistive Line Cord ... would it be legal today?

The nickname "curtain burner" came about when the line cord was neatly folded for appearance sake. Whether or not it was common is anyone's guess. If extended they only got warm, but are still not one of Engineering's finest ideas.
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Old 21st Apr 2019, 6:56 pm   #34
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Default Re: Resistive Line Cord ... would it be legal today?

In these days of "energy-efficiency" regulations a purely-resistive, multiple-Watt-wasting line-cord would no doubt fail to get past the Green-police.

[Admittedly, such a low-tech solution wouldn't inject much third-harmonic component back into the supply: this is a big issue in getting compliance-approval for power supplies these days]
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Old 21st Apr 2019, 7:28 pm   #35
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Default Re: Resistive Line Cord ... would it be legal today?

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Originally Posted by Pfraser View Post
For cautious experimentation: would it be feasible to coil non-resistive mains flex, in low-load applications, as a means of voltage dropping? Perhaps around a solid core at some point.
In theory, yes. In practice the cost, weight, and bulk of the length of mains flex that would be required make the idea a non starter.
It would have to be coiled up in such a way as to minimise bulk, but without any risk of any part overheating.

A small external transformer would be more sensible, cheaper, and more efficient.
Or a dropper resistance in a ventilated box.
Or trace heating wire.
Or a lamp, or a series of lamps.
20 volt, 3 watt, MES Christmas tree lamps might suit for a set that uses 0.15 amps. Probably about 6 such lamps in series, but start with more than that and monitor the actual voltage carefully. Clear lamps are available so as not to look too Christmassy.
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Old 22nd Apr 2019, 9:12 pm   #36
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Default Re: Resistive Line Cord ... would it be legal today?

Given that most of the sets using line cord droppers were American midgets designed to run on 110V supplies, my solution was to remove the decayed dropper entirely, replacing it with a straightforward mains lead terminating in an American plug. This plugged conveniently into a small auto-transformer with 110V output. (In my workshop anything with an American type plug is assumed to require a 110V supply.)
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Old 23rd Apr 2019, 8:00 pm   #37
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Default Re: Resistive Line Cord ... would it be legal today?

some of them of course use a 'tapped' (3-core) line cord which require more lateral thinking to replicate.
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