can't figure correct resistance for teflon coated copper wire

I have a spool of Teflon coated copper wire. I am trying to determine the resistance of a one foot piece, but both of my meters read differently. How can I get a reliable reading? I am using DVMs. Thanks in advance.

Reply to
Runner
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If solid wire, measure the diameter of the copper wire. If stranded, count the number of strands and measure the diameter of each strand of wire. Also, measure the length of the wire. Hopefully, you have more than one foot of wire. Then, go to a wire table, such as: Determine the resistance per 1000ft or per km from the table. The math is: resistance/ft * number_of_ft_length and you have the resistance. That should give you an answer without the ohms-guesser.

The problem you're having is typical of trying to measure fairly low resistances with commodity hardware (i.e. a multimeter). To do it accurately, you'll either need a milliohm meter: a Kelvin bridge: or possibly an ESR meter: The ESR meter might be a problem because it uses AC at 100KHz, which is not exactly the same as the DC resistance. The cheaper devices will also not work above about 10 ohms. For short lengths of wire, the contact and lead resistance to the probes become a major source of error, which will require a Kelvin bridge to eliminate.

If all else fails, PTFE insulated wire usually has the manufacturers name and product number marked somewhere. Find these, search for the specs, and you should get a number for ohms/ft or ohms/meter.

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

Alternatively, if you know the length of the spool (because it's new and untouched, or at least measurable in some way) measure the whole spool's resistance, and then divide by the length. Hopefully, the far end of the wire is poking out somewhere!

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--------------------------------------+------------------------------------ 
Mike Brown: mjb[-at-]signal11.org.uk  |    http://www.signal11.org.uk
Reply to
Mike

Hmmmm... If it is sufficiently important to know the precise and accurate r esistance of a given (and very short) length of wire, then that method will not work very well. I have never, ever come across a factory-sealed spool of electrical wire that is within even 1% of the nominal length, with the t endency to run a bit long rather than short. And even shop-cut lengths tend to have a bit added for waste and crimps.

And here is another issue: I keep a pretty good Fluke meter which gives a resolution of +/-0.5% in ohms in ranges above 32 ohms, and +/- 2% at the l owest range. Which may not be good enough in this application.

Again, if it is sufficiently important, then the correct instrumentation an d the calibration and set-up of same is critical.

Peter Wieck Melrose Park, PA

Reply to
Peter W.

rather than short. And even shop-cut lengths tend to have a bit added for waste and crimps.

For most at home the best way for very low resistance is to have an ampmeter and voltmeter and varitable power supply. Put as much current as you can through the wire and measure the voltage across it and ohm law the resistance.

Reply to
Ralph Mowery

This is what I ended up doing, although I decided to just power the entire 60 foot spool instead of the single one foot piece. I found that, at 12 V, 7 A was used, so that's where I started.

That was too much power, at least IMO, for the electric blanket I was making, so I kept checking current and voltage until a more reasonable

55 W was reached. This meant using 20 feet of the wire and powering at 5 V (11 A).

I'm under the blanket now and it's toasty. Not hot, but warm. I will be continuing to check connections for excess heating, but I repurposed the original Sunbeam connections and those are more than adequate and saved me a lot of time. The only source of moderate heating outside the blanket is the power wires to the blanket. Since I used the Sunbeam wires, their gauge is a bit thin. If it becomes an issue, I will upgrade to a thicker cable.

The Teflon wire is hot glued to the old blanket. I wasn't sure if this would work as I feared glue melting, but so far so good after some hours. At 84 watts though, it might have been a different story. Originally, I was going to go with the full wire spool and the 84 watt version, but I wasn't sure how I was going to vary the output. All I have on hand was a PWM 8 A 12 V lamp dimmer, used for dimming LED arrays, but it burned out almost instantly. I wasn't surprised of course, but thought I'd give it a try. Any ideas as to how to vary the power welcome.

Reply to
Runner

Yep. All within the accuracy range of the instruments. Which compound, not cancel, when multiple instruments are involved.

Peter Wieck Melrose Park, PA

Reply to
Peter W.

Dude! I just..........I mean it boggles the mind. You made your own electric blanket with hot glue and duct tape, and you're trusting your life to it?

I hope you live alone. I'm pretty sure you made your own smoke alarms but still.

#darwinawards

Reply to
Tim R

Run an amp through it and measure the voltage drop - same principle as many older micro-ohmeters.

Helps if you have a fairly precise amp.

Get to use all your meter ranges and sockets, in the process. Lots of room for mistakes, though.

RL

Reply to
legg

Not duck tape, hot glue only. Well, if you can demonstrate to me a name brand that lasts more than part of one season, I would buy it. I've bought several Sunbeams and all were returned/ exchanged because they don't last. I also went with Biddleford and same thing. Bought blankets today are junk and I am on limited income to keep shelling out for them.

I didn't mention the power supply, but it has overload protection along with current sense, which is adjustable. If the blanket suddenly starts drawing more than it should, the power goes off. There are also fast blow fuses on both lines.

Reply to
Runner

You did that through a 1 ft length of wire? P = E * I = 12v * 7A = 84 watts In a 1 ft length of wire, that should have gotten the wire red hot (depending on wire gauge and contract resistance).

I have a small complaint. If you expect usable answers, it's really helpful if you would describe:

  1. What problem are you trying to solve?
  2. What do you have to work with?
  3. What have you done so far and what went wrong? My initial reply assumed that you were doing some kind of measurement. I replied accordingly with the instruments required. My answer would have been very different if you had supplied the details and numbers.

Hint: Real electric blankets use nichrome or some form of resistance heating wire. See list at step 6. "DIY Heating Pad - (small Electrical Blanket)" Any clues as to the source and tupe of "teflon insulated wire" you're using? If its copper wire, be advised that soft copper wire work hardens fairly easily. Just bend it a few times and it will break.

The wire is not made of teflon. It's probably copper, nichrome, or something similar. The insulation is Teflon.

Make sure you're using high temp (195C gun) glue. Low temp (130C gun) will work, but methinks high temp is a better choice.

The standard high temp hot melt glue melts at about 200C. That's quite a bit warmer than the 25C to 55C range of a typical electric blanket.

Queen size electric blankets typically draw about 60 watts or 100 watts for a twin size.

A light dimmer should work. If you try to use a linear regulator, you'll end up dissipating quite a bit of heat in the regulator. The switching regulator in the light dimmer is far more efficient.

It should have worked. However, when the wire is cold, the resistance is rather low and the inrush current is rather high. When it gets warm, the resistance increases, and the operating current is reduces. My guess(tm) is that the initial inrush current exeeded the 8A rating.

Learn by Destroying(tm). Ok, what have you learned?

Nope. I can't do much without detailed specs on the wire.

- 25C resistance/ft

- 60C resistance/ft

- Type of wire used.

- Length of wire used.

- Estimated blanket insulation value.

- Thermostat system used.

- Power source. 12V, 117VAC, or variable power supply?

- Over-temperature protection (thermal fuse)?

A Google search might help: The commercial controllers usually have the power control and temperature regulating devices included. If one of the controllers from your failed blankets is still available, it might be worth testing and trying. Or, roll your own: Note that the design is for a 117VAC blanket, not 12V.

Be careful and have a fire extinguisher handy.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

ate resistance of a given (and very short) length of wire, then that method will not work very well. I have never, ever come across a factory-sealed s pool of electrical wire that is within even 1% of the nominal length, with the tendency to run a bit long

or waste and crimps.

es a resolution of +/-0.5% in ohms in ranges above 32 ohms, and +/- 2% at the lowest range. Which may not be good enough in this application.

on and the calibration and set-up of same is critical.

Re: varying power. Since the resistance of copper goes up with temperature (R is ~ temperature ) controlling the voltage might be best. (no thermal runaway) Are you using AC or DC power. AC might be easier, get a transformer (maybe filament transformer?) and drive it from a variac. DC will be trickier ('cause that power has to go somewhere) and some PWM scheme might be the way to go. Or buy a variable DC supply... you can use it for electroplating when not staying warm. :^)

In general I'm wondering why not try a heating pad and a nice down comforter?

George H.

Reply to
ggherold

12 VDC.

AC might be easier, get a transformer

I had originally considered this, but variacs in general are not isolated from the AC line and I felt I would have needed an additional isolation transformer, something I don't currently have and they seem to be quite costly.

I am currently using the PWM controller with the DC lines to the blanket. So far so good. I have been monitoring temperature of the blanket wiring. It does get up to 180 F, but I am using a timer to cycle the system on and off so that the blanket is not running continuously. I'm not sure how much I will have to vary the duty cycle yet as the house hasn't reached its wintertime lows, but the hope is for a 33% time on and the rest off. Just have to wait and see. I'm currently using a mechanical timer, but I may switch out to a programmable one I have here.

I did this several years ago. No heating pad, but two stacked down duck filled comforters. I still have this set up on the bed in the other room. Took a long time to heat up, but once warm I couldn't cool down so ended up having to turn a box fan on at full blast at 3 AM in a 47 F bedroom! I suppose I could set the fan up on a timer to do this automatically, never thought of that.

Reply to
Runner

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Has the ability to provide a short-term boost without the worry about duty- cycles and such. No controllers, timers or other needlessly complex devices.

My wife and I like a cold room, but we temper that with the number of permi tted pets on the bed. We have 180 pounds of available pets, being 95/25/20/

20/20 (Golden/Scottie/Maine Coon/Main Coon/Main Coon Mix). And where the ex pression "Three dog night" came from.

Once again, demonstrating that this venue exists to provide needlessly comp lex solutions to otherwise simple problems using the most obscure means-and

-methods presented with the maximum amount of obfuscation using as many wo rds as possible.

Peter Wieck Melrose Park, PA

Reply to
Peter W.

rate resistance of a given (and very short) length of wire, then that metho d will not work very well. I have never, ever come across a factory-sealed spool of electrical wire that is within even 1% of the nominal length, with the tendency to run a bit long

for waste and crimps.

es a resolution of +/-0.5% in ohms in ranges above 32 ohms, and +/- 2% at t he lowest range. Which may not be good enough in this application.

ion and the calibration and set-up of same is critical.

Just to add a bit of information to this guidance, joining nichrome wire t o nichrome or copper can be a bit of a challenge if you are not aware of th e metallurgy issues. It does not solder with lead free rosin core solder. C onnections to nichrome wire are often done with a crimp style connector. T his may not be a workable solution for an electric blanket, depending on th e area where the splice is applied. Nichrome can be soldered but you need a acid type flux and the degree of su ccess is somewhat dependent on the type of nichrome (A: has no iron and beh aves like stainless steel, B: contains some iron and can be more easily sol dered). I've had good luck joining both types using Ruby fluid flux and in one case using flux for soldering copper pipe, e.g. plumbing flux. Some people hav e had good luck with 60/40 acid core flux but I have not tried it. My gues s is that it would work better on C type nichrome. I also have used a small tack welder but its not a common household or shop tool. If you look on the web am sure you will find lots of alternatives. J

Reply to
Three Jeeps

:

curate resistance of a given (and very short) length of wire, then that met hod will not work very well. I have never, ever come across a factory-seale d spool of electrical wire that is within even 1% of the nominal length, wi th the tendency to run a bit long

d for waste and crimps.

ives a resolution of +/-0.5% in ohms in ranges above 32 ohms, and +/- 2% at the lowest range. Which may not be good enough in this application.

ation and the calibration and set-up of same is critical.

nt

m

d

e

to nichrome or copper can be a bit of a challenge if you are not aware of t he metallurgy issues. It does not solder with lead free rosin core solder. Connections to nichrome wire are often done with a crimp style connector. T his may not be a workable solution for an electric blanket, depending on th e area where the splice is applied.

success is somewhat dependent on the type of nichrome (A: has no iron and b ehaves like stainless steel, B: contains some iron and can be more easily s oldered).

se using flux for soldering copper pipe, e.g. plumbing flux. Some people ha ve had good luck with 60/40 acid core flux but I have not tried it. My gues s is that it would work better on C type nichrome.

op tool.

whoops - typo - I have used 60/40 acid core with some success...(sorry, wro ng sentence when doing 3 things at once...) J

Reply to
Three Jeeps

Why not use a proper thermostat?

I don't recommend electric blankets. For a multitude of reasons.

Add or subtract blankets, as required.

RL

Reply to
legg

I just bought a new one. I turn it one 20-30 minutes before I go to bed. I turn it off when I go to bed. But when I go to bed, the bed is nice and warm. Beats the shit out of laying there swearing for 15-20 minutes waiting for the bed to finally get warm.

--
"I am a river to my people." 
Jeff-1.0 
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Reply to
Fox's Mercantile

If a Sunbeam, good luck keeping it working. I went through three in five years before finally going the DIY route. I'm not sure what's happened to them. I couldn't even get any of the ones I bought to last out the season. I think it has something to do with the current circuit they use and the blanket wiring. Growing up, my parents and I had their blankets and they lasted 15 years before I unfortunately tossed them a decade ago.

Reply to
Runner

With the blanket? Maybe, but not sure how to incorporate or what to use.

Reply to
Runner

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