can't figure correct resistance for teflon coated copper wire

[Runner]

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.

[Jeff Liebermann]

On Thu, 29 Oct 2020 21:55:51 -0400, Runner <run...@aolnospam.com>
wrote:

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:
<https://www.powerstream.com/Wire_Size.htm>
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:
<https://www.google.com/search?q=milliohmmeter&tbm=isch>
a Kelvin bridge:
<https://www.google.com/search?q=kelvin+bridge+ohmmeter&tbm=isch>
or possibly an ESR meter:
<https://www.google.com/search?q=esr+meter&tbm=isch>
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.



--
Jeff Liebermann je...@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

[Mike]

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

[Peter W.]

> 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!

Hmmmm... If it is sufficiently important to know the precise and accurate 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 spool of electrical wire that is within even 1% of the nominal length, with the tendency 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 lowest range. Which may not be good enough in this application.

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

Peter Wieck
Melrose Park, PA

[Ralph Mowery]

In article <593b6d9c-02ba-4ec4...@googlegroups.com>,
peterw...@gmail.com says...

>
> Hmmmm... If it is sufficiently important to know the precise and accurate 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 spool of electrical wire that is within even 1% of the nominal length, with the tendency 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 lowest range. Which may not be good enough in this application.
>
> Again, if it is sufficiently important, then the correct instrumentation and the calibration and set-up of same is critical.
>
>
>

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.

[Peter W.]

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

[Runner]

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.

[legg]

On Thu, 29 Oct 2020 21:55:51 -0400, Runner <run...@aolnospam.com>
wrote:

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

[Tim R]

On Friday, October 30, 2020 at 1:30:43 PM UTC-4, Runner wrote:
> 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
>

> 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

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

[Runner]

On 10/30/20 4:05 PM, Tim R wrote:
> On Friday, October 30, 2020 at 1:30:43 PM UTC-4, Runner wrote:
>> 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
>>
>> 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
>
> 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?

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

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.

>
> #darwinawards
>

[Jeff Liebermann]

On Fri, 30 Oct 2020 13:30:37 -0400, Runner <run...@aolnospam.com>
wrote:

>On 10/30/20 1:21 PM, Ralph Mowery wrote:
>> In article <593b6d9c-02ba-4ec4...@googlegroups.com>,
>> peterw...@gmail.com says...
>>>
>>> Hmmmm... If it is sufficiently important to know the precise and accurate 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 spool of electrical wire that is within even 1% of the nominal length, with the tendency 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 lowest range. Which may not be good enough in this application.
>>>
>>> Again, if it is sufficiently important, then the correct instrumentation and the calibration and set-up of same is critical.
>>>
>>>
>>>
>>
>> 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.

>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.

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).

>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 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.

>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.

Hint: Real electric blankets use nichrome or some form of resistance
heating wire. See list at step 6.
"DIY Heating Pad - (small Electrical Blanket)"
<https://www.instructables.com/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 Teflon wire is hot glued to the old blanket.

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.

>I wasn't sure if this
>would work as I feared glue melting, but so far so good after some
>hours.

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.

>At 84 watts though, it might have been a different story.

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

>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.

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.

>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.

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.

>I wasn't surprised of
>course, but thought I'd give it a try.

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

>Any ideas as to how to vary the power welcome.

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:
<https://www.google.com/search?q=electric+blanket+controller>
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:
<https://www.qsl.net/ve3lny/blanketcontroller.html>
Note that the design is for a 117VAC blanket, not 12V.

Be careful and have a fire extinguisher handy.

[gghe...@gmail.com]

On Friday, October 30, 2020 at 1:30:43 PM UTC-4, Runner wrote:

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.

[Runner]

On 11/12/20 9:27 AM, gghe...@gmail.com wrote:

>
> 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.

12 VDC.

AC might be easier, get a transformer
> (maybe filament transformer?) and drive it from a variac.

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.

> 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. :^)

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.

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

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.

>
> George H.
>

[Peter W.]

https://www.amazon.com/long-hot-water-bottle/s?k=long+hot+water+bottle
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 permitted 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 expression "Three dog night" came from.

Once again, demonstrating that this venue exists to provide needlessly complex solutions to otherwise simple problems using the most obscure means-and-methods presented with the maximum amount of obfuscation using as many words as possible.

[Three Jeeps]

Just to add a bit of information to this guidance, joining nichrome wire to nichrome or copper can be a bit of a challenge if you are not aware of the metallurgy issues. It does not solder with lead free rosin core solder. Connections to nichrome wire are often done with a crimp style connector. This may not be a workable solution for an electric blanket, depending on the area where the splice is applied.
Nichrome can be soldered but you need a acid type flux and the degree of success is somewhat dependent on the type of nichrome (A: has no iron and behaves like stainless steel, B: contains some iron and can be more easily soldered).
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 have had good luck with 60/40 acid core flux but I have not tried it. My guess 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

[Three Jeeps]

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

[legg]

On Fri, 13 Nov 2020 12:18:15 -0500, Runner <run...@aolnospam.com>
wrote:

Why not use a proper thermostat?

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

Add or subtract blankets, as required.

RL

[Fox's Mercantile]

On 11/13/20 12:57 PM, legg wrote:
> I don't recommend electric blankets. For a multitude
> of reasons.

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
WA6FWi
http:foxsmercantile.com

[Runner]

On 11/13/20 3:07 PM, Fox's Mercantile wrote:
> On 11/13/20 12:57 PM, legg wrote:
>> I don't recommend electric blankets. For a multitude
>> of reasons.
>
> 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.
>
>

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.

[Runner]

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

[Fox's Mercantile]

My previous one was a Sunbeam, it went tits up after three years.
The new one is a Biddeford. Unlike the Subeam, it's soft and fuzzy.

[Runner]

I didn't have much luck with Biddeford either. I tried one of their
heated mattress pads. It didn't last a season. Hopefully, you'll have
better luck with their blankets.

[Runner]

I never did go with the nichrome wire. I have some, but used it three
years ago to make some telescope dew heaters. The idea was to wrap the
front of the telescope with nichrome embedded in a fabric with just
enough wattage to keep dew away. Works great at 5 to 10 watts. For
that I believe I crimped it to the supply wires. When I started on the
blanket, there was no way I was going to try using nichrome for
something that large and went with some 22 ga Teflon coated wire I had
around for years. So far, it has been working out well. I can get up
to 80 watts, but I will mostly be running at half that or a little over
as there's much less heating to deal with. Today, the dimmer arrived
and I tried out a jacket I also wired. It too reaches 80 watts but
again I will only need about 50. 80 watts seems a bit too much even in
a cold house.

Initially, to hold the Teflon wiring in place, I used hot glue.
However, when 80 watts passed through, some of it began to not only
outgas but melt so I removed all of it and went with nylon ties. I
first tested the nylon and a bit of blanket fabric by heating in the
oven at 90 C for several hours and both came out fine. I was a bit
concerned as nylon seems advertised to hold well up to like 85 C, but I
don't see any issues.

Only other issue was temperature. The longer the blanket is on at the
max, the warmer it slowly creeps up. So, I have to either run it at
less power, which I will normally do, or have a duty cycle using a
timer. I have since incorporated a timer for when it gets colder in
here. I'm going to try for a 33% time on and 2/3 time off and see how
it goes. Someone has suggested a thermostat, but not sure how other
than maybe using a probe attached to the blanket wires and then a relay
turns the blanket on and off as needed.

[ohg...@gmail.com]

Sunbeam blankets don't flat out quit, they slowly get colder over time. Not sure why that is. It's like they're programmed to do so. Too bad because their controllers are great. I switched to their mattress pads and they get inert over two or three seasons. Looking for another option right now.

[Runner]

Exactly right, don't just quit but lose warmth over time. Out of the
three Sunbeam blankets I owned and two heated throws, they all did this.
This was my main inspiration for my DIY revamping. There's actually a
fair amount of online info on the Sunbeams if you do a patent number
search. A circuit block will come up along with at least one schematic.
I thought the culprit might have been a small SMD voltage sampling
transistor inside the little box where the cord plugs into the blanket,
but replaced that and still no heating. I also swapped out controllers
for a different one to no avail. So, that left the blanket itself. I
suspect the changing of wire resistance from repeated heating is what
does it and of course there's no solution without either a revamping as
I have done, or a new blanket.

Patent numbers show that someone definitely designed their latest
circuitry. IMHO, probably due to lawsuits and other worries from the
past, it was overdesigned and thus now stops working after short order.
There is no easy solution unless one wants to try other brands and take
a chance that you might come across one that actually keeps working.

[Peter W.]

I don't get it. We purchased two Sunbeam extra-large heating pads with five (5) year warranties. One failed in short order and was replaced in short order. The other is going strong after four (4) years. We purchased through Amazon so there would be no nonsense with losing receipts. From what has been discussed here, they seem to fail on an hourly basis and no one invokes the warranty??

[Runner]

Three of my Sunbeam blankets failed within the five year warranty
period. I returned and exchanged each for replacements direct from
Sunbeam. However, the big catch is with the shipping. Even with
blankets that fail, Sunbeam does not pay shipping cost to ship the
defective blanket to them, and when paying $15 for insured shipping,
this almost defeats the entire process when the blankets were purchased
on sale for around $20 to $25. Besides, by the third time, Sunbeam had
become far less responsive and I had to contact them a lot more to get
them moving on the final blanket. And, yes, they were strict about
receipts, blanket tags, and such so don't forget anything and keep
copies of all.

Why all this and not have a blanket that simply works? Not for a single
season, but for years like they once had. Beats me.

[Dave Platt]

In article <ros07f$acu$1...@dont-email.me>, Runner <run...@aolnospam.com> wrote:

>Why all this and not have a blanket that simply works? Not for a single
>season, but for years like they once had. Beats me.

It is, unfortunately, "the race to the bottom". Price and features
drive sales; long-term reliability does not.

Sunbeam (and most other brands) thus have an incentive to "buy cheap",
from companies which are skilled at building stuff which will last
_just_ long enough. They count on most of the devices being out of
warranty by the time they fail, and on most consumers being unwilling
to go to the trouble and expense of keeping receipts and asserting
their warranty rights. That way, they get to sell you cheap things
every couple years, and collect a small profit each time, rather than
sell you something once every 10 years for only a slightly higher
profit.

Sadly, it shows the truth of an old rule of thumb. "When somebody
asks 'Why X, it doesn't seem to make sense', the answer is most
probably 'Money'."