On 10/30/20 1:21 PM, Ralph Mowery wrote:
In article ,
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.

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.