On Fri, 30 Oct 2020 13:30:37 -0400, Runner
wrote:
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.
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.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558