Would like to use Nichrome wire to make a small heater

GerGa · 10-06-2009, 09:18 PM

Hello all, I was thinking the other day how much I want a heated steering wheel in my car so I searched the web and came across Nichrome wire which is used in toasters etc. I have come across some websites that have a table of how hot various gauges of wire get per 120v and at various amperage's. My first choice of a power source is a battery from an r/c car, rated at 3.3ah and 7.2v (6x1.2v cells). Judging by the typical operating temps of 400F and up I think the battery will be sufficient for getting a proper temp to warm my hands in the winter.
I do although have no idea how to supply the power, could it be just a simple pot. or some sort of basic voltage regulator?
Right now I am only worried about first of all getting the electrical things worked out, but I think for a steering wheel the best way to do it would be to coil it around, which would be my guess of around 7.5ft of wire( I coiled string around it).
This website has quite a bit of info, but I can not buy from them and I understand there are variations of nichrome(different rho value or resistivity). This is another site with a nice table of different gauges http://www.interfacebus.com/properti...rome-wire.html .
The best I have gathered from the web is the rho value is 675ft*ohm/cm at 20C.
I know doing this will be largely by trial and error, but I need somewhere to start off, like what size wire and what kind of power source to use.
Also I am thinking that around 80-150F is a completely sufficient temperature.
Thanks in advance, Greg

Oh BTW I am doing this becuase I have renauds syndrom(extremities hate the cold).

Yoyizit · 10-06-2009, 10:11 PM

no burn ever at 42°C(108°F)
burned in 30 sec at 54°C(129°F)
5 sec at 60°C(140°F)
1 sec at 71°C(160°F)
But these values are only for skin with normal blood flow. You may not have that.
http://en.wikipedia.org/wiki/Raynaud's_phenomenon

There was another post about using nichrome wire. It was kind of tricky to find the right wire size.

For a prototype I'd use the car's 12v electrical system and wrap several inches of the wheel circumference with a few dozen 1/4w resistors wired in series.

Depending on the pitch of the winding 1/8w into each resistor should be warm enough unless the wheel dissipates some of the heat.

For 25 resistors @ 1/8w each the total power would be ~3w. 14v squared divided by 3 = 65 ohms, about 2.7 ohms for each resistor.

Probably you should go smaller than 2.7 ohms and add a series adjustable voltage dropping resistor to give you some latitude on the heat generated.

Try Hosfelt, Digi-key, allelectronics, Mouser or Jameco for resistors in quantity.

Once you zero in on the winding pitch and watts per inch you can pick the Nichrome to do the same thing.

A more sophisticated version would use thermistors and feedback to maintain a fixed temperature. Depending on the thermal inertia of the wheel it might take a while to get to a comfortable temp., with or without feedback.

vsheetz · 10-06-2009, 10:58 PM

A cursory Google search show many varients of heated steering wheel covers. Perhaps using a commercially made unit, or a commercially made unit as the basis with self-modifications would be a way to go.

GerGa · 10-07-2009, 12:40 AM

Im not really understanding your calculations, I dont know what kind of resistor you are suggesting.
Also I thought the equation is p=I squared times R. Where did you get 14 squared divided by total power? Oh wait you did power equals volts squared divided by resistance but I have never seen resistance or resistors expressed in watts..
Sorry Im confused I have no idea what you did..

micromind · 10-07-2009, 01:09 AM

Quite a few years ago, I made some heated mirrors using nichrome wire from a 1500 watt household heater.

12 volts applied to about 4' of the wire resulted in burning the frost off the mirror in less than a minute. If left on for about 5 or 6 minutes on a warm day, the mirror is just about too hot to touch. The little loops of bare wire on the outside of the mirror will get too hot to touch in about 30 seconds.

I'm not sure how long the entire wire was, I seem to remember using about 1/2 of it for both mirrors though.

Rob

Gigs · 10-07-2009, 08:19 AM

It's funny how no one ever admits they are building an herbal vaporizer.

Yoyizit · 10-07-2009, 01:21 PM

Quote:

Originally Posted by GerGa

Im not really understanding your calculations, I dont know what kind of resistor you are suggesting.
Also I thought the equation is p=I squared times R. Where did you get 14 squared divided by total power? Oh wait you did power equals volts squared divided by resistance but I have never seen resistance or resistors expressed in watts..
Sorry Im confused I have no idea what you did..

The resistors are the upper two devices. A 1/4w resistor is about 1/4" long.
http://www.willcoxonline.com/X10mods/3components.jpg

P=I²R = V²/R so
R = P/I² = V²/P

On second thought, get a heating pad or blanket, wrap some of it around the wheel and adjust the power for comfort.

At this setting, if the blanket is taking 80w and the blanket is one square foot (144 sq. in) and your hand grip is 12 sq. in. then you need a heater that spreads the heat and gives you 0.6w per sq. inch [80/144] and 6.7w [12(80)/144] for one hand.

P = VI, I = P/V, so 80w @ 120vac = 0.7A. You'll need an ammeter to check this current draw and you can assume you have a constant 120vac.
For a given I, your power in watts @120v is (120)I.

Finding a suitable material with the right sheet resistivity to generate this heat at 12 to 14vdc is another problem. A wire or resistor concentrates the heat in a small volume so this is kind of opposite to what's desired.

You also might need slip rings of some kind to get the power to the rotating steering wheel.

If you have a local chapter of Volunteers for Medical Engineering and they are willing to take on this project the whole thing may be free.

GerGa · 10-07-2009, 04:44 PM

Well I am not too worried about the heat will be distributed right now since the hardest part is the electrical. But at quick thought I am thinking something along the lines of something metal wrapped over the wheel like flashing tape, etc..just something metal to distribute the heat although nichrome tape would be awesome but there are none to little choices for that that I have seen.Yes I realize I probably shouldn't have something conductive touching the nichrome/connection. But whatever that doesn't matter right now..
I realize now that xxwatt resistor means it's power limit and are you saying that I should probably get a resistor with a rating of 1/4 watt limit and that at (14?)v(why 14 instead of twelve?) from the car shod produce an 1/8w each for 25 resistors to get sufficient heat, which at 25 resistors is a resistor rated at under 2.7ohm. And you are saying that to produce sufficient heat at whatever volts you mean is probably not quite 1/8 watt, or a 2.7ohm resistor. How much less resistant a resistor do you reckon I might try out(of course at 25 resistors). I don't really mind wasting a little money on trial and error since resistors are pretty cheap.

I am somewhat familiar with electronics, I have seen and soldered some components such as resistors before and also have been in school for EET for only under a month now so I have only learned ohm's law, power law, coulombs kvl kcl and a couple others I think and we have done some studying with series and parallel circuits with resistors. I should have recognized the derivative of the power law but I have only seen that a little bit.

Thank you very much for helping me out,
Greg

Gigs · 10-07-2009, 05:57 PM

Car batteries at 13.8 volts normally, more when they are charging.

Yoyizit · 10-07-2009, 06:18 PM

Beg or borrow a heat pad and ammeter first to get some idea of what power is necessary.

Yes, auto voltage may go to 15.5v in the winter to maximize battery charging while not unduly shortening the life of the battery.

For predictable results we may want to stabilize the voltage fed to this at something under 12v so we get predictable results. An LM317 comes to mind.

A 1/4w resistor dissipating a 1/4w gets pretty hot, so for a WAG I figured 1/8w. That will halve the temp. rise above ambient for what 1/4w will give you.

Temp. rise above ambient is proportional to watts dissipated into a volume with some surface area. Doubling the area should cut the temp. rise in half.

GerGa · 10-07-2009, 10:22 PM

So how does a voltage regulator circuit kit that utilizes the LM317 sound? Although I don't need that huge heat sink, I think this one looks perfectly functional:http://cgi.ebay.com/Voltage-Regulator-Kit-AC-DC-in-DC-out-Based-on-LM317_W0QQitemZ260479385900QQcmdZViewItemQQptZLH_D efaultDomain_0?hash=item3ca5c8012c
With your guess at of a total of 3w the current would be something like 213mAh at 14v and I read up on the LM317 and apparently it's minimum current is 150mAh which seems kind of small so I should be fine with that.

Yoyizit · 10-08-2009, 12:38 PM

Quote:

Originally Posted by GerGa

I think this one looks perfectly functional:http://cgi.ebay.com/Voltage-Regulator-Kit-AC-DC-in-DC-out-Based-on-LM317_W0QQitemZ260479385900QQcmdZViewItemQQptZLH_D efaultDomain_0?hash=item3ca5c8012c

That thing has way more parts than you need and all "the heavy lifting" has been done for you.

With electronic heater controls I'd be concerned about the Load Dump Transient that vehicles have.
National Semiconductor has app. notes online and some of their stuff that is supposed to run off car voltages have a few extra components to suppress this transient. The least you need is a series diode with a PIV of 1000v because the transient goes way negative.

I'd first do some kind of prototype with just a heater and maybe a series dropping resistor. The watts/in² required is the most uncertain part of this project.
Imagine or measure or calculate the surface area of a 25w incand. bulb. It's way too hot to touch, so it gives you some idea of how many w/in² you need. Try two in series (1/4th the power) to try to bracket what kind of power you need. Give them a few minutes for the glass temp. to stabilize.
By mixing and matching lamps in series you might get pretty close to a comfortable temp. Remember that an incand. lamp acts like a resistor whose value is current-dependent. See
"Voltage, light output, and lifetime" in
http://en.wikipedia.org/wiki/Incandescent_light_bulb

A Variac would come in handy for this experiment but they are pretty hard to borrow. You could use a lamp dimmer but then you need true RMS meters to measure the watts inputted.

GerGa · 10-08-2009, 02:33 PM

Ok I definetely would not just do some guessing/calculations and just slap it together and hope it works. I will do as much as I can to figure out how to do this(right power for the right heat according to how it is distributed). What about these power supplies, that say they have load dump protection for cars, one ihas adjustable voltage and one is 6v only.
http://www.powerstream.com/dc6.htm

http://www.watt-power.co.uk/PSD05%20Vehicle%20DC-DC%20Converters.PDF

Yoyizit · 10-08-2009, 02:52 PM

http://en.wikipedia.org/wiki/Load_dump
I think a 1000v PIV diode, a 35v Zener and a resistor or fuse will probably take care of this transient.

Then the 317, 2 resistors, 2 small caps and possibly a heat sink [by mounting to metal in the engine compartment] and you're done.

GerGa · 10-08-2009, 08:09 PM

So how could I go about finding out where and in what orientation to connect the diodes? Also how do I choose what diodes - just on allelectronics there are two 1000v diodes, 1 and 3 amp, and also I have not had much luck in finding a zener diode in 35v except here which is 5w but I am not sure what power rating to look for; is that one fine?
Also how could I go about figuring out what kind of fuse or resistor to use and how to hook that up?

And your mention of the 317 and a couple resistors and caps seems like the standard circuit design for a v-regulating circuit that uses the 317 - (just trying to state my understanding)