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Old 10-13-2009, 11:36 AM   #31
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Would like to use Nichrome wire to make a small heater


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Originally Posted by fireguy View Post
Cabela's ragg wool/thinsulate gloves $15.00
Thanks but if you had such a problem with the cold you would understand that gloves never work. Mittens are superior. Also this is a chance in the day to warm up my hands since most building are too cold summer or winter(over airconditioning). God now ssome people probably think im a snob..

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Old 10-13-2009, 11:47 AM   #32
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Would like to use Nichrome wire to make a small heater


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So your saying trials with resistors will give me a starting point for trials with nichrome?
If I wanted to mock up 1 foot of resistors like a 21awg nichrome wire which is .831ohms/ft http://www.heatersplus.com/nichrome.htm , then I would connect a bunch of resistors 1 foot long to make .831ohms of resistance(using whatever combination of series and parallel is needed)? Correct?
I am also fuzzy on how a resistor is going to produce a similar amount of heat, or might there be not too much difference and it mostly comes down to having resistors about the size of wire and similar resistance per length..?
Edit: I tested some resistors and I got them pretty hot, they are 1/4w resistors. I did some calculations and I really see why you said that maybe I should only do part of the wheel, it takes so many ohms to get low enough wattage. With the resistors I guesstimated with an ohm value a reasonable watt value for the whole steering wheel, within my limitations of volts. I came up up with 300ohms per foot at 8ft to give the right power amount for the whole wheel. But, nichrome is much less resistive about 8ohm/ft at 31gauge and 27ohmft at 36gauge;this is way too little(therefore way too hot), and an only an unreasonable amount of wire would up the resistance and down the wattage;within my voltage limitations.
So now to find resistance wire that has very high resistance...
Very high gauge wire and a close wind seems the way to go; best heat distribution for the power I need, but god is the wire so expensive...I really need high resistance
wire..

EDIT #2 - Oh ****. I can use resistors. doh.
Resistors give much more flexibility in the resistance value than nichrome but they don't easily wrap around curved surfaces.

A 1/4 w 1 ohm resistor with 0.5v across it gets pretty hot. With 0.35 v the temp. rise above amb. is only half as much. If you knew the thermal resistance, θ, of your resistors in C/w you could calculate the rise in free air.
http://en.wikipedia.org/wiki/Thermal...in_electronics

Metal alloys are much more conductive than carbon composition resistors.

One way to use skinny wire is to wind it around an elec. insulating flexible core so your actual wire length would be very long but the length of this "heated string" would be only a few feet.

BTW, #30 AWG copper wire wrap wire is 10' per ohm but it's delicate.

8' of wire @ 20w @ 8v = 3 ohms = 3/8 of an ohm per ft.
One thing spreadsheets are good for is error-checking, once you get them debugged.

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Originally Posted by fireguy View Post
Cabela's ragg wool/thinsulate gloves $15.00
The heat problem comes mostly from the inside.

Last edited by Yoyizit; 10-13-2009 at 12:15 PM.
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Old 10-13-2009, 07:49 PM   #33
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Would like to use Nichrome wire to make a small heater


I don't know, I had 7 360ohm resistors in parallel, which is about 51.5 ohms. These are on a pcb. Judging by their size and spacing and the heat I felt at my max of 12v, Ive established they are a good estimate for each 2 inch segment of resistors 8 feet around the wheel. I will most likely have a little something on the wheel to insulate from the resistors(just in-case its too hot for the wheel), some sort of heat conductive material sandwiching the resistors(perhaps aluminum tape, and yea the resistors have to be electrically insulated..), and lastly a cover over that made of acrylic or leather or what-ever will make it look like a real steering wheel(but of course not too thick). I don't thing the resistors being a weird shape over the wheel will be a problem, these 1/4inch ones are not huge, the cover over them will smooth it out, and I can deal with a little bit of bumpiness.

Considering all that, plus warm up time; I may want to go ~slightly~ higher in power at 12v. And an adjustable supply is really needed for an adequate warm-up time(ability to turn down power once at a comfortable temperature).

So say I want an 8foot stretch coiled around the wheel at 12v, at 51.5ohms/2 inches, which is 2.8w per 2 inches, and to get my total watts I can just multiply that by 6 inch=16.8w per foot, then at 8' 16.8x8=134watts, correct?

The thing is I don't know how to go from there to figure out how to wire up a proper amount of resistors to get my desired watts per foot.
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Old 10-13-2009, 08:15 PM   #34
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Would like to use Nichrome wire to make a small heater


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Originally Posted by GerGa View Post
I don't know, I had 7 360ohm resistors in parallel, which is about 51.5 ohms. These are on a pcb. Judging by their size and spacing and the heat I felt at my max of 12v, Ive established they are a good estimate for each 2 inch segment of resistors 8 feet around the wheel.

8foot stretch coiled around the wheel at 12v, at 51.5ohms/2 inches, which is 2.8w per 2 inches, and to get my total watts I can just multiply that by 6 inch=16.8w per foot, then at 8' 16.8x8=134watts, correct?

The thing is I don't know how to go from there to figure out how to wire up a proper amount of resistors to get my desired watts per foot.
2.8w each 2" of length for proper heating
7 ea. resistors in parallel for each 2" of length for proper heat distribution
8ft x 12"/ft = 96" of length
96/2 = 48 resistor clusters of 7 resistors each

each cluster needs 12/48 v across it to generate 2.8w.
R = V^2/P = [(12/48)^2]/2.8 = 0.022 ohms
7 ea. resistors x 0.022 ohms per resistor cluster = 0.154 ohms, max, for each resistor, for 2.8w min. for each cluster.

Each "resistor" could be 0.154 ohm x (10'/ohm) = 1.54' of #30 wire wrap wire or some shorter length of Nichrome wire larger than 31 gauge.

check
48 x 2.8w = 134w.
and
R = 144/134 = 1.07 ohms total 8' strip resistance

BTW, this network is equivalent to 8' of 7 ea. paralleled strands of resistance material, with each strand having a resistance of 48 x 0.154 ohms = 7.4 ohms.

check
7.4/7 = 1.06 ohms.

12v/1 ohm = 12A, so we are out of the LM317 class unless we use some tricks.

#40 AWG copper would be approx. 1' per ohm, so it is just about the right resistance per foot for this application. But, it would be delicate.
The strands in lamp cord are probably larger than #40 so then you would need less than 7 parallel strands, but your heat distribution may suffer.

This problem is easy in theory but finding the parts for this and keeping the bookkeeping straight is proving difficult.
If you program a spreadsheet with all of this, the bookkeeping problem disappears and you can plug in many different lengths and resistances until you find some optimum between price, availability, durability, etc..

And I don't think you can solder Nichrome.

Last edited by Yoyizit; 10-13-2009 at 08:50 PM.
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Old 10-13-2009, 10:29 PM   #35
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Would like to use Nichrome wire to make a small heater


Hmm that estimate was a little aggressive but I didnt realize it would be 12A. I'd like to limit it to less than 10A considering the lighters in cars are usually 10A.
I can go less on the power,2.3w/ft is too much.
I am thinking the nichrome at around 30 gauge with a 10foot or longer length sounds good. I will have to look at it more for trying to get the amps down and consider possibly not heating the whole wheel.
Thanks
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Old 10-14-2009, 03:40 PM   #36
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Would like to use Nichrome wire to make a small heater


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Originally Posted by GerGa View Post
Hmm that estimate was a little aggressive but I didnt realize it would be 12A. I'd like to limit it to less than 10A considering the lighters in cars are usually 10A.
I can go less on the power,2.3w/ft is too much.
I am thinking the nichrome at around 30 gauge with a 10foot or longer length sounds good. I will have to look at it more for trying to get the amps down and consider possibly not heating the whole wheel.
Thanks
12v/[<10A] = >1.2 ohms.

As you narrow down the design you may bump into other tradeoffs. There are plenty of surprises left in this project.

If you really need 12A or more you can use an aux. battery and a current-sharing arrangement.

I'd wind the wheel with something close to what you want and actually try it. The power, etc., can be worked out later. There may also be a seasonal variation in how much power is needed.


Last edited by Yoyizit; 10-14-2009 at 03:44 PM.
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