I'm designing a device which will be used to test the reaction of various materials when a red hot heat source is brought to within about 1/16" of the surface of the material.

I'm using a 1" length of 29 gauge nichrome wire bent into a very tight "V" shape... the pointy end of which needs to be glowing red hot, but NOT the upper half of the nichrome legs. It is connected to a power source which allows me to adjust the voltage, thereby adjusting the temperature.

The problem is that just the pointy end should be red hot, and the rest of the nichrome needs to be cooler. I have seen similar devices where only the point is red hot, but in my case the entire length of nichrome glows fairly evenly. Turning down the voltage does not help, it just lowers the overall temperature evenly.


+ - nichrome resistance wire connected to variable DC
| |
| | <-- this part should be cooler
| |
| | <-- this part should glow red-hot
V
________ surface to be tested for reaction to heat


So here is my question:
Is there a technique to adjust the resistance (or somehow treat) the nichrome wire so only the point glows red and the upper 1/2" remains cooler? I know it can be done because I have a sample of a component apparently made of resistive wire which does just that. My part has the same physical shape and appearance, but mine glows evenly. Perhaps another material like stainless steel would give the desired results? Any ideas?
Thanks,
Steve

 

Making it thinner at the point should help. You could bend it and then file part of it away.

 

Wrap the nichrome wire in aluminum foil very tight, or use copper/litz wire to wrap the leading portion of the heating wire.

 

You can also use 14 gauge copper wire as a support for the long straight sections. As said above, wrap the nichrom wire around the sold copper wire. The sold copper will be a good conductor to keep the straight sections from getting warm but it will also be a good heat sink for the legs of your "v". You should be able to adjust the voltage to get just the tip of the V red hot.

 

Follow the method used by Weller soldering guns.

Two conductive tubes going close to the surface, they don't need to be as large as the soldering gun tubes are, and then have a screw or clamp to mechanically and electrically mount only the nichrome V element ends in a repeatable and consistent manner.

This would also make it easier to change the tip if it breaks, etc.

 

Making it smaller at the point and reducing the drive current would be the simplest method as Markd77 first pointed out

 

A different material might be better, if it had more thermal conductivity. Then the "leg" area would lose heat to the incoming conductors, while the tip would be the hottest part. You'd probably have to play with wire thickness versus resistivity to get this to work right. Or could you arrange an air jet to carry heat away where you don't want the wire to get too hot, but leave a small part sheltered and thus hotter?

 

As an alternate you might find usefull an RC engine glow plug, running at 1-1.5V, 3 to 4 A, like the one here . about $10

 

Hi Steve,

Cut two pieces of nichrome wire to the desired length. Then with some power applied bring the tips together so they just touch. There will be a small arc as they fuse together forming a poor quality weld. Bring the power up until the tip glows almost white hot and then slowly back off to red hot. This completes weld and anneals the join.

This creates a high resistance weld that overheats and glows just at the weld point. The weld can be broken easily, so handle with care. Fortunately, it doesn't take much to make another one.

The holder for the wires should a good one that holds the wires precisely and with a small contact pressure that can be easily controlled. Adjust the temperature (colour of the glow) buy adjusting the voltage.

Good Luck,
Ifixit