Resistance wire question (nichrome)

[steveh]

SOLVED: Resistance wire question (nichrome)

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 this 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

 

[BobK]

Use a larger wire (non-nichrome) for all but the point.

Bob

 

[steveh]

Hi Bob, thanks for the help. Yes, that would work, however the connection would be problematic because solder would melt, and a mechanical connection or crimp tends to loosen up with time due to expansion from the heat. I was thinking in terms of increasing or decreasing the resistance of a single length of nichrome but not sure how this can be done.

 

[BobK]

You could twist the nichrome wire with some copper wire ending the copper just before the point.

Bob

 

[duke37]

You could thin down the part that is to be bent. Get out your big hammer or gind it.

You could use copper leads but Nichome is a pig to solder. I used to use silver solder or braze to coat the wire which could then be soldered.

 

[steveh]

Hi Duke - filing down the part that is to be bent (to increase resistance) works, but it weakens the wire too much and is not very precise... I need to make a lot of these. Is there a simple way to *decrease* the resistance of the section of nichrome that is NOT bent? Like some sort of DIY conductive coating?

 

[BobK]

You could try plating silver onto them.

Bob

 

[duke37]

You could coat the wire with silver solder but you will need a oxy/gas torch to get the temperature.

You could spot weld to thicker wires using capacitor discharge.

You could buy some bits for a directly heated solder gun.

 

[steveh]

Problem solved: Using a slightly larger gauge of nichrome wire and filing down the tip to obtain the desired resistance in the tip region consistantly works pretty well... thanks for the help guys.