Heating Rod - Need Help with Design

Hey everyone,

I'd like to design something that looks like a soldering iron where you are able to plug in the unit and get the tip of a metal rod to heat up to around 400 degrees.

My main objectives are:

1) Easily portable and non-cumbersom
2) Get the tip of whatever I'm using (the tip of a soldering iron for example) to 400 degrees, would be nice to have a temperature readout or a known position on the pot where 400 can be obtained.

Not really sure where to start, maybe take a part an existing soldering iron? Any help appreciated, thanks.

Google "digital soldering station".

One of the problems you'll be facing is getting it hot enough while keeping it insulated, yet providing good thermal conductivity.

You could use a length of nichrome wire wrapped around an insulated tube, and power it using a low-voltage transformer that's isolated from the mains. You could control the temperature using a bimetal switch, or perhaps a thermocouple driving a comparator circuit and SCR to turn the power to the tip on and off, controlling the temperature.

Ideally I'd like to have a pen sized device and have the tip of the pen heat up to 400 degrees. How much current do you think that would require? If I ran it off batteries, would that even be feasible? Last thing, how long do you think it would take to heat up to 400 if it were pen sized off watch batteries probably.

A lot depends on the use for this device. You could probably use watch batteries to heat a small flashlight bulb filament to 400 deg, but that's about all. Very fine nichrome wire, around 36 ga might heat up, too, but only for a few minutes at best. In both cases, the cool wire will not be very resistive. Your batteries will have to supply at least tens of milliamps of current to heat the wire/filament, and be able to have enough voltage to keep sufficient current through the element to maintain the temperature.

If you don't mind an external battery pack and larger batteries, it might be possible - it all depends on the application.

You might be thinking of the "Cold Heat" soldering tool.
http://www.thinkgeek.com/interests/giftsunder20/69d3/

I bought something similar many years ago, but I wasn't very satisfied with it. I have not used this particular tool, so I have no idea how well it does or doesn't work.

I once had a doctor use a battery-powered cauterizer to "melt" through my thumbnail in an attempt to relieve pressure from blood built up behind it. It got red hot, but the tip was not a rod, and I doubt the battery lasted very long. They are one-time use, throwaway devices, so battery life isn't an issue. Cost either, for that matter, since insurance pays for most of them.
I don't know how you would control temperature, unless you measured resistance.
Gas-powered soldering irons are compact, but again, you somehow have to measure and control temperature.

http://www.nibsburner.com/systems_main.htm

See also: http://www.circuitcellar.com/library/print/0606/Malik191/Malik-191.pdf