electronics answer (long)
Pascal Oudet
>well, people not interesting in electronic stuff might skip this post (which would be better on rec.sci.electronics).
warning: don't use your working PC to do this, find an old power supply (there is always the risk of destroying it)
1- first, I recommend to use an old power supply, who has a ON switch on the primary. if it doesn't, there's a signal called "remote on" or "ps_on" on the connector. in ATX PSU (2*10 connector), it's pin 14 (no std color. pins 13,15-17 are GND, black). when PSU is plugged, but not ON, 2 signals have a voltage >0.
one is 5v standby, pin 9. the other is ps_on. short this pin to ground to power on. if any, the PSU fan must be spinning
(you can look at the pinout on ATX spec at
http://www.formfactors.org/developer%5Cspecs%5Catx2_1.pdf).
The same principle is valid for non standard PSUs, just find the 2 wires having non 0 voltages. (ps_on is usually lower than 5V sb)
5V outputs have red wires, 3.3V are orange. 12V is yellow.
you can connect your tip to 5V or 3.3V. if it doesn't get too hot, that's it. otherwise, you need to find a mechanism to adjust the power.
2- It can be difficult to locate inside the PSU the appropriate components to modify to adjust the voltage (playing on the regulation feedback loop). It can be dangerous too, as there is a high voltage too (up to 400V dc). So don't do this. moreover, the adjustable voltage will probably limited to 2.5V, which could be too high in some cases.
So I connected to the 5v output a PWM controler, based on a TLC555 (fixed frequency, variable duty cycle). the web is full of this kind of circuit. I attach one schematic as an example. I recommend to make it work at a pretty low frequency (~100Hz), to minimize commutation losses. you can play with this by adjusting the 22uF cap.
this circuit is powered by the 12V supply (yellow), and the output is connected to the gate of a N-mos with a Rds-on as low as possible.
Source is tied to ground, and drain to one pin of the burning tip. the other tip pin is tied to +5V (or +3V, choose the one which has the higher current rating). Mount the MOS (preferably TO220 package or bigger) on a good heatsink, as it will get hot.
PSU usually don't like to have this kind of switching load, the regulation gets crazy. so you need to add a permanent load on the 5V line, say 100 ohms.
You can even power a fan to blow away the fumes with the 12V line.
This circuit has probably not all the bells and whistles of a commercial one, but you can start woodburning for less than 5$.
one more trick: if you want to make your own tips, buy some Nichrome wire. this is a resistive wire used in heating devices.
Enjoy,
Pascal
