I have a 60W soldering iron, which is too much heat for typical electronic soldering purposes. I find myself constantly switching it on and off using a powerstrip to control its temperature so it does not get too hot. I was wondering if there exists some easy way to reduce its power output. Is there some type of circuit I can build which could limit its current draw to only 100mA (so it would be a 12W iron) for example?
Maybe I could wire a 40W lightbulb in series?
There are numerous ways of reducing power input to your iron. As others have said and you yourself have noted, this is more likely to be of value as a learning exercise than a major improvement to the iron.
A temperature controlled iron is a really really really good idea for quality soldering.
A "closed loop" temperature controlled iron regulates the power to achieve a desired temperature by varying power based on what a sensor measures.
A power controlled iron which is "open loop" (no temperature sensor) will vary widely in temperature depending on local environment -- eg air flow & drafs, contact wit metal stand etc.
However :-) :
Series Diode:
An extremely easy way to reduce power input is to switch a diode in series with the iron circuit.
Short the diode when full power is required.
Remember that mains kills.
Power input will be about 50% with diode in circuit.
Diode voltage rating must be mains suitable (say 2 x Vac) and current raing must at least equal iron max current
TRIAC CONTROLLER:
A cheapish and easy method is to use a TRIAC type light dimmer.
Wattage rating should be at least equal to iron max wattage unlikely to be a problem.
Series Light Bulb(s)
If wishing to experiment with series light bulbs place a lamp socket in series with the line.
Building this into an extension cord allows you to avoid hacking the iron.
Putting two sockets in parallel allows some interesting experiments.
Sockets with bulb out and iron plugged in will be as lethal as a normal lamp socket is. A shorting switch across the socket gives full power.
The high resistance-nonlinearity of tungsten filaments with temperature will be learned about this way :-).
Ditch it and buy a temperature-controlled iron. Even at lower power your iron will keep heating if you leave it unused for some time. This will go on until there's an equilibrium between generated heat, and heat drained to the environment. Problem is that at low temperatures little heat is drained, and the temperature will settle at a too high temperature. Most likely making your first solderings bad. While you're soldering sufficient heat is drained to the PCB and your components, and the temperature will drop, so that only after a few seconds you can solder properly.
A temperature-controlled soldering iron will reduce the power when the iron isn't used for a while, thus preventing thermal runaway.
In theory you can make your iron temperature-controlled, by placing a thermocouple near the tip, and switch the heating element by a triac, controlled by a control loop. I would just buy one, though.
I agree with Steven on the temperature control. If this is what you ultimately want then I would definitely buy one. You could make your own but it would require a resonable amount of time/effort/knowledge for little savings. You can pick up used Metcal stations on eBay for pretty cheap, the performance of which is a lot better than anything you are likely to throw together quickly. Of course if it is for the challenge/learning purposes then go for it.
To answer the power control question:
For lower power, the light bulb would work, but you wouldn't be able to adjust it (apart from swapping in different bulbs)
A simple Triac based light dimming type circuit would also work, and be adjustable (soldering iron would be the load) You could probably just use an off the shelf dimmer if you did't want to put one together:
This concept could be improved by adding a temperature sensor for feedback control (closed loop). You sense the error with a comparator or ADC combined with microcontroller and adjust the power accordingly.
For decent results probably a PID (proportional integral derivative) algorithm would be a good idea. A simple e.g. PIC16 can easily handle this at the speed necessary for this application. I used a similar setup to control a heated etch tank I built with nichrome wire element and PIC16F690/DS18B20 sensor.
Do be careful with any (especially non isolated) mains circuits such as the above.
Simple to complex power reduction:
Connect two (similar power) soldering irons in series.
Use an AC speed controller (motor speed controller).
Use a Variable Auto-transformer (Variac Variable Transformer, 0-130 Volt Output).
For (2) and for (3) test first using a 5W/15W 120v AC light bulb.
