# Modifying a soldering iron for lower power

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?

• Surely it'd be cheaper and easier to just go out and buy a lower-powered soldering iron - I mean, they're not exactly expensive... – Majenko Nov 12 '11 at 17:50
• You're right, but I might actually learn something by following through with this. The 60W iron is basically a giant 240Ohm resistor. Suppose I connect a 40W lightbulb which under normal circumstances functions like a 360Ohm resistor. So I end up with 600\Omega on 120V which is 0.2A. Voltage drop across iron will be 48V. That's 9.6W. Okay maybe i want a 100W bulb. – Steven Lu Nov 12 '11 at 17:58
• If you can live with halving the power you could put a diode in series. – Wouter van Ooijen Nov 12 '11 at 19:56

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 :-).

• I was puzzling over why a diode in series would halve the power, then I realized that this should have been obvious to me: it is a half wave rectifier. Also, no wasted energy. This is much more elegant than the light bulb idea I wanted to go with. Now if I can find some female mains sockets I'll definitely throw this together so I can tame this soldering iron so I can get some more use out of it before it burns itself to a crisp. – Steven Lu Nov 13 '11 at 7:03
• There would be a small amount of loss due to the diode's forward voltage drop. Make sure you use a diode that is big enough to handle the current of your soldering iron. – Robert Stiffler Oct 25 '15 at 21:11

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.

• +1 - I was using an inexpensive single temperature iron for years, thinking 'what difference could there possibly be? its a stick that gets hot' . After reading enough things that kept saying ' get a real, temperature controlled iron' I did. And you know what? HOLY FREAKING COW What in the world was I doing all those years?!? Bottom line. Choose a different project to learn resistance on and buy a real iron. – rfusca Nov 12 '11 at 20:01
• I will definitely invest in one of these eventually. I know that it will be very helpful compared to what I do have but I don't do a lot of soldering yet. Should I save up for a Weller digital soldering station (130\$) or would this one do the trick? amazon.com/Aoyue-937-Digital-Soldering-Station/dp/B000I30QBW/… – Steven Lu Nov 13 '11 at 6:55
• I accepted the other answer because it was the most helpful for improving my existing soldering iron (and answering my questions), but this answer is clearly the correct answer for others who are in my situation. I am just cheap and like to tinker. – Steven Lu Nov 13 '11 at 7:07
• @Steven - any temperature controlled iron will do, so the Aoyue should be fine. Weller is never cheap. – stevenvh Nov 13 '11 at 7:41
• @StevenLu - I have that very Aoyue, its nice. – rfusca Nov 15 '11 at 0:15

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.

• What's the diode bridge and R2, R3 for? I've never seen those in a dimmer. – Federico Russo Nov 12 '11 at 19:13
• @FedericoRusso - Good point, I was looking for a quick example and didn't notice that. I'm not sure what benefits it might have off the top of my head, here is the page I got it from (mentions using it to control soldering iron but nothing about the diodes) Anyway I changed it for a more "standard" circuit. – Oli Glaser Nov 12 '11 at 19:28
• @Oli Glaser Of course if it.... What? – AndrejaKo Nov 12 '11 at 23:07
• @AndrejaKo - I added the rest, thanks. It seems I lost quite a bit of this answer when I browsed to another page and came back. It usually saves it but in this case it looks to have got a bit mangled. – Oli Glaser Nov 13 '11 at 2:09

Simple to complex power reduction:

1. Connect two (similar power) soldering irons in series.

2. Use an AC speed controller (motor speed controller).

3. 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.

• Cool ideas. The first one is probably real hit and miss though. Having a thermometer attached is nice. Having it in closed loop feedback control is even nicer. – Steven Lu Sep 29 '14 at 13:18