Soldering iron came in a $14 USD kit off eBay. Lots of neat stuff, but the iron became so hot as to badly damage a DIY Kit. After a 2nd chance w/ the same results, it was hacked, Heating element of 216 Ohms directly connected to two "hot" wires of VAC ! Has to be runaway current. After much research but before any purchases, it was decided to add a CURRENT LIMITING Xc Capacitor of X2 class to one of the wires, 6uF should do the trick to maintain 5oo mA and 120 volts @ 60Hz.

Will this work?

Did lots and lots of research, and is better than impedance of shaving off 1 to 10 watts to heat energy. The capacitive reactance becomes the current limithing "resistor", but cannot find any schematic or datasheet to show how it is properly done.

Please advise.

  • 11
    \$\begingroup\$ The solution is buying a proper soldering iron, not hacking some piece of garbage from eBay. \$\endgroup\$
    – Matt Young
    Dec 26, 2015 at 21:01
  • 9
    \$\begingroup\$ Soldering iron for $14, come on, throw it in the bin before you get some real health danger. \$\endgroup\$ Dec 26, 2015 at 21:22
  • 4
    \$\begingroup\$ Tools like that just aren't safe, they're cheap for a reason. I've recommended some options down below. \$\endgroup\$ Dec 26, 2015 at 21:34
  • 2
    \$\begingroup\$ Life is too short for crappy soldering irons. Ditch it and get something temperature controlled, even if it's knock-off. Or skip the knock offs and go for a temperature controlled iron you can expect to last for decades, even if you buy it used. If throwing it away makes you unhappy, use it for wood-burning or something like that. \$\endgroup\$
    – Ecnerwal
    Dec 26, 2015 at 23:34
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    \$\begingroup\$ You can buy a 40W soldering iron for less than $1.20 including shipping from China (I know they're about 50 cents wholesale) but I'd suggest getting a brand name temperature controlled iron such as Hakko (Japan) or Atten (China) or Weller (USA or Germany). Might cost you $100-$200 but worth it. This kind of iron ramps up the power when you are soldering and cuts it back when idle. Some will even shut down when left idle for a period of time. When/if you can afford it, get a Metcal iron. \$\endgroup\$ Dec 27, 2015 at 1:58

4 Answers 4


Tools like that just aren't safe. They're that cheap for a reason. I recommend ditching that iron and spending a little more on something that's built a little more properly. Hakko and Weller have some simple entry level irons that cost anywhere from 25-50 dollars.

They're also a lot safer.

Hakko has a nice kit that is a full soldering station with a bunch of tips if you feel like expanding your tools a bit, but that starts around 100 dollars and might be more than you want to spend.

  • \$\begingroup\$ "Hakko936" is available on-line at about $35, I bought one and it seems real, or at-least compatible with real Hakko parts. \$\endgroup\$ Dec 26, 2015 at 22:36

the problem is almost certainly not thermal runaway, nichrome wire (which is what almost all heating elements are made from) has a very small positive termperature coefficient of resistance, the resistance increases slightly, but not enough to bother with.

216 ohms on 115V is about 60W, 60w is going to be too hot for use on electronics without some sort of thermostat to regulate it. if you reduce the power to 15W it'll be usable, but still not as good as a thermostat controlled iron, and will be slow to recover whilst being used.

  • \$\begingroup\$ I have thermisters w/ associated resistors. So, does anybody have a circuit schematic of a temperature controlled soldering iron w/ readout? (Readout means display.) Was thinking of going in that direction, anyway. \$\endgroup\$ Dec 27, 2015 at 4:54
  • \$\begingroup\$ you can buy temperature controller on-line for about 15 bucks, \$\endgroup\$ Dec 27, 2015 at 4:56

If money is a problem and you can't afford a better iron then a lighting dimmer switch will reduce power to this iron (or a better iron).


simulate this circuit – Schematic created using CircuitLab

Basic dimmer circuit.

Note that the 216Ω resistance value will increase as the iron heats up.

Your dimmer will not be calibrated and will have to be adjusted by trial and error.

You're working with mains voltage. Be careful and check the earth connection has low resistance between the element case and the mains plug earth pin.

  • \$\begingroup\$ Nichrome does not change resistance much between room temperature and red-hot, \$\endgroup\$ Dec 27, 2015 at 1:02
  • \$\begingroup\$ Yes, compared with many of the common pure metals. e.g. Copper's coefficient of resistance is about 0.004 /K whereas nichrome's is 0.000,4 /K. It is still positive, however, unlike carbon, germanium and silicon which have negative coefficients. Table of resistivity. \$\endgroup\$
    – Transistor
    Dec 27, 2015 at 9:26
  • \$\begingroup\$ Okey-dokey! Bought several DIY Light-Dimmer Module Circuit kits @ 200v & 500w or 2.5 A. After I put one together, I can possibly modify the next one more to my liking and far less powerful! It is built around the BT136 Triad. Great job guys! \$\endgroup\$ Dec 29, 2015 at 10:28

Assuming an AC voltage of 120 Vrms, the 216 Ohm load maintains a current of about 500 mA. Running the voltage up to 170 Vrms with the same load, the current runs up to over 750 mA. Thus the Power consumption ranges from 66w upwards to an astounding 134w - hot enough to take out circuit tracers in an instant! Flux vaporizes instantly leaving behind a porous mound of ugly looking Sn/Pb solder that lands halfway up the component stem!

Is there any protection from runaway Voltage in the Mains?

  • \$\begingroup\$ Why have you posted a question as an answer to your own question? And what is 'runaway voltage in the mains'? \$\endgroup\$
    – Transistor
    Dec 27, 2015 at 11:15
  • \$\begingroup\$ Had to get the thought in there some kind of way and run away voltage is ... I dunno. \$\endgroup\$ Dec 29, 2015 at 10:16

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