DIY Cordless Cold Heat Soldering Iron




Introduction: DIY Cordless Cold Heat Soldering Iron

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Traditional soldering irons use a heated tip to melt solder in order to make electrical joints, while this works well when you have a mains outlet around, its not that practical when you're out in the field. Anyone who's done anything RC will know the frustration involved in a joint coming loose or a wire breaking off in the middle of a day out. You can buy a couple of different models of cordless soldering irons online but the main drawbacks are either lack of power or short battery life, the cold heat soldering iron addresses both of these problems. If anything it is perhaps a bit too hot and since it is only on instantaneously, you can make loads of joints (100+) before the battery runs flat.

The cold heat soldering iron works by essentially "short circuiting" the battery across the solder joint, the solder acts as a resistor, heats up really quickly and melts to form the joint as in the normal soldering process. The concept is similar to a traditional soldering iron in the way the tip is heated although in this case, the heated part is the actual solder and not a soldering tip. What's even better about this project is that it can be built using things lying around at home, there really is nothing complicated about it.

If you enjoy this Instructable, please vote for it in the Build A Tool contest!

The general concept for the soldering iron tip is based upon - DIY Cold Heat soldering iron by photozz.

Step 1: What You Need to Build a Cold Heat Soldering Iron

Here's a basic guide for what you'll need to build your own cold heat soldering iron, feel free to improvise and make use of components, wires and plugs you have lying around the house, nothing fancy is required for this Instructable.

  • A Scrap Piece Of Copper, Aluminium Or Brass Tubing (8cm/3")
  • A Thin Piece Of Scrap Glass (Or Mica, Plexiglass, Acrylic etc)
  • A Short Length Of Ripcord Or Any 2 Strand Wire
  • Some Pencil Lead (Graphite) Refills
  • Heat Shrink Tubing or Insulation Tape
  • LiPo Battery Plug To Match Your Battery
  • A 1000mAh 2 Cell Lipo Battery (Or Similar Capacity 2 Cell)
  • A Piece of Wood, Pen Or Dowel For The Handle - Alternately, There Are Plans In Step 5 To 3D Print A Case For The Tip & Battery

Step 2: Prepare the Soldering Iron Tip Components

The tip is the most important part of your soldering iron so it pays off to take some extra time to get it right.

Start off by cutting your scrap piece of tubing into two equal sections of about 4cm or 1 1/2" long, this doesn't need to be all that accurate but it needs to be long enough to adequately support the pencil leads and absorb some of the excess heat created when soldering.

Now strip the plastic insulation off of the ends of your ripcord to expose the wire, the exposed wire should extend about halfway into the tubes with the edge of the insulation up against the tube ends.

Push each wire into the tube and then use a hammer or a pliers to squash the tube flat over the wire. Flatten the whole length of both tubes, these are going to be your contacts onto the pencil leads.

You need to score the tubing so that the pencil leads are held firmly and in the centre of the tube contacts. Use a pair of side cutters, a craft knife or a Dremel to cut or score a line down the centre of each contact as shown in the image, the line doesn't need to go the full way down the length of the contact.

Now cut a strip of glass from your sheet of glass, the piece needs to be the same width and length as the flat tubes. In this case, the flattened tubes were about 1cm (2/5") wide. Use a glass cutter to score the glass and then break off a strip of the correct width before breaking it to the correct length.

The glass strip should be a similar size to the flattened tube contacts when you are done.

Step 3: Assemble the Soldering Iron Tip

Assemble the tip by putting the glass strip between the two contacts with the score marks facing inwards towards the glass. Insert the pencil leads between the contacts and the glass, lining them up with the score marks, and then wrap the tip up with insulation tape or put a piece of heat shrink tubing around it.

Once you are happy with the position of all of the components and the alignment of the pencil lead, heat up the heat shrink tubing with a lighter, heat gun or blow torch briefly to shrink it together and lock the pieces in place.

The pencil leads should be close together but not touching each other when you are done. You will now need to be very careful when handling your soldering iron tip as the slightest bump will break the pencil leads, they are really fragile.

On the other end of the ripcord lead, you need to solar on your battery plug. The polarity (positive + and negative -) of the battery plug don't matter for this tip so you can solder either lead onto either of the plug terminals.

The tip and electrical connections are now completed.

Step 4: Testing the Cold Heat Soldering Iron Tip

Once you have completed your tip, you should probably test it and check that the connections are made correctly before you go through the effort of mounting it onto a handle or inside a case.

To test the tip, make sure that the pencil leads are not touching each other or anything conductive, they should preferably be raised a little off the counter or work surface. Now plug the tip's lead into your charged LiPo battery.

With the battery plugged in, take a piece of thin strand solder and touch it against the two pencil leads simultaneously. It should smoke a little bit and then the solder will begin to melt. A little spark or the tips of the pencil leads turning bright orange is quite normal, just don't allow the whole pencil lead to turn orange.

Shown in the attached video is my test of the tip, I plugged the tip into a power monitor to check the voltage and maximum current when soldering to ensure that the current was not exceeding the batteries' limit.

The battery voltage was 8.33V which is normal for a fully charged 2 cell Lipo and the maximum current drawn to melt the solder was 5.30A which is well below the batteries 20A limit. With a 1000mAh battery, you should get about 11 minutes of soldering before the battery needs charging. That's 11 minutes of actual contact between the two pencil leads, which is a lot of soldering time, around 130 joints if each takes 5 seconds or so.

Step 5: Mounting the Soldering Iron Tip

Once you are happy with how your tip works, you can mount it onto a handle or rod so that it's easier to use. A round wooden dowel works well as a handle and is relatively inexpensive. Tape the tip to one end and the battery to the other end, you'll now have a neat pencil like portable soldering iron.

I decided to take it one step further and 3D print a case to house the tip and the battery. The case is printed in two parts, one is the actual case in which the tip is mounted and the battery fits into and the other is the end cap to close off the battery compartment.

You can download the 3D print model files here.

The case was designed for a 0.4mm nozzle printer but it can be printed with any printer with a suitable build volume. The material used was HIPS with a nozzle temperature of 225°C (437°F) and a bed temperature of 95°C (203°F).

Attached is a timelapse video of the case being printed.

Once you have printed both pieces of the soldering iron case, the tip needs to be installed. The tip fits tightly into the front of the case, you may need to remove the pencil leads to push it into the case and then replace them afterwards.

Step 6: Using the Soldering Iron

Although the battery is effectively not in circuit when the soldering iron is not in use, you should always keep the battery unplugged when you are not using it to prevent something short circuiting the tip. A stray tool or screw in your toolbox may fall across the pencil leads and could cause a fire.

You can use any two cell battery you already have for your RC hobbies or whatever is available from your local RC store, it doesn't need to be a high spec LiPo battery. A two cell battery has enough voltage and 1000mAh is enough capacity for a fair bit of soldering before it needs to be charged. To get more soldering time out of your soldering iron, use a battery with a larger capacity - 1500mAh or 2000mAh. We don't recommend increasing the voltage (number of cells) above two, this design works quite well between 5V and 10V so a 2 cell LiPo is perfect.

To solder a joint, simply hold the soldering iron in one hand as you do with a regular soldering iron and then solder in another hand, gently touch the tip onto the joint and bridge the two pencil leads with the solder wire. The solder will melt and the joint will be formed. Do not press down on the pencil leads as they are extremely fragile and will break. It may be tempting to push down harder if the tip doesn't heat up at first, rather move the solder wire around to a better angle, see the attached video.

Step 7: Some General Usage Tips

  • Don't attempt to solder any sensitive electronic circuitry with this soldering iron. There is a voltage difference across the tip which may damage sensitive components.
  • Remember not to over discharge your LiPo batteries, use a battery monitor to alert you when the battery is running low.
  • Keep the tip away from anything conductive when the battery is plugged in and always unplug the battery during transport.
  • Work quickly, once the joint is hot, push the solder you need into the joint and stop quickly. The tips of the pencil leads may start glowing but don't let the whole lead heat up, it will start to damage the heat shrink or insulation tape you used to hold the tip together.
  • Don't press down on the tip, the pencil leads are really fragile and will break if any pressure is applied to them. Rather move the solder around until it touches both ends of the pencil lead, this will cause it to melt.
  • Try making a cap to protect the pencil leads when you're transporting the soldering iron, this way you can leave it loose in your toolbox.
  • You could use an RC brushed ESC to vary the voltage and current supplied to the tip for smaller or larger jobs, a servo tested could be used to give the ESC the reference signal.

Please remember to vote for this Instructable in the Build A Tool contest if you enjoyed it.

If you enjoyed making this tool, you may be interested in Building Your Own Solar Panel From Scratch or have a look at our list of interesting Arduino projects to tackle.

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    36 Discussions

    Would you happen to know the temperature generated at the tips of your soldering iron? Please advise. I think this is modeled on resistive tweezer technique--without the tweezer action.

    1 reply

    Hi Mkrobert,

    That's actually quite a difficult question and it depends very much on what you soldering and its thermal characteristics. The two rods essentially heat up the item being soldered rather than get hot themselves. The level to which the item heats up would depend on its resistance, its thermal conductivity and its thermal inertia. Since you are constantly supplying energy to the solder joint, it theoretically would keep getting hotter and hotter until you remove the tip. The thermal conductivity of the metal in the components and joints however would draw heat away from the joint and eventually it would reach some sort of equilibrium.

    ignore the nattering nabobs of negativity this is a great idea RadioShack has something similar to this. And I believe that use alkaline batteries I'm definitely going to build this I'm always looking for better ways to solder

    1 reply

    Hi KerryS39,

    Thanks! Yeah the RadioShack version is where the idea came from, it's just quite pricey for what it is. Good luck with the build!


    1 year ago

    IDK how you solder, but most normal people solder that way, that the solder tip is only contacting one pad at the time. Which means that you can solder electrolytic caps, chips or anything. Since the current is flowing through the pad there is no way that you could damage component in the process.


    1 year ago

    Something that no one has mentioned is that there is a potential (no pun intended) of damaging attached circuitry as the solder tip is going to have 6 or so volts across it. This could damage a lot of components on a circuit board if used there. It is an interesting idea for soldering wires together. I can see that there would be a bit of a learning curve to using it.

    I also think that it might be interesting to see how it would work on coaxial connectors. These require a lot of heat to solder, but as the heat isn't being transferred from a soldering iron tip to metal, it may work quite well. One problem I have always had with soldering outdoors is that wind will draw a lot of heat away from the tip, sometimes making a joint impossible. This method should be able to overcome some of this problem.

    I am going to give this a shot for cables, and other items, but I doubt that I would risk using it on a circuit board.

    1 reply

    Well, actually the author stated in the fist part of step 7 that

    "Don't attempt to solder any sensitive electronic circuitry with this
    soldering iron. There is a voltage difference across the tip which may
    damage sensitive components."

    so you're probably right to be cautious.

    However I wonder how sensitive the components need to be in order for them to be damaged? It should be a voltage divider between the other side of the tip and the components other side (if it's at all connected to the other side), which should make most of the current to favour the path through the tip rather than the component. (I apologize if I'm unclear; English isn't my first language.) With a maximum starting point of just over 5V and 5A there shouldn't be much that goes through the component. And since it's battery powered it is virtually ripple free DC that is rather benign to most components. Any ideas on that?

    I suppose that the only way to really know is to test on a couple of components that you can afford to loose, and if I build this or anything similar I'll make sure to do that. I've got a substantial pile of electronic scrap that my wife wouldn't mind if I reduced somewhat...

    short circuiting a Lithium battery in this way is a VERY BAD idea do a google search for short circuiting lithium polymer batteries. There is serious potential for this to literally explode in your hand.

    5 replies

    He is into radio control, meaning he would be pretty experience with Lipo and safe use. Additionally, he mentions the current that this drew in use, about 5a I think, which is well within the specs of most Lipo cells designed for radio control. 1a cell discharging at 5a means 1/5 of an hour it would last 12 min, pretty much the time mentioned in the article (that would be 5c discharge, cell was rated for 20c, or 20a for a 1 amp battery). I'm sure we have all seen small radio control helicopters or quads that discharge their whole capacity in half that time, meaning they are discharging at twice the rate of this tool, or closer to 10c....and perfectly safe. I don't see any issue with this tool, it's a lot safer than many rc planes

    I am a Mechanical and Electrical engineer. It is just not something that people that are unknowing can really safely build. Yes his ratings are ok but if someone builds this and does not know what the C rating is and uses too small of a battery. You know something about it the hobby RC community does as well but someone that is not involved in RC more than likely has no idea what those ratings mean. There are plenty of batteries out there that do not have a high enough rating to run this and then you do have exploding batteries. Even major manufacturers have problems with it look at the Samsung galaxy note 7, they banned them from going on airplanes because they blew up unannounced.

    Hi sephiroth,

    Perhaps I shouldn't have used the term short circuiting (that's why it was in inverted commas), there just aren't any traditional resistor components used, but the battery is not really being short circuited.

    As shown by the power meter, the soldering iron only draws 5.3A which is well below the batteries rated 21A maximum current draw and far from the 31.5A danger zone.

    LiPo batteries only blow up when you draw too much current from them (or damage the plastic casing by hitting or breaking them). The blowing up is due to you exceeding the internal temperature required for thermal runaway (about 80C) which usually starts occurring at about 150% of the rated current. Once you exceed this temperature, the reaction in the battery become self sustaining and no longer requires current draw or short circuiting. The casing swells up and the heat melts the plastic, the plastic then ruptures and the lithium is exposed to air, this is then when it explodes as lithium is extremely volatile.

    Resistance in the soldering iron is provided by the ripcord, the connection to the tube, the tube, the tube connections to the graphite rods and by the solder. The highest 5.3A current draw is during the first few milliseconds of contact and drops significantly, down to about 2.7A once the rods and solder are heated due to their internal resistance increasing with heat.

    So there is very little risk of the LiPo battery exploding and even less so than with RC aircraft which run close to the rated current of the battery pack.

    It probably isn't quite exactly a short circuit, given that the graphite has a little teensy bit of resistance (taking a value of around 30 ohms for a 6cm 0.7mm mechanical pencil lead, perhaps a rough estimate would be 1-5 ohms, judging from the current and voltage stated above). But it's true that lipo batteries are not toys; maybe a safer option/improvement would be a bunch of supercapacitors wired in series.

    Battery aside, I'm also wondering about the quality of the solder joints - seems to me that the joints wouldn't be particularly strong, but I don't have enough experience to judge based on the videos.

    Yes but even the E cig batteries blow up and the resistance on those coils is much higher than the graphite in this project. NiCd or lead acid battery chemistry would be much much safer for this project than LiPo. The lipo have a very small window for charge and discharge where they are happy the rest you will get a visit from some very angry pixies if not right away after a few uses.


    1 year ago

    Also, a protected button switch is mandatory, otherwise the tips are alive as long as the battery is in place. A good possibility is a small push-button with a shield around the activator. This could control a high power mosfet with extremely low on resistance in series with the tip.

    A good solder joint requires the wire or copper to also heat up to bond with the solder. How well does this heating of solder alone, make for a good solder connect. By heating solder alone you may end up with what is called a cold solder connection that can fall off the wire or pcb pad. Also a NiCad battery would be somewhat safer the li-ion for this type of use.

    5 replies

    I agree, it does look like a cold joint. I don't think that should be inherent to this design though, but rather a matter of practice with the equipment. If the tip where to touch the leads and/or other points to be soldered as well as the tin then they should heat up as well. It should also be possible - with some training - to heat the leads or whatever is being soldered with the molten tin if that is preferable to avoid overheating sensitive items. If you follow the link the authors provided as credited for giving them the idea you will see that it is possible (although that particular iron isn't battery powered the tip and science is the same).

    If the size isn't deterring then there are 6 V MC batteries (found some just over 15x5x10 cm / 6x2x4 "), although I believe most are 12 V. The 6 V should be pretty perfect for this, and could be found for equivalently a couple of tens of dollars (at least in Europe - I'm from Sweden). That would also make the iron a lot easier to hold - it could be pen sized (albeit with a power cord sticking out the end).

    I would think that the heat from the solder itself would heat up the metal enough for it to bond properly. I often make joints by having the blob of solder hanging off the tip of the iron, holding it there long enough for the whole part to heat up (1-2s normally), as long as it's a hot enough iron. I prefer this for soldering direct to battery cells as it doesn't heat the cell up as much as transferring the heat through the end of the cell to the area being soldered.

    Hi Discostu956,

    The solder does heat up the copper underneath it pretty well but you can also increase the heat on the copper track by touching it with the pencil leads, the copper and the solder will then heat up the same way.

    Thanks for the comment.

    If you solder to battery cells, don't use a low power iron. Use a big ole solder gun, as this heats fast and more heat to a small spot. You don't want to heat more metal then you need and fast heat then cool is best for a battery. You should tin a spot on the cell first then cool cell, come back and solder your wire making sure the solder flows well, and cool cell quick. Battery will be fine this way.

    If the solder looks like it is balled out and not like say wax flowing up a lit candle wick, then the solder is not bonding to the connection. So if you don't heat both the wire and the connecting pad, the solder will not flow properly and bond to all metals ( PCB pad and wire) . I spent 3 weeks in electronics school, just on soldering and got a class 2 NASA certificate. Take some practice to get it right. I would not use this method of soldering since you cant well control the heat. In an emergency, you may get by, but not for a permanent solder.