How to build a low voltage soldering iron

Build a low voltage soldering iron with a copper rod, some nichrome wire and thermal insulator.

Although soldering irons are quite cheap, widely available and come in various shapes and sizes, let’s try to make one from scratch. This involves some basic knowledge of electronics and some DIY abilities. However, please note that building electrical devices carries risks, and I recommend exercising caution to fire, burns, or other hazards.

This article will describe some simple to build soldering irons that can provide 15 – 60 W and are powered at low voltage (5 – 20 V, depending on heater wire you use). This means you can power it with any power supply that meets these requirements (a computer power supply or notebook charger will be a good choice because it is also short circuit protected).

Old literature

The idea of building a soldering iron came from some old hobbyist literature published in 1987. In those times, such tools were expensive and hard to find in Romania. For reference, here is the original drawing of a DIY soldering iron^[1]:

DIY soldering iron project from old Romanian literature 

The author claims this DIY tool can be made from pieces of garbage, it is lightweight, low power and low voltage (making it suitable for powering from a transformer - this will also provide mains insulation). He continues this project with the plans for winding that suitable transformer with multiple outputs and an enclosure, therefore building a complete soldering station. Here is how I would modify his "design":

Another DIY soldering iron variant

The most difficult part of this project is to find a suitable insulator between heater wire and soldering tip. In fact, in that book the author claims he actually used Mica sheets which he carefully rolled over drill bits of decreasing diameters. The few Mica pieces I had came apart when I tried this.

That being said, here's a simplified explanation of how you might build your own soldering iron. Keep in mind that this is a very basic and rudimentary guide, and the components you use must be carefully selected for safety and functionality:

Materials

1. Soldering tip: Get a copper rod thick enough and use a file to shape one of its ends (make it sharper or give it a flat, screwdriver like appearance).
2. Heating element: You can use some Nichrome or Kanthal wire to heat up the copper rod.
3. Electric insulator: Because the tip and heating element are made of electrical conductors, some sort of electrical insulator must be placed between them. This material must have good thermal conductivity and it must be heat resistant. Alumina and Mica are good options.
4. Handle: A heat-resistant handle to hold the copper tip with heating element.
5. Power Source: A short-circuit protected power supply to provide the necessary voltage and current. This could be a low-voltage transformer or a power adapter.
6. Switch: An on/off switch to control the power supply to the heating element. A better alternative to the switch is a PWM controller.
7. Thermocouple: An optional component which can be placed in thermal contact with the copper rod to monitor solder temperature.

Materials for heating element

In the above photo, besides solid copper conductor, nichrome wire and alumina insulator there are also some other parts. I will use wire end clamps to hold things in place and the diamond cutting disc is for the alumina tube. This is a hard material which can only be cut with diamond discs.

Steps

Starting with the copper rod, it is very easy to acquire from any hardware store. Look for solid core copper conductor wire of 10 square mm (I found it in local stores as FY/H07V-U 10 mm²). Get one meter. The 10 square mm core has a diameter of 3.57 mm which is enough for a soldering tip. While you are at the hardware store also get some wire end clamps with screw and some metallic L-shaped corner brackets.

Next comes the difficult part of obtaining the insulator. The best material for this purpose is Aluminum oxide or alumina (Al₂O₃). Mica is usually available as sheets which will delaminate and break if bent. Insulating tubes made of alumina can be found in household appliances used for heating (some water boilers). For the purpose of this post I got mine from AliExpress, where you can find them in various inner and outer diameters. Obviously the inner diameter must be as low as possible to allow you to insert the copper tip with the lowest possible air gap.

Nichrome wire is widely available in household heaters. An old hairdryer is the perfect choice. You are going to wind this wire over the alumina insulator. The number of turns depends on available space. It is recommended to cover a length of 4 to 7 cm of insulator to have enough thermal contact. It is a bit tricky to get it right because total length of Nichrome wire determines optimal working voltage as well as maximum power. This is why I recommend powering the heater with a PWM controller.

Start by securing the Nichrome wire to the copper rod with a screw clamp. Then insert the alumina tube and wind Nichrome over it. Leave enough space between turns so they don't touch each other. At the other end of the copper rod use screw clamps to prevent the alumina tube from sliding out and to be able to attach this assembly to the handle.

Built heating element test

I successfully melted solder (common Sn60Pb40, 1 mm thickness) with the above setup. Alumina tube is 50 mm long (I cut one tube in half) and there are 17 turns of Nichrome wire over it. The wire I got has 0.47 mm in diameter when measured with a caliper. It seems to provide enough heat to actually solder with this thing when powered at 14.3 V, drawing slightly less than 4 A (57 watts). Nichrome glows slightly and this is normal. It shouldn't light up like a bulb filament!

Heated Nichrome glows in the dark

I don't like those screws on the clamps. And there is actually a problem about replacing them: they are using non standard thread sizes.

Once you got the tip and heater ready, all you have to do is attach them to a handle while trying to avoid heat conducting elements. Use the screws from the clamps as attachment points for a metal bracket. An example is provided in the top photo.

Alternative materials

1. Copper wire for the heating element. When cold, Copper has very low resistance. It could be a possible alternative but only if you power it using a current limited (or constant current) power supply.
2. Rolled silicone pads as insulator. These pads are commonly found as insulators between heat producing semiconductors (CPUs, power transistors) and heatsinks. As a temporary solution, they may be used, but keep in mind the maximum temperature rating is usually 300°C or below.
3. Steel as soldering tip. Heat promotes iron oxidation and soon it will begin to rust. More important than this, solder alloy will not stick to it.

Example of a heater made with rolled silicone pads as insulator

Overview

Overall the build cost of this soldering iron is higher than buying one. However it was a fun project, it can be successfully used for soldering and if you already have the required parts from various appliances, the build cost will decrease.

Keep in mind to always prioritize safety and be cautious when working with heat and electricity. If you're unsure about any step or material, consult with someone experienced in electronics or consider using a commercially available soldering iron for reliability and safety.

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¹ George Dan Oprescu. Caleidoscop de electronică in Colecția Cristal, published by Albatros in 1987, page 236.