Making a Resistance Soldering Unit, and maybe a spot-welder too…

December 25, 2018

Most ebike builders have never seen a resistance soldering unit, so they don’t know that it’s an option when you are moving up to soldering larger connectors. If you want to buy one, they are expensive, but…they are surprisingly easy to make if you are the type of person who is handy around a shop.

What the hell is an RSU?

The type of readers we have here already know how to use a soldering iron. You plug it in, the tip gets hot, and by pressing the hot tip against two parts that you want to join, the parts will become hot enough that when you touch the joint with a section of solder, it melts and flows into the joint. Pretty straight-forward, right?

An RSU will heat up two metal pieces by passing current through them. It is the same phenomenon where our ebike electrical connectors get hot when we run too many amps through connectors that are too small for the amp-load. You touch two conductive probes to either side of the pieces you want to join, and then you pass high amps through them (by turning on the RSU with a foot-switch). Since the probes are actually touching the work-piece, you don’t need high volts. It is the amps that creates the heat.

To give you an idea of what’s possible, the commonly used parts can provide up to a max of 700A at 2V. Of course the actual amount of amps you end up getting is easily adjustable. That same 1400W can be wound for 120A at 12V.

How a Transformer works

The heart of an RSU is a simple transformer, and they can be cheaply found inside a discarded trash-day microwave oven. There are many youtube videos on how to safely harvest a Microwave Oven Transformer  (MOT).

For your safety, the most important thing to remember is that microwave ovens have large capacitors that will hold a 2,000V charge, even after the microwave is unplugged.  Wear rubber gloves when dis-assembling a microwave, because if you accidentally touch the capacitor connectors, they can definitely kill you, and…even if you live, it will definitely hurt.

If you don’t want to wait to find a discarded microwave for free, you can simply go to ebay and search “Microwave Oven Transformer”. The MOT from a very common 800W microwave might work fine (2V X 400A), but I recommend getting a larger 1400W-1500W unit (with ebay candidates, I had to google the part number to find the wattage). This is because you can configure the RSU to output lower amps, but the max amount amps that are possible are limited by the physical size of the MOT. If you start big, you can then adjust the amps downward to whatever works for the job you need.

I am going a little overboard on the MOT size because I also want to use it as a spot-welder for fat copper electrical buses on a custom battery pack, but that’s an article for another time. If you are certain you only want a heavy-duty soldering station, an 800W MOT will be fine, and easier to acquire, too…

A common bar magnet, with iron filings to show the shape of the magnetic field. N means North, and S means South

The Earth has a three-dimensional electromagnetic field that is roughly shaped like a doughnut (also called a toroid). If you place a simple bar magnet under glass and sprinkle the top of the glass with iron filings, the metal flakes will align with the invisible magnetic field in a shape that is very similar to the Earth’s field. In the picture above you can see the classic “double loop” that is formed when you are only showing a two-dimensional slice of that field.

In this article, I will use the terms steel and Iron interchangeably, but steel is simply iron that has had about one-third of one percent of carbon mixed-in with it, which makes the iron physically stronger, but doesn’t change its magnetic properties. The MOTs we will be talking about typically use a core that is made from a stack of thin steel sheets that are called laminations [each lamination is dipped in a clear insulation lacquer, so they are not electrically connected to each other].

The first step is to understand that if you take a coil of insulated copper wire (magnetic wire looks bare, but has a coating of clear insulation on it), and when you pass an alternating current through it, it would be called an “air core” solenoid. It will produce a magnetic field. The air-core style can have its AC alternations switch back and forth very rapidly without overheating (high frequency). These are used sometimes in radios.

However, the magnetic field from an air-core solenoid is very spread out (weak, like my knees). But…if you insert something made of steel into it’s center, it becomes an iron-core solenoid (often used as an electromagnet which can be turned on and off as needed, like a motor). Adding iron to the center causes it’s magnetic force to be more focused and more concentrated. If you also surround the coil with steel laminations that have the classic “double loop” shape I mentioned before, the steel core will draw-in the entire magnetic field so that it is flowing only through the laminations, instead of through the surrounding air.

The characteristic of iron and steel that draws-in and channels a magnetic field is called it’s  “permeability”.

The current coming out of the common homes’ wall sockets in the USA is about 110V of alternating current (AC), which changes its direction 60 times a second (60-cycle). A typical wall socket might have a 15A breaker on it for protection, meaning that if you multiply the 110V X 15A, you can plug in an appliance that draws a maximum of  roughly 1600 Watts.

This is why the biggest common microwaves are 1500W, and if they are running and you turn on a second appliance, sometimes the breaker trips and has to be reset. A microwave that needs more power than this will likely be wired for using 220V AC, so it can use fewer amps to do the job. If your entire home is wired for 220V (like Australia) then you would need a 220V-input transformer, but…the MOT that I am recommending for North American hobbyists who want to make an RSU is a 1500W unit that is wired for a 110V input.

The current that’s passing through the primary coil uses the phenomenon of “inductance”, and it will “transform” the current into a magnetic field. IF…you place a second coil next the first coil, the magnetic field will induce a current in the secondary coil, without any physical or electrical connection between the two coils. Because the primary coil and the secondary coil are not electrically connected at all, the only connection between them is the pulsing magnetic field that they are both located inside.

A standard issue MOT with the high-voltage coil removed

In the pic above, you can see the “double loop” of the magnetic field in the shape of the steel lamination stack that forms the core of the transformer. The high-voltage secondary coil has been removed, and the spaces that are designed for the coils to be inserted into them are called the “windows”. The straight bundle of laminations shown in my hand are the magnetic “shunts”, which will not be needed.

The laminated core of a MOT. There are several styles, and this one is called an E / I for obvious reasons. You can see the remnants of the original welding that held them together, at the top and bottom of the joint.

In the pic above, everything has been stripped from this MOT core, and the E / I sections have been separated by grinding off the welds that previously held them together (I don’t recommend separating them). The relative efficiency of this style of transformer is only average, but it is common because the coils can be machine-wound and the majority of the assembly can be partially automated, making them fairly affordable.

Removing the high-voltage secondary coil

The high voltage side of the transformer typically produces roughly 2,000V (whether the input is 110V or 220V). You must never plug it in while the stock high-voltage coil is still in the transformer, since 2,000V will absolutely kill you. In the pic above, the HV coil of the transformer is covered by additional insulation, which looks like stiff paper.

I feel that using a hacksaw is the easiest way to remove the HV coil. Be very careful not to cut or nick the 110V input coil. The HV coil can easily be recognized because it has the same mass as the low voltage coil, but it is made up of thousands of tiny strands, much smaller than the low voltage input side.

110V on the left, and 2,000V being pulled out on the right. Roughly one volt per strand (be aware, not all transformers use one volt per strand, when you are performing calculations)

The two coils need to be roughly the same volume to get the maximum effect, measured in Watts of power. When you input 110V at 14A, you get a magnetic field that has roughly 1500W of energy. The adjacent coil will convert that pulsing magnetic field into an output AC current. It does NOT change the number of Watts, so…if you use many strands of fine wire on the secondary to raise the voltage, the amps will go down to balance out.

So, if increasing the number of strands will raise the volts and lower the amps, then we can use this phenomenon to raise the amps, but…doing that will also lower the volts. Fortunately, lowering the volts is an additional benefit, because that makes the resulting device much safer (other than the danger of high heat).

If you want to get the maximum amount of power out of the RSU that we are going to make, you must fill the entire transformer window with the maximum copper mass that will fit. However, it will not hurt anything if you use a smaller output coil (you can even use two separate coils in the output window). This is the easiest way to adjust the output of amps, by swapping to a smaller output coil.

I recommend somewhere between two to twelve “turns” in the custom secondary coil, and doing that will result in 2V to 12V on the output (regardless of how thick the wire is), and…be aware the actual resulting voltage will be approximate. The size of the copper mass on the secondary determines the total Watts, and the Watts divided by the volts will determine the number of amps that will result.

[Be aware, all secondary coils will output alternating current / AC]

The 3V magnetron heater power supply, normally placed between the primary and secondary coils

One example of the inverse relationship between voltage and amps is the magnetron heaters’ power supply. Between the primary and secondary coils is a tiny third coil that can be discarded. They might have just a few turns, since I have seen several with only three turns in the coil. This configuration would result in about 3V of AC current, but…since it has a very low volume of copper mass passing through the transformer windows, that means that it will have low Watts too, resulting in low amps.

Once you have a transformer and you have removed everything from the core except the primary input coil, you will need to add a new custom secondary coil through the windows, and that coil must spiral in the same direction as the primary. If you get that backwards, the output will be very low, and the transformer will get hot.

The absolute highest possible amps you could get would be a from using a fat copper bar that’s bent into a “U” shape, and…since it only makes one turn through the windows, it would be a one-volt output that provides roughly 1400A. That configuration would be unrealistic, so the most common secondary uses two turns of fat welding cable (as shown on the header pic at the top of this article). Welding cable has a tough but thin insulation cover, so the majority of its diameter is all copper. Welding cable also typically uses a very flexible multi-stranded type of wire, which is beneficial in a variety of ways.

Commercial RSU’s

A transformer-based RSU is not a new thing that hobbyists have discovered, a company called “American Beauty Tools” and also “Luma” has been making them for years for industrial use. The company “Micro Mark” also sells RSU’s to model train enthusiasts. However, the 250W RSU from American Beauty Tools is over $500!

Here is blog from a hobbyist who shows how to make a DIY RSU (click here).

A very professional looking DIY Resistance Soldering Unit

In the pic above, a hobbyist took a small MOT and swapped-in a custom 6-volt secondary, then also added a motor speed-controller that was made for a router/saw. Doing this allowed him to adjust the 110V AC power going into the primary, which adjusted the output amps to anything up to roughly 50A, for a total of 320W.

Since copper electrodes tips might melt (from heat) and then stick to the workpiece, in this build he has used carbon gouging rods, which come with a conductive copper skin (click here). They are light and brittle, and I have easily sharpened the tips of these with a pencil sharpener. A second option for high-temp soldering (or spot-welding) is to use expensive tungsten rods, but their high resistance means they will get very hot.

Carbon Gouging rods are cheap. They can be found online, or at a local welding supply shop.

For your first RSU / Spot-Welder, I’d like to suggest making holders that use a 1/8, 3/16, or 1/4-inch diameter rod. It is very easy to source copper, tungsten, and carbon gouging rods in those sizes.

[Soldering irons use a steel tip, but we are not trying to get the RSU probes hot, the electrical current will be what we use to get the work-piece hot]

For small soldering jobs, copper is the most affordable tip, but at the higher amp-levels, the tips will melt enough to stick to the workpiece (6-ga solid grounding wire is roughly 3/16″, found at hardware stores). Tungsten is expensive, but it’s high melting temps mean it will not stick to the workpiece, but…it has high resistance, so it will get super hot with frequent use. Carbon gouging rods are fairly cheap, and very easy to shape the tips.

Here is a short video from the Luma Electric company, showing their industrial RSU in action (click here). Notice in the pic below that the tip is so hot, it is actually glowing. Having this amount of concentrated heat means that soldering can take place very fast, so the heat does not spread very far through the workpiece. The electrode here is a 1/8-inch carbon rod.

The Luma industrial RSU. The second electrode is clamped to the metal vice to complete the 6V AC circuit

Here is another short video showing an RSU in action (click here).

And finally, a third short video showing fat cables having lugs attached by an RSU (click here).

Also, when soldering thick conductors, a normal soldering iron would struggle to get the workpiece warm, because the copper mass would act as a heat-sink…which means the copper mass will be pulling the heat away from the joint, and spreading it out faster than it is being applied. An RSU can apply very high heat to a specific spot before the heat can be wicked away.

Another major benefit of an RSU, is that it is typically actuated by a cheap foot-switch. This leaves both hands free to hold the electrode and feed solder into the place that it is needed, or to manipulate and reposition the work-piece. Some types of tasks allow the use of conductive tweezers so that the two electrodes also physically clamp the workpieces together before/during/after the heat is applied by actuating the footswitch (as seen in the videos just above).

Even though two electrodes need to touch the workpiece to complete the circuit, one of the electrodes can be clamped to it, so you only need to use one hand to touch the second electrode to the spot that you want to get hot…

[If you sometimes find that you need a “third hand” to feed-in the wire-style solder that is common, give “solder paste” a try. You apply the solder paste to the joint, connect the two parts, then apply heat for a few moments to melt the solder]

Standard soldering irons are a little slow to heat up when plugged-in, especially the large ones that are needed for big jobs. An RSU can get very hot very fast, and then it cools down fairly quick too. They can be pulled off the shelf to do a job, and immediately fired up with no wait.

A Spot-Welder

If you are certain that you only want an RSU, a very common microwave size you could easily find is 800W, and it’s MOT is about half the size of the 1500W unit I am recommending. If you take 800W, and then wind the secondary output coil to 12V, the resulting amps would only be 66A (or, maybe use six turns of secondary coil, making 6V / 130A), which is still very useful. 800W is very powerful for a soldering tool. In fact, It might be a good idea to try out everything on a free smaller microwave first, while you keep your eye out for a large one.

A professional looking DIY portable spot welder.

I’ve added the pic above to show what a common portable spot-welder looks like. This is the type that would be used to melt two metal pieces together with 1400W. Most youtube designs use wooden arms, with the welding cables running alongside the arms, down to the electrode tips.

Spot-welding two mild steel nails together with 1400W. Notice the yellow wooden arm, and the cable that runs alongside it to the tips.

The pic above shows two very important things to notice. Mild steel nails melt at 2500F (1370C), which is what you can easily achieve when you are using 700A. The other thing I want you to notice is that the point of contact is getting hot SO FAST, that the person is holding the nails with his bare fingers. Of course he would have to immediately set them down after the weld is complete, since the high heat would migrate fairly quick to the place where he is holding the nail.

If you have ever had to wait a long time for your soldering iron to heat up the large connectors that you are soldering, an 800W RSU might be a handy addition to speed things up.

Once you get it working for the types of jobs you do, I’d recommend making a wooden box to house the RSU, or possibly buying a plastic tool-box. This will prevent anyone from accidentally dropping something conductive into the 120V AC connections (like a screwdriver, or pliers).

Soldering materials for plumbing

Some plumbing jobs use copper pipe, and their joints need to be soldered. It is normally done by using a hand-torch for the heat. But when you have a copper pipe joint located in a tight spot near some wooden studs…the torch might start a fire. So, plumbing is one of the places where an industrial RSU would be used (click here for an example). I mention this because a casual search will turn up soldering supplies at your local plumbing hardware stores.

A plumber using a propane torch to solder a copper pipe

The type of flux used by plumbers is a very aggressive acid-based type, and is very bad for electronics. Electrical connectors might not be harmed by plumbers flux, but I don’t like having any around to mix up with my electrical repair supplies.

The same goes for the solder used for copper pipe. Plumbers in the past used a 50%-50% mix of tin and lead (Sn/Pb), and now water-pipe is required to use “lead free” solder (98% tin). The best solder for ebike electrical connectors is the 63/37 type (and 60/40 is also good). I have used the modern SAC305 “lead free” solder, and all I can say is…when the government completely bans lead solder someday, I will stock up with enough 63/37 to last the rest of my life.

Rosin-based flux (dried pine tree sap) is common and works great for electrical parts. If you are struggling to get a good solder connection, make sure you are using good flux. If you are not using flux, your life has been sitting on a throne of lies…

Let’s wrap this up

I don’t really “need” a spot welder, or an RSU. But…as long as I enjoy experimenting, they can be very handy to have. I sourced a large 1500W transformer because it can do two jobs. One of those jobs is a 700A spot-welder that can melt steel parts together, and the other is a mild and adjustable RSU for those jobs that my 100W soldering iron is struggling with on occasion.

I also have one of those pocket spot-welders from kWeld, and it can weld 0.20mm thick nickel ribbon to 18650 cells very well. However, my future plans include building some high-amp battery packs, and if you are frequently drawing more than 20A per cell, the nickel acts more like a resistor than a conductor (some 21700 cells can safely provide 30A peaks).

I may experiment some with brass for the battery tabs (cheaper than nickel, and 20% more conductive), but for the main bus material?…I really like copper for it’s low price and heat-sinking abilities. This is because I recently discovered that most builders (like me) didn’t realize how much the bus material can be used to help draw heat away, and cool the cells.

A copper bus-plate with nickel tabs for spot-welding to 18650 cells

In the pic above I found an example in industry where they are using copper buses, but right next to the cell is a nickel tab, so they can still use existing factory spot-welders (nickel will spot-weld easily).  The connections between the copper and nickel around the edges requires much more heat (enough to damage the 18650 cells), but…the nickel tabs can be attached to the copper buses in a separate operation, and then allowed to cool before spot-welding the nickel to the cell-tips.

The method they used was an expensive laser-welder to connect the nickel/copper joints, but I’m not about to buy one of those! Copper is notoriously hard to spot-weld onto the nickel-plated 18650 shells, but it can be done with expensive equipment. I still have high hopes for some experiments that are coming up, where the copper bus is “DIY nickel-plated” in the hopes that the nickel-plate will make spot-welding the copper easy, but…we’ll have to wait for those results.

Wish me luck!

If you are not going to build your own battery pack (which means you are sane), I have two battery packs from Luna Cycle that I am very happy with.

Written by Ron/spinningmagnets, December 2018

Grew up in Los Angeles California, US Navy submarine mechanic from 1977-81/SanDiego. Hydraulic mechanic in the 1980's/Los Angeles. Heavy equipment operator in the 1990's/traveled to various locations. Dump truck driver in the 2000's/SW Utah. Currently a water plant operator since 2010/NW Kansas


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