DIY Resistance Soldering

There is a method of soldering called ‘Resistance Soldering’ that is very popular among Model Railroaders. Basically it involves running a lot of Amps at a Low voltage through two contact points. The resistance between those points cause the area to heat up. THe contact point may consist of a contact ground clip and a carbon probe. It might also be two Stainless steel (or nichrome) probes. This method is sometime used for soldering surface mount components. The primary supplier of this equipment is American Beauty Tools

However their quality equipment is very expensive. There are also some DYI Resistance Soldering projects listed at various web site that involve modifying a available transformer (old Microwave, Battery charger, soldering gun, UPS). As I was looking at these I wondered if the same thing could be done digitally with a battery as supplying the power. My reason for a using a battery is that they can provide lots of AMPS for short durations and I happen to have several 18650 battery packs for ebikes. It would be convenient to be able to use them at a location where house current was available.I do not see why a 12 volt automobile battery could not do the same thing.

So I thought I would post the question here and let someone point out the gross ignorance of my idea … and hopefully explain why. The basic concept is to use an Arduino PWM output to limit the Voltage and Current going from the battery through a circuit with unknown resistance. The poorly drawn block diagram illustrates the concept. I have chosen 1-10 volts and 1-30 Amps as my limits. More amperage would require a shunt rather than Hall chip. It would also be getting too close to the 35 Amp limit of my Battery Management system.

I know that PWM can be used to control voltage but I have seen very little about using it to control Amperage. If I understand correctly there are two main parameters to the PWM control: frequency and duty cycle. I am wondering if one of these corresponds to current and the other to voltage.

No. PWM puts out ON-OFF cycles of VCC (5V on Uno) and 0V. How long 5V is by the duty part of the frequent cycle. Default PWM is close to 500Hz ON-OFF for some pins and 1000Hz for another on Uno.

If you PWM led13 with a short duty cycle and wave the board fast you will see dots, not a solid weak trail. On the Arduino main site are explanations of PWM when you want more facts.

PWM for resistance soldering makes sense, pulse that heavy current. You will need a hefty FET to drive with the Arduino PWM, the FET takes the load.

Did you see this article? https://softsolder.com/2010/09/08/resistance-soldering-transformer/

Another way to get short bursts of current is to build up a capacitor bank using big caps and find a way to charge it as fast as you make joins and cool contacts. On Youtube I've seen a 6-pack of soda can size caps used to make a car battery equivalent. That should be able to smoke a jumper much less solder it.

I wonder is it wise to "abuse" a Lithium Ion battery in that sort of application? I would feel much more comfortable using a lead-acid battery. This might be a good application for a worn-out car starter battery.

This is probably a stupid question, but if you are using a battery why do you need any electronics?

...R

I suspect the car starter battery would work well as a power source but I would have to go out and buy one as I do drive a car any more. I do have some spare 36 Volt battery packs and they would be very handy (and a LOT lighter in weight). What I am trying to figure out is HOW to avoid 'abusing' the pack

I am an electronics expert, but I'm NOT a resistance soldering expert and I don't think I"ve ever intentionally* resistance soldered anything.

My reason for a using a battery is that they can provide lots of AMPS for short durations and I happen to have several 18650 battery packs for ebikes.

Make sure the batteries are rated for high-current (or short circuit) operation. "Bad things" can happen if you "abuse" modern high-energy batteries.

I do not see why a 12 volt automobile battery could not do the same thing.

Well, you're probably not going to hurt a lead-acid car battery... The are "abused" every day... But unless there's something to limit the current, 500 Amps or so through a small wire or a small "connection" will vaporize things. In other words, you are likely to explode whatever you're trying to solder, or the wires supplying the current!

I assume a proper resistance soldering setup has current limiting and/or voltage/current control. If you use a resistor for current limiting with a car battery, you're going to need a (physically) big resistor because the resistor is going to dissipate more heat/energy than the solder joint. (If it all works correctly, the solder joint should get hotter than the resistor with the energy concentrated in a small spot.)

  • I once unintentionally un-soldered some batteries with excessive current! :D

"I assume a proper resistance soldering setup has current limiting and/or voltage/current control. "

I would make that assumption as well. As to using a Lead Acid car battery: I have literally witnessed someone connect two batteries (24 Volts) and use a set of jumper cables to stick weld. I do NOT want to weld so a reasonable method is needed to limit the current and voltage. Too high a voltage and you get an arc welder. Too high a current and you get burning tool (and perhaps a damaged battery).

The problem is how to implement it. If I understand you folks correctly then with PWM (and burly MOSFET) one could control the total amount of energy applied over a given period of time but one could not limit either the peak voltage or peak current. Obviously I had the wrong idea :(

Power Transistors ?

lewtwo: I suspect the car starter battery would work well as a power source but I would have to go out and buy one as I do drive a car any more. I do have some spare 36 Volt battery packs and they would be very handy (and a LOT lighter in weight). What I am trying to figure out is HOW to avoid 'abusing' the pack

You build the capacitor pack and keep it charging between solders. They weigh much much less than a car battery.

Best part, you can make one for 5V or 3V, maybe use multi-Farad super caps in parallel to get the current you desire but really I dunno if super caps are completely unsuitable -- may get damaged by too fast discharge.

It sounds like a fun project to play with but don't expect to get a useful soldering station at the end of it. It you actually want that, then it will be cheaper to just buy one. Yes, even the really expensive ones. You are going to be buying MOSFETs in quantities of one or two and then buying more when you blow up the first one. Your parts cost will eventually exceed the cost of a pre-made one.

If I was stuck in the outback and I had a lot of Arduinos and suitable MOSFETs, then I wouldn't use them. I'd just pick a longer piece of wire to act as a low-value resistor and control the current that way.

You will have fun working out how to measure PWM currents with a hall effect device. That's going to be extremely useful education for whatever you build next.

I picked up a Stahl temperature control iron and station for $20 because they had a defect that turned out easy to fix. The heating element wasn't all the way in. Solution: open it up, push till it clicks and glue the wires (I used hot glue) in place. Works like a charm, even the temperature control.

MorganS: It you actually want that, then it will be cheaper to just buy one.

Did that ... well I found a used one (HOTIP H-101A) on ebay. I am waiting for it to arrive (and hoping it actually works). I think what I will find inside is a multi-tap AC transformer. Alas it will nut run on battery power.

It was worth asking the question because I got a bit of an education :)

I thought about hanging a DP50V5A or DP50V15A Digital Power Supply off the battery. The 5 Amp model would require too high a voltage. The 15 Amp model might work but I am not real certain (this application might appear to be a short circuit to it).

If I was starting from scratch to build an AC unit then I would probably acquire a used Weller D550 soldering gun. They are rated for over 300 watts at somewhere less than 1 Volt AC. Then I could use the Arduino to regulate the primary power duty cycle.

Whereas a $10 15W pen only solders...

unless you're doing surface mount I miss the big deal.

If your reason to build one is to use it when there is no mains power available (you mention this in the post) there are easier solutions, such as a butane gas powered soldering iron.

GoForSmoke:
Whereas a $10 15W pen only solders…

unless you’re doing surface mount I miss the big deal.

Surface mount is not something that I do. My eyes are too poor, patience too little and I have no equipment suitable to the task. On the other hand though hole boards are becoming about as common as hen’s teeth. I already have a temperature controlled soldering (some chinese knock off) and a Weller 8400 gun for heavier stuff.

It has been suggested on the endless-sphere ebike forum that resistance soldering might be an alternative method for construction of battery packs as opposed to spot welding. The primary advantage is being able to use lower resistance copper tabs/buses rather than the nickel tabs/buses currently in common use. As far as we know no one has actually tried yet principally due lack of equipment. Thus it is only conjecture at this point. Bear in mind that these battery packs sometimes have more than a 100 18650 cells and involve buses carrying upwards of 50 amps at voltages over 72 volts (and those are NOT the extremes).

I think that it might be a preferable method for soldering heavy 10 and 12 gauge conductors to small XT-60 and XT-90 connectors but that is only conjecture as well.

My design school teacher had a soldering gun that was a 60W incandescent light bulb enclosed in polished sheet metal with a handle and trigger and a cone on the front with the point cut off making a small hole. When the bulb turned on there was a spot of heat instantly just past that hole. Bulb off, no heat except the cooling join. It's like having a magnifier and sun with a switch.

I wonder if a tiny inductive heater could work?

GoForSmoke: I wonder if a tiny inductive heater could work?

Lots of ways to produce heat ... remember that we started with rubbing two sticks together. :)

By inductive heater I assume that you mean a coil that rapidly changes the magnetism of the target material. At one time I worked in a factory where they used that method to heat 2 inch diameter steel rods before slamming a head forming die on them. Inductive heating requires a ferrous metal. The only ferrous metal in this application is the steel shell encasing the battery and that is bit where we want the least amount of heat.

I note microwave ovens a similar method to heat food ... or any material with metal or water molecules. However the only 'phaser' pistols I have seen have been on Startreck.

Inductive heating could be used to heat a ferrous rod that transferred heat by contact but that would be little different form existing soldering irons.

Let me know when you can solder by rubbing two wires together! :o}