Resistance Soldering

© 10-Dec-2000 20:17 Dr. Spiff


Table of Contents


I first became aware of resistance soldering when I took a 1 week course leading to a NASA level 1 soldering certificate. The principle is very simple.

  1. All metals have some resistance [ohms] to current flow [amps].
  2. The voltage that is dropped across a resistance creates current. [volts = amps * ohms]
  3. This voltage times the current is watts. [Power (watts) = volts * amps]

For example, a 1 foot long piece of 16 AWG copper wire has .00409 ohms resistance. If you put 6 volts DC across this resistance, you will generate a current of 6 volts/.00409 ohms = 1467 amps. To figure the heat generated, it is 6 volts * 1467 amps which is 8802 watts or 8.8 kilowatts. This assumes that your 6 volts has an unlimited current supply. Not likely, but it could happen.

After that aside for the "class clowns", back on topic. What does a very hot piece of copper have to do with soldering? Not much, until you consider that any metal to metal joint that you would like to solder will exhibit a resistance. If it resists, you can heat it. And if you can heat it, you might be able to solder it. It ain't rocket science is it?

So how do you get a controllable source of volts or amps? Remember the 3 variables mentioned above, volts, amps, and resistance. The resistance is going to be a given for any particular joint you wish to make, so the only variables you have control over are voltage and current. And given that we are trying to maximize bang for the buck, controlling voltage is much cheaper than controlling current. A transformer will allow us to control the voltage we put across the joint which will, in turn, control the current.

There are 3 parts to the system, a transformer, a switch to control the voltage, and a means of applying the voltage to the joint.


Transformer (XFR):

There are lots of ways to skin this cat. You can find a low voltage, high current transformer, or you can make one. Back in the days of tubes there were filament transformers which put out 6.3VAC which were center-tapped to give you ~3.15VAC. I have successfully soldered using a filament transformer with the center tap on a 6.3VAC 20amp secondary. But the truth is that these transformers are thin on the ground. When was the last time you replaced tubes in your TV?

Another approach is to rewind an existing transformer. The power xfr in a microwave oven is a pretty stout unit with most of them in the range of 600 to 1000 watts. And the xfr is usually the last thing to go when an oven dies. You can usually get a great price on a dead oven, FREE. But you still have to rewind the xfr since the stock unit has a 110 volt primary and a multi-kilovolt secondary. No sweat.

Identify the secondary winding. It is the winding NOT connected to 110 volts. Rip it out. Go to a motor rewind shop, or someplace that has magnet wire. This is coated copper wire which you will wrap around the now vacant space of your microwave xfr. How big is this wire? As large as you can get since it is going to carry a great deal of current albeit for a very short period of time. I have found 8 AWG magnet wire at the motor rebuild shop in my small town. You should be able to do the same. You won't need much since the ratio of primary volts to secondary volts is the same as the ration of primary turns to secondary turns. Let's say you have 110 turns on the primary at 110 volts, 3 turns on the secondary would give you 3 volts. Remember you want to keep your secondary volts down below 6 volts and preferably closer to 3 volts.

I use the "wrap and cut" method to wire my xfr. I wrap some wire onto the secondary side of the core. I test the output of this secondary to see what I've got. And I'm usually too high, so I use a sharp pointed probe on my voltmeter to "hunt" back down the wrapped wire to see where I ought to be. NOW I cut the magnet wire loose from the coil.

What if you have found a transformer that has sufficient capacity, say 30 or 40 amps, but it has a 12 volt secondary. All is not lost. Your xfr is properly defined as having a 110 volt primary amd a 12 volt secondary. What would happen if you fed it 55 volts instead of the 110? That's right you'd get 6 volts out of the secondary. But that is still too high, so you just keep reducing the input voltage until you get the output voltage you want. How? Use another transformer to feed this transformer. You can either get or build a transformer that drops 110 to 25 that feeds this one and you are set. [HINT this is exactly what a Variac or autotransformer does for you.]

One of the things to keep in mind in all of these gyrations, is that the product of the voltage and current is the same for each winding. Let's go back to the xfr with a 12 volt 40 amp secondary. This works out to 12 volts * 40 amps = 480 watts. The primary winding has to supply this power plus whatever is lost in the core. So the primary is going to have to supply 480 watts plus some loss percentage. We'll call it 550 watts just to be on the conservative side. That means the primary will have 550 watts / 110 volts = 5 amps. This will become important if you have one xfr feeding another and when we talk about the control mechanism.

Joint Connection:


Now that you have the juice, you have to get it to the joint. From the xfr to the hand piece you will need 2 cables to conduct the full current that you will be applying to the joint. Using 18AWG lamp cord is not a good idea because it may well melt in your hands. Go to your local welding supply house or golf cart service center and see if they have some 6 AWG welding cable. You'll only need 10 to 15 feet of this cable. While you're there, pickup some lugs to attach the cable to the xfr and to the hand piece.

As mentioned earlier, the heat comes when current travels between 2 points of contact. There are 2 ways to make this contact come about. You can use the probe and clip technique or you can use the tweezers technique. Which you use will be determined largely by the joint you are making.


The requirements of the handpiece are that it be electrically non-conductive, able to resist heat, provide an attachment point for the probe and the cable from the xfr, and be comfortable in your hand. Sounds kinda' like an arc-welding stinger doesn't it? That is what you are trying to emulate. There are lots of possibilities including wood, fiberglass, and PVC pipe.


My experience is that bare metallic probes tend to stick to the items being soldered. Clearly, a less than optimal experience. Go back to your local welding supply house for the solution. Get some "carbon gouging rods". These are rods of carbon encased in a copper jacket about a foot long and come in several diameters ranging from 1/8" to 1/4" and beyond. Get several since they are cheap and fairly fragile. The soft carbon can be shaped to any kind of point you desire so you can "customize" your probes.

Here is the design for the Mark 2 handpiece. This utilizes a piece of graphite golf club shaft and a brass fitting which allows a nominal 1/8" gouging rod to be placed either axially or laterally in the handpiece. The rod slips into either of the 8-32 tapped holes and is held in place with a thumbscrew inserted into the other hole. The cable is pinned into the 5/8" brass head while the other end is connected to the xfr.

The picture at left is of my handpiece which holds 1/4" nominal gouging rod. The construction details are the same except that the holes are tapped 1/4-20 instead of 8-32.






Probe and Clip

To use the probe and clip method, terminate one of your cables with an alligator clip and the other cable with a probe handpiece. The clip attaches to your work just like a grounding clamp used in arc-welding and the probe is placed against the work. When the control switch is closed the current will flow between the clip and the probe's point of contact.


The tweezers consist of a handpiece with 2 probes, each of which is connected to your xfr. The tips are placed on either side of the joint you wish to make and the current flows between the tips when the control switch is pressed. The tweezers are more difficult to make since you are bringing 2 cables into something which has to spring apart.

This just in... The tweezers are doable with a little persistance. The heart is a spring clip similar to those that were once used by UPS drivers to hold their clipboard pages together. This clip is stainless-steel, about 6" long and 5/8" wide. The cross-hatched pieces are any insulating material, wood or plastic. The probe bodies are pieces of 1/4" brazing rod, drilled and threaded 8-32 using the same cross pattern as the single probes. This holds short pieces of 1/8" gouging rod. The other ends are threaded 1/4-20 so the cables from the xfr can be clamped to them. The insulating blocks are drilled or routed with a 5/16" hole.The probe bodies are covered with a couple of layers of heat-shrink tubing to get them to 5/16" OD or a snug fit in the insulating blocks.

The blocks are attached to the spring clip with tiny self-tapping screws. Mine are about 1/16" dia. and 3/8" long. (scrounged from defunct hair dryers, I think.) The photo at left shows the set of tweezers constructed as mentioned above.






You now have juice and a connection to the work. How are you going to control the flow of current? A switch downstream of your current xfr requires a switch which will handle 100 amps. Very expensive. But an upstream switch will get around this difficulty.

If you can walk and chew gum, use a foot switch as shown at left. This lets you keep your hands steady and your focus on the work. Get everything in position and zap! A tap of your toe and the joint is soldered.


If you feel you need more control, or are having difficulty with that gum, put a microswitch on your handpiece. This switch actuates a closes the contacts of a low voltage (6, 12, or 24 volt) relay. The downside is that you now have to provide a low voltage xfr and a low voltage relay. The upside is that you don't have to hunt under the bench with your foot for the switch.



Using your new resistance soldering unit

Solder fundamentals

Joint prep and cleaning are fundamental to a good solder joint regardless of the method used to apply heat. If you are not prepping your joint before applying your solder, you are not getting good joints. Period.

Your "fit-ups" should be as close as you can get them. Solder will bridge voids, but you shouldn't count on it.

Don't mess with the work until it has cooled. All solder goes from liquid to solid as it cools, and if you disturb the joint before it is solid, you will get a "cold" joint which is prone to failure. Using eutectic solder, 63/37, shortens this plastic phase, but doesn't eliminate it.

Soldering procedure

Once you have cleaned up the pieces you are going to solder together and jigged them in some way so they won't move while you are soldering them, you think you are ready to solder. But wait!

Are you making multiple joints? Do you have a plan in mind so you don't un-solder the 1st joint when you go on to the 2nd joint? Are you sure you want to do this?

Because the gouging rods are soft carbon you can easily shape them to suit the work. File a "V" shape notch in the probe tip to get good contact on a corner or a thin wire. Use a "C" notch to get good contact on a tube.

Place your probe(s) around the joint, push the switch, apply solder. That's all there is to it.

If you push the switch and don't get any heat, first check your joint setup. The 2 pieces of metal MUST be in electrical contact with each other. Physical contact is not enough, oxidation on the surfaces to be joined can stop or retard current flow. Next check the tips of your probes and "touch them up" with some sandpaper.

Remember that the current, and consequently the heat, will flow between the 2 points where the probes contact the work. You want to have these points as close to the joint as practical. This avoids over-heating the work and un-soldering already finished joints. Remember...

NO electrical contact, NO current flow, NO heat

Send me photos of things you have assembled using you new resistance soldering unit, and I'll post them on the sample joint page. email me with questions or comments