Resistance soldering is a highly touted method, sometimes useful for hobby work. An electrical current is flowed through one work piece, the joint and then the other work piece. Current through a resistance generates heat. Since the joint has a relatively high resistance, most of the heat is generated there. Very frequently the joint resistance is too high and there is insufficient current with too little resultant heat. Flux may help or hinder the heating, since most resins are relatively poor conductors and acids are good ones, Heat sources can range from a low voltage transformer with tweezer type electrodes or clamp and probe electrode to a variable transformer with meter, foot controlled timer and a wide variety of tweezer, dual or clamp and probe electrodes. With proper use all ranges of soldering temperatures can be achieved, even on difficult work.
Caveat: Sufficient
uncontrolled heat can be developed to brittlize or melt brass castings. On
my first try, I found out the hard way. Caused by poor contact, arcing can
produce undesirable pitting of work or cause tip erosion. To reduce
possible arcing, tips should never be moved or removed while current is
applied.
There seems to be some misunderstanding about the process and requirements, even among those who have used the method for years. First, only current through the resistance at the joint causes the desired heating process and nothing else. The current path is a closed loop through the transformer secondary winding to a terminal, then through a lead wire to one probe. After passing through the work, the path is through the other probe and lead to the other terminal to complete the loop. Current will always follow the path of least resistance, which frequently is the shortest. It is also constant throughout the loop. Everything in the loop has resistance, which contributes to limiting the loop current at a given voltage.
The most important part of the loop is the path through the work and joint. The path between the probes should be as short as practical , avoiding other joints. Using a clamp and probe usually makes this difficult to achieve, while tweezers and twin probes can permit short paths in tight spaces. The flow will be around the joint, if another, lower resistance path is presented. Keep in mind that the path through the stock material is resistive and will generate heat that can spread very rapidly. Examining the proposed current path before hand can save a lot of grief. With a good unit, this path resistance will largely determine the magnitude of the current.
The joint is the most critical segment of the loop. Its conductivity determines success or failure. Among novices, when power is switched on, common occurrences are that nothing happens or heat rises slowly permitting spread, or even to glowing probes. The adjoining surfaces must be cleaned of all contaminants and oxides. Restricted to the gap, there must be a conductive path between them. This may be a flux or better , a solder paint or paste. Normally probes will be on opposite sides of the joint; but in some cases they may be used as tweezers to insert or remove small detail parts. Usually the heat from the part will melt most solders.
External components are also important in that they influence current. All should be of sufficient size to permit the joint resistance to limit the current magnitude. Constricted probe points increase resistance, generating heat in both tips and adjacent surfaces on work. This may be an advantage, if points are close to joint, but not to the tips themselves. Tip contact with flux and solder must be avoided. Both tips and work contact surfaces must be clean.
Leads should be flexible and of sufficient size to avoid excessive heating. For those requiring 10 amps this would mean a #16 or better a #14 wire.
Before discussing power and the transformer an understanding of the circuit parameters is in order. The heat energy produced is equal to the loss in the resistance or W = I2 R. Where I = the current in amps and R = the resistance in ohms. If the current doubles the heat energy quadruples. The temperature rises with time dependent on the mass of the object heated. This is analogous to a car. A heavier car requires more energy (gas) and time to reach speed.
In any given circuit, the current is determined from I = E/R, where E = the voltage in volts. By doubling the voltage, the current doubles and the heat energy quadruples. The voltage can control the heat energy applied, but not the temperature. Given enough time, a lower voltage can still melt the stock. It might be noted with a fixed voltage that by cutting the resistance in half, the current doubles and the heat energy also doubles.
A side effect of voltage is arcing when the probe tip is near, but not touching the work surface. When two opposite charges are separated by air, an electric field forms between them with a field intensity of E/l in volts per meter. As the distance decreases with a fixed voltage, the intensity increases to a break down level, where arc arc occurs. Doubling the voltage will double the break down distance. When the probe point is removed from the surface, arcing occurs immediately, ionizing the air, producing a much lower resistance and larger current than that of an approaching tip, due to inductive kick. The cutoff distance is much larger than that of the normal break down point. The higher the voltage the greater the current and time, creating greater pitting of work surface and erosion of the tip. To avoid this, current must be switched on after contact and off before breaking contact. However with higher voltage, any accidental break with power applied will cause greater damage.
In the simplest system, the power supply consists of a transformer which is normally rated in output voltage and amperage. Since this is not a regulated device, the voltage is the minimum value at the rated current. The amperage is the maximum safe working value without overheating the windings over extended usage. This does not imply that the current will be at this value with every soldering shot. The actual current will be determined by I = E/R, where R is the total resistance of the current loop. Many joints will draw less than 1 amp, even with a 50 amp transformer . Unless you plan to use brazing with large masses, high currents are not usually required. For this type of work a torch usually performs better. Overheating metals produces oxidation and evaporates flux, which is not conducive to good solder joints.
Since current is the work horse, maximum wattage ratings may be misleading. Power is P = I x E and the voltage must be known to determine maximum current available. A 250 W unit could yield up to 100 amp at 2.5 V or 50 amp @ 5 V. Both are ludicrous and dangerous. Neither the tips nor leads could handle these currents for a small fraction of a second. Since R = E/I, the first would require a total loop resistance of .025 ohm and the second .1 ohm. Good unit resistances are in this order, but fortunately joint resistances are much higher.
Using add-on refinements, there are two ways to control heat for any given joint resistance, based on current or time. Current is determined by voltage and joint resistance. Changing the voltage with selectable transformer taps will vary the current. A much more expensive choice for continuously adjustable voltage is a variable transformer or variac with a meter to know where the setting is.
Since temperature rise is time dependent, a simple on/off switch can be used with observation of the soldering process. In many cases, pulsing provides a good control. A foot switch is usually more convenient. For repeated work, a timer switch can make life easier. Timed application can also help reduce unit temperature during frequent uses.
With their ability to hold, in many detailing cases, tweezers are more convenient than probes. Although fine control is lacking, a simple tweezer type unit may serve the purpose. Thinner, wire nail tips handle more delicate jobs, while heavier carbon tips are used for larger work. Over 40 years old, a very simple type has a tapped transformer with 3 or 6 volt outputs. There is a triggered switch, which may be pulsed to control heat. Over the years, contacts had to be cleaned and burnished to remove pitting. The plastic enclosure cracked, requiring repair with epoxy . Resin tends to collect on tips, insulating them and must be cleaned off regularly.
Note: Adjust brightness and contrast for optimum viewing.
Simple solder tweezers
Micro-mark offers a newer version that appears to be identical, hopefully with improvements.
For more serious work and at much greater expense, full blown units are available. With foot switch controlled adjustable timer and current adjustment, repeatable processes are handled more easily. This requires a learning process with practice on scrap plus notes on successful settings.
Note: Adjust brightness and contrast for optimum viewing.
American Beauty resistance soldering unit.
The upper right knob adjusts voltage read on meter below it from 0 - 3 V to control heat. Time can be set from 0 - 15 sec, indicated by needle on meter and adjusted by knob below. A toggle switch enables a spring loaded timer start solenoid or turns it off. Time can be totally controlled solely from a foot switch on floor for pulsing operation.
A wide variety of tweezers and probes are available in different sizes. Maximum wattage varies with size. Care must be used to avoid flux and solder contamination on tips which can reduce contact and cause erosion. With some practice, high heat can be applied rapidly by pulsing, without disturbing other joints.
Note: Adjust brightness and contrast for optimum viewing.
RESISTANCE PROBES
Small tweezers.
Medium tweezers.
Large tweezers.
Twin fixed probes.
Single large probe.
While the smallest tweezer is useful for delicate work, the medium one is very useful for assembling frogs and other track work. The upper probe has two hot tips to bridge joins in tight places; while the large lower single tipped one requires a clamp elsewhere.
Soldering proceeds normally, but some fluxes are not very good conductors and reasonable metal to metal contact is required for heat producing current conduction through joint. Some parts can be readily heated and plucked for removal with tweezers. With a steady hand and careful release, they can be set in place.
A more recent entry into the field, the Coldheat battery powered soldering "iron" is useful for quick, small, wire type work, like handrails or piping. The bifurcated tip with an insulating separator, acts as a twin probe and should be used as such.by spanning the gap between pieces. In addition to the supplied bevel tip, a chisel and a conical tip are available. The temperature can reach 700o in 0.3 sec. This is more than sufficient for soft solders. The quantity of heat supplied is low, which limits the size of work. Low temperature solders will expand this. With the elimination of cells and addition of leads, this could be made into a useful, delicate work handpiece for a resistance soldering unit.
Note: Adjust brightness and contrast for optimum viewing.
Coldheat soldering tool.
Resistance soldering is not a panacea. There are many jobs where other devices are superior.
SOLDERING-HEAT SOURCES
BACK TO METHODS INDEX
