resistance soldering

I would guess because the charger would be pulling a load with nowhere to pass it on. So it would turn to heat where you don't want it - smell of frying transformer windings.

The 'input' (115 VAC, likely) is relatively low current (perhaps one or two amps), and easily switched.

The 'output' (to the soldering appliance) is very HIGH current (like

20-50 amps), and difficult to switch properly. It would take a BIG switch to handle such current properly (like in a Frankenstein movie), not the usual little 'foot switch'.

Dan Mitchell ==========

marv>

Resistance soldering apparatus is normally straight 'AC'. I can see no advantage to the added complexity of using 'DC' for this purpose.

The relative arcing depends a great deal on voltage, current, and frequency. A lot also depends on the relative inductances in the circuits being switched. Low frequency (less than 25 hz.) 'AC' acts pretty much like 'DC'. At the normal power line frequency, 50 or 60 hz., there is some difference, that is occasionally important as in 'AC' vs. 'DC' welding. Truly high frequency 'AC' arcs EXTREMELY well ... making sparks several feet long! It's beginning to try to 'broadcast'! It's near impossible to turn it 'off' with a normal switch arrangement.

At even higher power levels, as in commercial substations, arcing is a BIG problem, and you need special breakers (air blast quenched, etc.) to cut the current flow.

High power (voltage and/or current) 'AC' and 'DC' are both difficult to control, and high current is what we're talking about with the output of resistance soldering apparatus. That's high current in relative terms of what most are used to in model railroading. 'Breaking' a 50 plus amp inductive circuit under load is a different animal than switching a model railroad block or turnout.

Dan Mitchell ==========

Charles Kimbrough wrote:

What you describe might work for VERY light soldering, but five amps is nowhere NEAR enough for soldering rail. Good Grief! We run five amps to some of our TRAINS (four powered units on 15 lighted passenger cars), continuously! The rail doesn't even get noticeably warm! With DCC some of the larger clubs are carrying 10 amps or more in the track. NOT good when you get a short circuit, but even then it's not the rails that get hot and 'cook'! Power trucks and wheelsets, yes.

For soldering rail properly you'll need 25 or so amps. With resistance soldering (and other forms too, to do it right) you have to get the joint hot FAST. It should be almost like spot welding. 'Psst', and it 's done. That takes POWER.

Dan Mitchell ==========

marv>

I think the arc that people were referring to is in the switch when it breaks the circuit.

Please check this site on building a resistance solderer.

I believe everything I have will work with one exception. This site plainly states the foot switch should be in the primary [plug side] line. The other sites I found did not make this point clear.

Thanks for all the comments.

Marvin & Sue

A starter relay from a Ford automobile easily handles three times that current and only takes half an amp to energize. It can be had for less than $15 at your local auto parts store, and even has a diode across the coil to supress back EMF.

You don't need a huge switch to handle 50 amps.

Yes, but you can also get an arc if you purposely or accidentally (easy to do) release the contact pressure on the soldering probe or tweezers. Also if you inadvertently get paint or oxidation on the contact surfaces. It's quite easy to get an arc TO the piece you're soldering to ... and this is RARELY good (often burns a small 'crater' in the workpiece). Excellent electrical contact is neccessary!

Like **ALL** soldering, resistance soldering requires CLEAN metal, a CLEAN soldering appliance, and proper technique.

Dan Mitchell ==========

JimS wrote:

In a car it's also in a 'DC' circuit with little inductive reactance. And, how many 'cycles' (on/off) is the relay rated for? A few times a day? It's easy to trip a resistance soldering unit 50 or 100 times in one soldering session. A few soldering sessions could put many years of service on the relay contacts. How much soldering are you planning on doing? Lots of things work 'sort of', for a while.

Still, it might work. And there are LOTS of other switches that WILL handle the 50 amps. Most are big and expensive.

But **WHY** would you want it to do it? It's **SO** much easier, safer, and more efficient to just switch the primary, as many have pointed out!!

Dan Mitchell ==========

Phil Dumpster wrote:

In article , snipped-for-privacy@umflint.edu by Daniel A. Mitchell dropped his wrench, scratched his head and mumbled,

(snip)

Refer to Nikolai Tesla and the great Colorado experiment:

Bob

Hi Marvin,

As an (ex) Power Transformer Design Engineer, I can agree with some comments made over the last day or two, but disagree with others!

Here are a few comments that I hope will be helpful to all interested in Power Circuit design....

Transformers, if designed and wound correctly, usually draw far less current when 'idle' - under no, or light loading. It will, therefore, do no damage to power-up the Transformer without loading it. IF there is appreciable heating under no-load conditions, it is being driven at too high an input voltage, and may need to be connected to a higher input-voltage terminal/tap. The output will still provide a high current, but the whole circuit will be running at better efficiency.

The absolute best Transformer for this kind of job is a Toroidal. The design is the most efficient traditional Transformer type, and can be designed with the minimum of wire required to carry the currents. Efficiency is very high, and thus heating is kept to a minimum. They are also easy to modify for the required output Voltage and Current. They are often supplied as kits (do Radioshack still do these?) for the user to finish off, to their own specification.

Switching is almost always done on the primary/input winding. This is especially true when the secondary is to provide a high current! In Low-Voltage High-Current circuits, standard types of switches do tend to have some appreciable resistance, which at high currents will create heat. This in turn increases the resistance of any copper wires and components in the vicinity, making even more heat!!

Copper winding wire increases it's resistance by roughly 0.4% per degree centigrade, which soon adds up with a significant running temperature rise that may approach 70 degrees at full load... so things can reach the point of thermal breakdown very quickly, in a circle where : Heat=More Resistance=More Power-Loss=More Heat!

Some switch contacts may 'weld' together, if used to control the high currents at the output, so it is nearly always a more reliable option to place the switch in the live input supply lead, with a suitable suppressor. If switching HAS to be done in a high current lead, use a 'Contactor' with the correct current rating, rather than a standard switch. The Contactor coil can then be activated by a footswitch, keeping the heavy current path completely separate from the footswitch contacts.

Another possibility is to use an electronic light dimmer card to do the switching on the input winding. This would also separate the actual switch contacts from the welding circuit current altogether! Just replace the rotary dimmer control component with an on/off switch.

One last 'rule' - you can never have too much cooling! Use lots of heavy metal panels to mount power components on, shunting away heat, and include some good Cooling Fans to blow all of that hot air away!

David.

Thanks for your comments. I had already decided [due to other comments] to put the foot switch in the primary side. I have all the parts I need to build the resistance soldering rig, and hope to get it working in the next week or so.

Marvin