Advice on Resistance soldering

[BufferParticipant @buffer]

Hi

I want to make a resisitance soldering unit for building my tender bodywork and other projects. I was trying to do something similar to Gerald Wingrove as described in his book. I know about using an isolation transformer and don't stick your finger on the mains etc

The advice I was looking for is what is the best or simplest method to control the primary side of the transformer in order to vary the heating on the secondary side. I have just read an old article in MEW about a band saw welder but the control method is a bit sketchy.

I know some people may be a bit reluctant to give this sort of advice as there are a lot of plonkers out here but I am not really doing anything different than making an instant heat soldering iron but with a variable heat control.

Thanks for any help.

Rich

[BufferParticipant @buffer]

[Michael GilliganParticipant @michaelgilligan61133]

Richard,

May I suggest that you look at the other, currently active, thread on the same topic: **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=136086&p=1

… and especially, follow the link I provided to a previous one.

The "American Beauty" devices are 'definitive' and there is a load of information on their site.

.

The best way to vary the mains side of your transformer would, I think, be to use a Variac [*]  … but you may get away with an electronic "drill speed controller" or "dimmer"

.

MichaelG.

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[*] https://en.wikipedia.org/wiki/Autotransformer

Edited By Michael Gilligan on 09/06/2018 21:39:46

[BufferParticipant @buffer]

Michael

Thanks. When i used the search in the top right corner it looked to me as if nothing came up but I now see it does.

regards

Rich

[SillyOldDufferModerator @sillyoldduffer]

Posted by Richard brown 1 on 09/06/2018 20:44:11:

…

The advice I was looking for is what is the best or simplest method to control the primary side of the transformer in order to vary the heating on the secondary side.

…Rich

Unfortunately I don't think there is a straightforward safe way of controlling power input to the primary of a transformer without modifying the coil or front-ending it with another transformer like a variac – not cheap!

The most usual solution is to control power output by tapping the secondary side of the transformer and/or by combining a number of secondary windings. Or by moving the magnetic core. I assume you are reusing a transformer that doesn't have either option!

A dimmer or drill-controller might work, but I wouldn't do it. They work by chopping the mains up making sharp waveforms that aren't really compatible with transformers. Chopping produces voltage spikes that can puncture a transformers internal insulation.

In total plonker territory, you could make a rheostat from a bucket of salt water and two electrodes. Lowering the electrodes into the water varies the resistance and could be used to control power fed to a primary. This method was much used to control lights in the early days of electrified theatres. Didn't last long because of the many electrocutions, gassings, fires and operational inconveniences. DON'T! Buckets of water were soon replaced by properly designed wire-wound rheostats, much safer, but expensive and difficult to find today.

The other thread mentions using filament light bulbs as resistors. That's a possibility, but designing and wiring up safely requires a level of skill. It also mentions rewinding Microwave Oven Transformers. The construction of the examples I've looked at all made it easy-ish to remove the EHT winding and replace it with a home-made secondary. Winding a new secondary is straightforward because you only need a few turns of thick wire, and you can tap it every turn if you want. (Professional units all seem to have three taps between 2v and 5v.) You don't touch the mains primary: wiring it up safely still requires care (box, fuse, switches, earthing etc. etc), but there's less to get wrong.

Be quick though – transformers in microwave ovens are disappearing in favour of a switch-mode EHT supply that cannot be adapted.

Usual warnings about messing with mains electricity! In particular be sure you understand your limitations. Fools jump in where angels fear to tread. Confidence is no substitute for knowledge and experience.

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Dave,

Thanks for elaborating upon my "… you may get away with…"

MichaelG.

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P.S. … Richard has not yet told us what currents he has in mind

… If reasonably small; it may be practical to adjust the output side electronically.

Edited By Michael Gilligan on 10/06/2018 10:38:21

[Frances IoMParticipant @francesiom58905]

I doubt if any microwave oven made in last 15 years contains a transformer – I know that when I had to replace my 1980s very heavy model about 15yrs ago I was surprised at how little the replacement weighed.
If you know of one it is probably worth buying a replacement for the value of any re-usable transformer

[BufferParticipant @buffer]

Hi

I already have a microwave transformer and have taken out the secondary winding.

I don't know what current I need so cant answer that one I'm afraid. I am guessing a power output of 200-300 Watts.

I wanted to do this after seeing the Gas Turbine Builders Association stand at the Warwick Model Engineering show. They had a spot welder on their stand that was being demonstrated welding stainless steel. It was made from a microwave transformer. It used a timer from Maplin to control the on/off time of the weld. I have the plans for this welder from the GTBA but i would like to use this to just heat brass for soldering and not welding. Hence my question about controlling the output so I dont melt my brass.

Thanks for your advice above

Rich

[Joseph Noci 1Participant @josephnoci1]

Richard, Heat is a bulk effect, and the heat input to the job can be controlled by the time the current flows for – High current for a short time or opposite. I would put two to four turns of light welding cable – the 80 amp or 100 amp type, with many copper strands making it easy to bend over the core. Start with 4 turns, and fit a 30 amp Solid State relay on the mains input – they are controlled with a 0 to 5v or 0 to 24v DC input line, isolated from the mains. They are also Zero-volt switching so clean and low EM noise.

Make up a few test samples to solder. Fit a pushbutton the the +24V ( or whatever) control on the relay and activate the button for a short pulse – 1/2 second or so, If the sample vaporises, remove one turn from the transformer, and try again. Repeat…When you manage a 'controllable' button press period, ie, in the 1/2sec to a few seconds range, the the transformer output voltage is kind of in the ballpark. The fit a simple part-seconds timer in place of the pushbutton so your hand are free to do the work – maybe activate the timer with a foot switch.

I have attached copper studs to brass sheet in this way, and found it easiest to apply up to 4 or 5 short pulses, each heating the job up more, and stop pulsing as the solder liquifies nicely..

Joe

[SillyOldDufferModerator @sillyoldduffer]

Interesting development : spot welder vs soldering. Many similarities but the application of heat is rather different.

1. Both use a transformer to step down voltage and step up amperage. It's amps that produce a heating effect, and you only need enough volts to cause current to flow. Depending on the job, this could be as low as 1V or – unusually – as high as 6V.
2. For resistive soldering, you want sufficient heat to raise a substantial mass of metal to soldering temperature, say 250C. The amount of current needed depends on the size of the job. Electric soldering irons are typically 10W, 15W, 25W, 50W or 250W. Smaller irons for fine electronics, the bigger ones for coarse electrical work or de-soldering. For modelling, (more metal requiring heat than electronics but not massively so), I'd suggest a Resistance Soldering Unit delivering 200W would do most jobs, and likely less.
3. As Joe says, a good way of controlling the amount of heat is to vary the length of time power is applied. When soldering, the amount of heat delivered can be controlled by the operator lifting a hand-held electrode off the job as soon as the solder melts or by turning off. A foot-switch is convenient. You don't need a timer, just enough amps to melt the solder in the first place.
4. Spot welding is rather different. Welding requires a much higher temperature, and it's important to remove power as soon as the metals reach welding temperature, say 1300C. If overdone the small mass of metal between the electrodes melts leaving a nasty hole. The best way to control spot welding is with a timer – human reaction times are too slow. Electrodes pinch the metals to be joined and a pre-calculated pulse of power is applied, just enough to create the weld. Pulse length is looked up in a table, or found by experiment. Spot welding is ideal for repetitively joining identical thicknesses of sheet metal together. Not like soldering at all.

To be clear

• Soldering – apply carbon electrode to the job and operate foot switch. As soon as the solder melts, switch off or move electrode to heat further along the job. Semi-skilled, no timer.
• Welding – pinch work between tungsten electrodes and set timer to make a weld. Operate foot switch to deliver a single pulse. Move electrodes and repeat. Once set up, largely automatic – unskilled.

Dave

Edited By SillyOldDuffer on 10/06/2018 13:57:55

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Richard brown 1 on 09/06/2018 20:44:11:

I want to make a resisitance soldering unit for building my tender bodywork and other projects. I was trying to do something similar to Gerald Wingrove as described in his book.

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Not sure which of Wingrove's books [*] you're referring to, Richard … but the pictures here http://www.geraldwingrove.com/Falls_of_Clyde/Gallery-9.html#5 seem to show a pretty basic handpiece on quite a small scale:

http://www.geraldwingrove.com/Falls_of_Clyde/Gallery-9.html#5

Is this roughly what you have in mind ?

MichaelG.

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[*] http://www.geraldwingrove.com/Books/Books.html

Edited By Michael Gilligan on 10/06/2018 14:43:00

[Michael GilliganParticipant @michaelgilligan61133]

I've just found this **LINK** which might be useful.

Resistance Soldering: Transformer

MichaelG.

[BufferParticipant @buffer]

Joe thanks for the info, that seems like really good advice which I will probably try. I'm not really sure why you would need a relay isolated from the mains to switch it on and off. What is wrong with just a normal foot switch, switching the power to the primary side? I'm sure as long as all the electrics on the mains side are safely concealed the only danger as far as I can see is getting burnt by the secondary side and not shocked as its an isolated transformer and only a couple of volts. or am I missing something?

Michael That is exactly the thing I had in mind. Gerald Wingrove emailed me this morning and told me he used a variable transformer on his but it has very recently died in a puff of smoke. So he is getting an American Beauty but they are very expensive.

Thanks

Rich

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Richard brown 1 on 10/06/2018 17:18:47:

Gerald Wingrove emailed me this morning …

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Wow, Rich … I'm impressed !!

MichaelG.

[BufferParticipant @buffer]

Well i emailed him yesterday.

[Mike PooleParticipant @mikepoole82104]

It is possible to control the input to a transformer with a back to back pair of SCRs, this was wisely used for spot welder so. Another method is to wind another winding on the transformer and applying a DC to it which will saturate the core iron and limit the current transfer of the transformer.

Mike

[BufferParticipant @buffer]

Mike you lost me at scr! Rich

[Mike PooleParticipant @mikepoole82104]

I was in a bit of a rush to go out and didn't read that before posting. My iPad is sometimes helpful with its suggestions but that help just made gibberish.

We will try again, a silicon controlled rectifier (SCR) can be used to delay the point in the sine wave that the current is switched on so the amount of power can be controlled that is being input to the transformer which will be reflected in the current applied to the load. As both positive and negative half cycles need to be controlled a pair of SCRs will be required and a circuit to trigger the gates at the required time. This control technique was widely used for what was termed heat control on spot welding transformers.

If a transformer has an extra winding installed which a DC current can be applied to, then the transformer effect which relies on the magnetic circuit provided by the iron core of the transformer can be reduced. DC will have the effect of saturating the iron in the magnetic circuit so it will limit the transformer effect and thus the current produced in the transformer secondary circuit. This technique has been used for weld current control and drive control, even audio amps have been built with this technology. Look up saturable reactors and magnetic amplifiers.

Mike

[not done it yetParticipant @notdoneityet]

Wouldn’t a simple triac be the norm?

[Mike PooleParticipant @mikepoole82104]

Triacs need a bit more work to control reactive loads but would probably be ok in the application with a well designed gating circuit.

Mike

[Joseph Noci 1Participant @josephnoci1]

Richard, no problem using a foot switch directly, I suppose. Depending on the size of your transformer, it should work fine. There bigger you go though, the worse the arcing becomes at contact closure, as the contact closure/opening can occur anywhere on the slope of the applied mains voltage.

All the other suggestions of SCR's and triacs can also work, but there is a lot of buggering around, finding some resistors to drive the gate , where do I find a Diac, , etc, etc, etc – by the time you sort it you have lost interest!

You really do not need variable control of the mains cycle to control the heat in a setup you are after – duration of current flow is all you need.

This is a solid state relay – Just connect the transformer one wire to Neutral, the other to terminal 1 below, and terminal 2 to Live.

Connect a 9v battery via a foot switch to terminals 3 and 4, and no mucking about. The relay switches when the mains voltage crosses through zero, so no surges, etc. Relay costs a pound or two..

As long as you keep the high-voltage mains side covered/insulated, you are quite safe – the secondary will not hurt you..The solder joint may burn you, but that's your choice.

Joe

[BufferParticipant @buffer]

Mike, Joe thanks for your advice I will try the relay method.
Regards Rich

[Michael GilliganParticipant @michaelgilligan61133]

I don't know how widespread the problem might be, but this makes interesting reading:

**LINK**

http://www.instructables.com/id/The-inner-workings-of-Counterfeit-FOTEK-SSRs/

MichaelG.

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Edit: here is a direct link to the UL page with the photos:

https://www.ul.com/newsroom/publicnotices/ul-warns-of-solid-state-relay-with-counterfeit-ul-recognition-mark-release-13pn-52/

Edited By Michael Gilligan on 11/06/2018 10:03:46

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