Repairing thermocouples by spot welding

[Flyback]

Hi,
All out thermocouple ends have come apart.
We need to spot weld them.
But the welder costs £1800

https://uk.farnell.com/labfacility/xf-1250-far/thermocouple-welder-5-10-min-250vac/dp/2897470

So do you know what size capacitor is in there?, and to what voltage they charge it for the weld?
Also, why does the welding "connection" surface have to be a block of carbon, (which also costs £35)

 

[Nigel Goodwin]

Simple answer, don't buy such tools from Farnell (or RS) - why go to the most expensive supplier you can find?.

However, what are you doing to break the thermocouples?, and why not just replace them - preferably with ones that don't break.

 

[Diver300]

The welders may not discharge the capacitors directly into the weld, there may be a transformer.

Where I used to work we had a projection welder for putting the lids on DIL-14 oscillator. It could store up to 2500 Joules in a big capacitors and it discharged into the welding transformer, which was very near the welding jaws. The welding transformer had a single turn secondary, and was connected with about 500 mm2 of copper. The current peaked at around 100 kA.

 

[jpanhalt]

We've been around this track before with Flyback somewhere else. Not all spot welders are the same. Capacitor discharge welders have specific purposes. Others are transformer or inverter based. The latter can impart a lot of heat, and that can burn back insulation. I tried a capacitor discharge welder for battery tabs, and the welds were not reliable. With very fine wires, it might work better. The best DIY method I found for TC's is a TIG welder. In school, we used an oxy-hydrogen torch. My tools are shown in the first link. Examples of my practice welds in in the second. They are indistinguishable from the commercial TCs from Fluke.

The wires can be twisted but that is not necessary so long as they are very close to each other (it's a fusion weld without filler). Slide them in the slot of the copper strip and be sure that is in good contact with the grounded, heavy copper plate. With a low setting that gives a stable arc, strike an arc on the copper plate and carry it across the TC leads you want to weld. It is almost instantaneous.

https://www.eevblog.com/forum/index.php?action=dlattach;topic=316504.0;attach=1438987;image

https://www.eevblog.com/forum/index.php?action=dlattach;topic=316504.0;attach=1438993;image

Edit: Those picture came out way too small. Hopefully it is fixed.

John

 

[danadak]

Thermoelectric effects due to junctions = errors, page 24-58 on

See attached.