Welding thermocouples

I usually just silver solder the ends together with a small butane torch.

Greetings, all.

I'm looking for a better way to weld thermocouples than my present car-battery-and-carbon-rod method. The TCs are 24 gage type K (chromel-alumel). I'd like to keep this inexpensive, preferably from junkbox parts. (I only need to do this about once every month or so.)

Extensive Googling hasn't turned up any circuits, etc, but I gather that a capacitor discharge arrangement might be just the ticket. Anyone have any idea on voltage and charge? And, how to actually do it? I'm afraid that if I just poke the twisted wires at the capacitor terminal, I'll either weld everything to the terminal or spatter hot metal everywhere.

Or would I be better off to build a high-current supply and emulate the car battery? My main complaint about the car battery is that it is inconvenient to go out and mess around with jumper cables, etc, if it happens to be raining or cold.

Or, any better ideas?

Thanks!

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

Unfortunately 20,000uF of aluminum electrolytics at 6.25V will yield a really pitiful snap upon short circuit. That only corresponds to 390mJ of energy, with the majority of it going up in the ESR of the capacitor(s) and the contactor used to complete the circuit.

Now if you crank the voltage up to say 35V, perhaps crank the capacitance up to 47,000uF, use low resistance wiring all around, and replace the mechanical contactor with an IRF2804 MOSFET, you might have something that produces spot welds. Or perhaps you will have just made yourself something that makes a big mess or destroys itself.

Datasheet for MOSFET here:

John, I forgot to mention that this is for use in a pottery kiln at temperatures around 2200F. Too hot for silver solder!

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis

At a lab I worked we used the arc of a carbon-arc lamp fitted to an extremely elderly plate-camera microscope. Also worked for Pt/Pt-Rh. :lol:

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Hi, Bob. If you're welding the two wires, you want one and only one weld nugget. That would lead you to cross the wires like an "X", and place the junction of the "X" between the welding tips.

Fritz' advice above is good. When I wrote the first post, I was thinking for some reason about welding 30AWG wire (the most common at a prior job). Obviously you'll need a lot more energy for 24AWG wire. His advice of higher voltage and more capacitance is a good one, with either a series resistor or a voltage regulator to supply 12VDC for the automotive relay coil.

I'm not quite as enthusiastic about the MOSFET, first because of added circuit complexity, and also because of the possibility of exceeding maximum current. Of course, you can do that with a relay too, but in the event of failure you'll probably just end up with a spot-welded relay contact. Since we're talking about junk relays, you should be able to pop and swap easily and inexpensively. The series resistance of a homebrew setup, along with the inherent sturdiness of the larger automotive relays, will probably be enough to prevent this from happening too soon. If you pick a common type of car, you can snag junkyard automotive relays for a buck a pop and have as many usable spares as you want.

Whether you eventually find a dental/jewellers' welder (first choice, and you may also look for the smallest handheld or tweezer spot welders) or go homebrew, you should set up the weld schedule by looking at the welds produced. Your weldment should be a little more than half the wire diameter, with no signs of arcing or splashing. Tear the wires apart to examine the weld nugget. You'll have to experiment to find the correct voltage and pressure settings. Once you're close, tweak the pressure for final adjustment.

As to the welding tips, they're also called spot welding electrodes. Go to a welder supply house and get a couple with 1/16" flat tips. They're only a couple of bucks apiece, and they'll open a package if you ask nicely.

Good luck Chris

Chris, I haven't had any luck locating a dental welder, so I want to have a go at the "junkbox special". But I'm still not clear on the details of using your nice circuit. You mention crossing the wires instead of twisting, and you mention "welding tips". Is the crossed-wire junction supposed to be pinched between the tips, as in a spot weld?

Thanks, and sorry to be so dense. This is all really new to me and I haven't been able to find any good practical info yet... besides yours!

Best regards,

Bob Masta dqatechATdaqartaDOTcom D A Q A R T A Data AcQuisition And Real-Time Analysis