How To Tac Weld Thermocouple Wire?

even better (costs less). thanks

No. use copper. However, there is a special copper alloy that is designed for this sort of application. One old trade name is Elkonite and it's copper with 1-2 % of something added. I have used it and it performs no better than copper, but the tips do last longer.

Another technique which might suit you better is a custom-made spot welder. The one I made is from an old microwave oven transfomer with the HV secondary chopped off and rewound with about 4 turns of the biggest wire I could fit. The primary is switched with a solid-state relay (A Croydom 2410) driven off a 555 timer that can be set from about 50 mSec to 500mSec.

The work anvil is a solid slab of brass, and the spot probe is a hand-held small "pencil" of the copper-? alloy. While it's prime purpose is not to weld TC wires, it does a good job of that.. It takes a bit of practice to place one wire on top of the other, the probe on the wire junction and then hit the trigger switch (I use a knee-operated switch) but after a few false starts, you will get the feel for it. One variable that is important, and not that easy to control, is the pressure of the probe. I have been tempted to arrange a spring-loaded jig to get control of that, but the hand-held probe makes the whole device much more versatile, so I just live with the need to experiment a little more with new applications.

I have used this to spot-weld a TC pair onto a 1" dia fender washer.

Barry

a small benchtop unit with spring-loaded upper copper electrode is what i will attempt to fab up

thanks for the info

where is best place to put the control SCR? on the ground electrode or postive electrode? the weld is basically a short with some resistance, which yields a brief transient high current. what type of SCR is being used? is a

10A version good? something bigger ??

It should go like this: (View in a fixed pitch font)

VCC + Electrodes | ___ ----|___|---|------------ ---| | | | + | SCR Energy cap ### _|_ --- ___ V_A | Trig -|___|- / | | | Gnd | | ------------|-----------------------| created by Andy´s ASCII-Circuit v1.22.260103 Beta

I'd use the largest SCR I had access to, as the peak current will be pretty high. According to some notes I made at the time, the original shop-built unit used a 2N5169, a 200volt 20Amp pressed stud device.

Barry

so the "load" (the electrodes) should go before the SCR

+Vcc(cap)--electrode--tc wire--electrode--SCR-gnd

here's a pic of a DIY TC wire clamp, uses #4 solid wire soldered to some copper plated steel. then epoxied to the clamp feet. it pinches TC wire rather easily, i'm just waiting on a 55amp SCR to arrive so i can test it out.

(pic approx 100kB in size)

Correct

That looks a pretty neat solution. I may make on myself for the next batch of TCs

ok, so i got my SCR, wired it up, and........................ no go, not enough juice in the cap.

17k uF at 60v is not enough to weld 24ga tc wire. i'm gonna try adding another 17k uF to see if that will work. if not i'm gonna resort to something a tad better, a 555 variable one shot that feeds the gate of a N-MOSFET with a 12v car battery as the source. pleanty of current with the ability to time the pulse, etc.

adding

...sooo much easier just to pop it into an oxy-acetylene flame for a second or two !

AWEM

i no have oxy-acet setup. thus far the parts cost me only ~$25. adding a 555 and mosfet will only be another few $$ (and i have the battery).

Something sounds wierd there. 1/2 xCxVxV should provide plenty of energy, I have just finished stripping down few used one-shot cameras. The flash charge cap (320uF) typically still holds about 180 volts, and it's plenty to take the tip off a screwdriver.

Did the wire stick at all?

Barry

all i saw was a spark, but the wire did not stick at all. i'm using 12ga stranded zip cord between cap-scr-clamp, total electron path length is approx 36", perhaps the resistance in the wire is causing me the issue??

i added another 17k uF (power was 34k uF at 60v). the wire sparked and actually stuck to the copper. this jolt also killed the SCR !!

We have one of the machines previously mentioned at work. I had to make up a bunch of TC's for a project and as soon as I got the technique down, the rest was easy. Talking to others who also had problems getting the machine to help them do it right, I realized that the machine wasn't the trick as much as the setup. I start with twisting the wire ends together pretty tightly. About five or so twists. Clip the twisted part off except for about two and a half or so turns. I twist the wires together using the same tool (welder cable connected) that I zap it with so that nothing gets disturbed. We have an assortment of pliers for the job, but I like the ones that look like slip joint gas pliers. Lightly make contact between the twisted end and the other electrode. Don't push too hard. Make sure you're comfortable and don't keep pushing after the spark occurs or you'll flatten the ball. Use the same technique each time, trying it a few times before you change something else. There's a whole lot of variables in making a good joint, and you don't want to change too much at one time. With our machine, it took some fiddling to get the current and time between shots just right (it continues to charge even after the charged light went on.)