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Welding thermocouples

Discussion in 'Electronic Basics' started by Bob Masta, Jan 27, 2005.

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  1. Bob Masta

    Bob Masta Guest

    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
    www.daqarta.com
     
  2. I usually just silver solder the ends together with a small butane
    torch.
     
  3. Chris

    Chris Guest

    Hi, Bob. Thanks for all your perceptive posts -- glad to be able to
    help.

    If I were in your shoes, I'd be thinking about finding a small dental
    welder at a garage sale or on ebay. They're practically
    indestructible, already have the hardware built in, and will give you
    consistent welds. Also, you can get 'em cheap. They're just about
    perfect for welding T/Cs.

    There's more than just applying voltage in making an adequate
    resistance weld. Voltage losses and pressure on the wires to be joined
    are just as critical. When you get a small dental welder, you buy a
    consistent solution to these problems.

    However, if you're using a car battery and carbon rods now, you could
    probably do better with a home brew setup. Get a good sized 12VDC wall
    wart and a big automotive 12VDC coil relay (like the ones used to drive
    the starter solenoid) and do something like this (view in fixed font or
    M$ Notepad):

    | Ersatz C-D Welder Control
    | ____
    | + | | ___ +
    | o---o-o--|317 |--o-|___|-----. .----------o----.
    | | | |____| | 1K | | |
    | | | | .-. | | |
    | | | | | | o /o |
    | - C| | | |240 CRY1 / |
    | ^ C| | '-' / |
    | | C| | | o V
    |12VDC | | o-----o | Welding Tips
    | '-o | | +| X
    | | | | C ---
    | | o .-. | --- ^
    | |=|> | |<---' | |
    | | o | |1K | |
    | | '-' | |
    | - | | | - |
    | o---o-o----o-----------------o--------------o----'
    |
    created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

    A lot depends on your setup, but I'd start out with one or two 10,000uF
    caps for C.

    As far as it goes, I would really recommend using the money you saved
    and stopping by a welder supply house for a couple of small welder
    tips. They're not too expensive, and many shops will sell them in
    small quantities if you ask nicely.

    Try to set up something so you have a consistent pressure on the weld
    nugget through the weld tips (possibly a weight). And don't twist the
    wires before welding. Try to just cross them in an "X", so you only
    have one weld nugget. Adjust your voltage (1.2V to 6.25V will give you
    a 25:1 power range) so you get good welds.

    Of course, most caps aren't meant for crowbar-type discharge, but you
    said you were looking for a junkbox-type solution. Keep an eye on your
    caps, and periodically test them for damage. And remember to keep your
    welding tips clean.
    Glad to be of help to you for a change. Thanks again.
    Chris
     
  4. Chris

    Chris Guest

    You're right, Mr. Popelish. This is a better and more obvious "easy"
    solution to the OP problem than a homebrew CD welder. If there's any
    oxide on the surface of the T/C wires, it helps a lot to manually
    remove that with fine sandpaper before silver soldering.

    I still like my jeweller's welder, though. If you're sweating limits
    of accuracy, a weld will give you slightly better results than silver
    solder, I think.

    Chris
     
  5. Yes, especially if you heat a significant length of wire above the
    bead. I clamp both wires in a heat sink and just cross them. Then
    heat the extended ends till the solder bonds the cross. Then clip the
    ends off. Heating to red heat can slightly change the exact
    properties of the wires, but as long as this is kept very near the
    ends, it doesn't change the output voltage which develops along the
    wires as the temperature changes from the sensed temperature to that
    at the measurement circuit.

    But if the thermocouple will measure a temperature that has a large
    gradient along the wire, it is best not only that the couple is
    welded, but that it is welded to the surface being measured. The two
    wires can even be welded to separate spots on the surface being
    measured.
     
  6. Bob Masta

    Bob Masta Guest

    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
    www.daqarta.com
     

  7. 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:

    http://www.irf.com/product-info/datasheets/data/irf2804.pdf
     
  8. cpemma

    cpemma Guest

    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:
     
  9. Bob Masta

    Bob Masta Guest

    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
    www.daqarta.com
     
  10. Chris

    Chris Guest

    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
     
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