battery tab welder (scr question)

Discussion in 'General Electronics Chat' started by bomrat, Jan 29, 2010.

  1. bomrat

    Thread Starter New Member

    Dec 13, 2009
    25
    0
    hi, i am trying to build the battery tab welder on the internet.

    one small hickup, i purchased 2 scr's off of ebay. and they are different from the plans.
    the one in the plans is a scr
    the one from ebay is a phase control scr (it has another lead marked cathode potental)
    i hope this would work in my setup, i have a schmatic of the welder with the scr in place, and i moked up a scr with another lead marked cp for cathode potental. can i use this part?
    thanks

    datasheet for scr:
    http://www.digchip.com/datasheets/parts/datasheet/375/T600141804BT-pdf.php
     
  2. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,698
    904
    It looks to me like the lead labeled cathode potential simply is another connection to the cathode. You could use that and the gate lead for firing the scr. It should work.

    However, that schematic has a major flaw. Once you make the scr conduct, your 12V supply will be shorted across the battery tab. There is no way to turn the scr off, as current will flow continuously. I hope you have a way to protect the scr and power supply.

    John
     
  3. VoodooMojo

    Active Member

    Nov 28, 2009
    503
    53
    The SCR will turn off whenever the electrode is removed from the piece being welded.
    It will work fine. It is what is called "short circuit welding"

    the attached pdf is (one of) the most common scr used for diy battery tab welders
     
    Last edited: Jan 30, 2010
  4. awright

    Well-Known Member

    Jul 5, 2006
    84
    7
    No, no ,no. VoodooMojo, what if bomrat is using a car battery for his 12-volt source? (Which reminds me, always use safety glasses when doing resistnce welding because molten metal can spit out of the joint.)

    The schematic presented might work with a small bench power supply that was current-limited by design or simply by being a weak power supply, but the design principle is extremely unwise. Shorting a high current power supply could quickly burn out the SCR and crater the battery tab or even destroy the battery. Moreover, with the circuit shown, the energy delivered to the tab is heavily dependent upon the timing and technique of the operator, since no timer is provided.

    The energy stored in the 2 farad capacitor at 12 volts is 144 watt-seconds, which is, I believe, rather high but not an unreasonable capability for battery tab welding. So there is no need for the "short-circuit" of the power supply after discharge of the capaicitor. Of course, peak current might be limited by vagaries of manual force application of the weld tip to the tab or the presence of contamination.

    The simplest method of resolving the problem (which I maintain is potentially serious) is to insert a power resistor in series with the power supply. This should be selected to limit follow-on current to a safe value for the circuit components while still recharging the capacitors quickly enough for the next weld. More exotic approaches could add a power transistor to open the circuit from the power supply whenever force is applied to the weld tip. Having a force-actuated welding tip to trigger the weld is advisable anyway to be sure that the proper amount of force is applied to the tip before a weld can be initiated. Tip force is an important factor in weld quality, along with current profile. Some means of reducing the voltage or capacitance might be handy.

    awright
     
  5. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The negative side of the 3V battery should be connected to ground. Also, a resistor is needed between the battery and the SCR gate to limit the input current.
     
  6. VoodooMojo

    Active Member

    Nov 28, 2009
    503
    53
    I have no disagreement with you but his schematic states a 20 amp 12 volt supply. Not quite an unlimited source of current.
    I am no novice to welding and would not have suggested a direct line weld using automobile batteries. Although it is an often practiced technique when no other welding gear is available.
    I, myself, use a stud welder when I weld battery tabs, but then I guess I am just one of the fortunate ones.
     
    Last edited: Jan 30, 2010
  7. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,698
    904
    I don't think so. If the trigger voltage source (i.e., 3V battery) is grounded, then the full current from the capacitor will go through that puny switch, instead of the scr.

    John
     
  8. mik3

    Senior Member

    Feb 4, 2008
    4,846
    63
    The negative side of the battery is connected to ground and the positive to the gate via the switch and an additional resistor.
     
  9. JimG

    Member

    Dec 7, 2009
    53
    0
    I built a similar welder several years back and just used a DPDT switch that toggles between "charge" and "weld." Not very clever, but it is simple :)

    In charge mode, the switch carries the charging current. In weld mode, it provides the control voltage/current to the gate of the SCR (in series with a momentary footswitch).

    I've made thousands of spot welds joining 24 ga SS sheets with my welder. I am on my second switch (the first one was a toy and only lasted a year or so).

    Jim
     
  10. bomrat

    Thread Starter New Member

    Dec 13, 2009
    25
    0
    would if i put a timer circuit in line with the switch / scr gate lead.. that should protect the powersupply some.
     
  11. bomrat

    Thread Starter New Member

    Dec 13, 2009
    25
    0
    how about this design. took a shot at the cap and resistor specs for the timer...
     
  12. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,698
    904
    There are many ways to address what I saw as a problem. One of the simplest would be to use a DPDT relay (like a cheap automotive relay) operated by the welding switch, presumably a foot switch. When the switch is pressed the charge circuit will be disconnected and the firing circuit activated. So long as the switch is depressed, there is no problem and plenty of time to remove the electrodes from the work piece. Then releasing the switch allows the capacitor to charge and reset the system.

    A small resistor in the charge circuit will help protect your supply and limit current to the 20A or so it can deliver. Another small resistor in the gate circuit (probably less than 2 ohm) will keep the SCR in its safe operating area (see its spec sheet). You can probably get by without it, but I would definitely consider something in the charge circuit, as mentioned.

    John
     
  13. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    Hope this is not off topic, but on every battery I have ever tried it was possible to just solder to them, although some need a bit of scratching first and some flux.

    If this is for making RC racing batteries for higher currents then soldered copper braid is better than welded steel strip anyway as it has less resistance.
     
  14. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,698
    904
    I use both soldering and welding, depending on the situation. The OP's question was for welding. There is a certain thrill to doing that, I guess. NASA also uses welding: http://mmptdpublic.jsc.nasa.gov/prc/7188D.doc

    I have run into situations where the space is simply too limited to allow soldering with braid, such as when the batter pack must fit a premolded case, e.g, an Airtonics Vision 8SP transmitter case.

    John
     
  15. CMH70

    New Member

    Mar 11, 2010
    9
    0
    VoodooMojo, i have often wondered if a stud welder like the ones from harborfreight would work for battery tabs. Would you mind enlightening me on maybe which one you use and how you do it? My interest in this is for my R/C hobby batteries.
    thanks
     
  16. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    He flexes his muscles and the female tabs 'just melt'...all over the batteries.

    No but seriously, the stud welder I have would be no good for the job, as is.

    You could use it as a power source and use the hand held electrode method. However, maybe someone else has more insight into the task.

    [ed]
    Is this the one you are thinking of?
    http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=3223

    If so, I think it may be a trial and error type thing, The timing is preset to a 1 second cycle which may be too high for thin battery tabs. It is set to weld 2mm copper. And it's 10 pounds.. tough to be accurate.
    [/ed]
     
    Last edited: Mar 17, 2010
  17. CMH70

    New Member

    Mar 11, 2010
    9
    0
Loading...