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Help with a one shot device

Discussion in 'Electronic Basics' started by Jeff, Mar 4, 2005.

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  1. Jeff

    Jeff Guest

    Hello all,

    While I took a few circuits classes in college, they are failing me now
    that I need them.

    I want a timing or control device that when I push a switch, it
    energizes a normally dead circuit for the set time, then the circuit is
    dead again. I also need this thing on the cheap, if that is possible.
    I have been looking on google, but getting confused about limitations
    of things such as a 555 device, hooking up relays and timers, etc. I
    could build it if I could just figure the layout.

    The main circuit is a 120V, 15A input to the primary side of a
    transformer and this mystery black box I want, with the secondary side
    being around 4V, and many amps. The timing device needs to be on the
    input side of this circuit. This is a homebrew spot welder if you were
    wondering.

    I am pretty sure that I need a relay and a timer, but need some help
    there. The switch voltage can be either 120Vac, or from a DC battery,
    does not matter, whatever is easier/cheaper.

    Hope that is enough info to get some help. TIA.

    Jeff
     
  2. mike

    mike Guest

    I could assume you have experience here...but just in case you don't...

    There are a BUNCH of problems.

    I used to think that to spot weld, you just clamp the material and stuff
    a lot of current thru it for several seconds until it gets red hot.
    That works fine for things that can stand the heat.
    If you're welding something like battery tabs, thermal fuses, things
    that can't stand the temperature rise, you need LOT more current for
    much shorter time. So the level of difficulty depends on what you're
    trying to weld.

    You might find that the inrush current is higher than 15A when you short
    the secondary. My spot welder made from a microwave transformer draws 40A.
    You want to switch the circuit on at a zero crossing. If you don't
    you'll have all manner of grief with blown relay contacts etc.
    You want an integral number of input cycles. If you don't, depending on
    the hysteresis in the core of your transformer, you'll get random weld
    power. In the extreme case, you leave the transformer in a state that
    causes it to saturate with the next shot. This blows up whatever you're
    using to switch the primary.
    Much of this gets solved by using a solid state relay with internal
    circuitry to "close" on zero crossing. The SCR will turn off at a zero
    crossing too. I use a 240V 40A SSR.

    It's pretty simple to build the counter and control circuitry into a
    single chip. Mine just uses an analog ramp to synchronously drive the
    solid state relay. The control is analog, but it adds a complete cycle
    at a time at each threshold point. IIRC, I got best results on battery
    tabs using about 6 cycles of 60 Hz. But it can vary a lot with
    secondary resistance. Work HARD on minimizing secondary path resistance.

    I posted a circuit at the link below to show how simple the whole thing
    can be. I can send you the pal equations if you're interested.

    http://nm7u.tripod.com/homepage/weldsch2.gif
    I'm a fan of minimilist designs.
    The design ain't pretty, but it works.

    If I were to do it again, I'd probably use a PIC processor just cause
    they're easier for most people to find and program. Once you bite the
    bullet and build the PIC/AVR/Whatever you like programmer, you'll find
    that most anything you can do with a 555 is quicker/easier/better done
    with a microcontroller.

    here's a link to the whole shebang.
    http://nm7u.tripod.com/homepage/welder.html

    So, I strung it all together. Tried to weld battery tabs.
    Electronics works perfectly, but the welds were tricky and unrepeatable.
    What you really need is a LOT more current for a much shorter time.
    Got MUCH more consistent tab welds with a commercial CD welder.
    I got lucky. The CD welder cost me less than the wire I bought to wind
    the transformer secondary.

    mike

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  3. I was going to reply with the typical 555 timer circuit, but I think you
    can get away with a simpler circuit, which requires no separate power
    supply. Here is one:

    http://home.comcast.net/~rcmonsen/buttonrelay.GIF

    Timing is controlled by Ct and Rt. Before a button press, the gate of
    the mosfet is at 12V. When you press the button, it gets dragged down to
    gnd, and slowly comes back up by recharging the cap through the timing
    resistor. Since I don't know how long you want it to stay on for, I
    don't know how big to make the cap and resistor. In general, the thing
    will turn off about when t = Rt * Ct seconds have elapsed, but that's a
    guess based on a turn on voltage of 8V for the mosfet. That may be high,
    but it's in the ball park.

    One thing is that because of the configuration, the action of turning on
    the coil has the side effect of pulling down the 12V rail slightly, thus
    speeding up the turn-off of the mosfet, and likewise the turning off of
    the coil causes the +12V to rise a bit, causing the mosfet to turn on
    faster, so it shouldn't chatter when turning on and off. If you find
    it's chattering, you can use a smaller smoothing cap (C3), which will
    allow more movement of the 12V rail, and thus a quicker turn on and off.
    470uF is bigger than you probably need, but if it's too small, the relay
    may cycle on and off at 60Hz. Not a good thing.

    Standard precautions and disclaimer applies. You MUST ENSURE that the
    pushbutton is isolated from the user, probably using some kind of rubber
    cover. Also, make sure the the neutral and ground aren't swapped by
    testing the socket you plug it into.

    This assumes you are in the US. If you are in europe, don't use this
    circuit without modification by somebody who knows what's up with the
    local power.

    --
    Regards,
    Robert Monsen

    "Your Highness, I have no need of this hypothesis."
    - Pierre Laplace (1749-1827), to Napoleon,
    on why his works on celestial mechanics make no mention of God.
     
  4. Jamie

    Jamie Guest

    there are ready made timers designed to wire in or plug into a
    socket that operate from 120 volts and gives you relay contacts.

    the key words are
    ONE SHOT relays.
    DELAY off Relays.
    --
    in any case, if you want to use a 555 timer, get your self a simple
    wallwart @ apox 12 vdc,300 Ma or more supply, a 555 timer ic, and a

    12 v coil relay with dual poles that are 10 amps ech.
    use both poles together to double up the current and some caps/snubbers
    across the contacts to absorb the flyback.
    or use a smaller relay to drive a large mercury relay which handles
    those kind of loads just fine.

    the 555 timer is to run in one-shot mode, this mode forces you to
    release the trigger before you can put the timer into a short on state
    again.
    http://www.uoguelph.ca/~antoon/gadgets/555/555.html

    there you will find info on how to construct it.

    P.S.
    the relay coil should have a diode across it to absorb fly back
    energy.
    also use something like a 2N2222 transistor to drive the relay coil
    from the output of the timer.
    make sure you drive the transistor hard enough to saturate it, this
    will keep the heat down.
    i am sure others here will offer you drawings, i don't have that
    ability at this time.
     
  5. Jeff

    Jeff Guest

    Jamie,

    Thanks, I will look for more info on one of these ready made units to
    see what the cost would be. Very nice website link too.
     
  6. Jeff

    Jeff Guest

    Mike,

    Thanks, I am making mine from a MOT also. Had not really considered
    the larger fast amp draw. Great info on the whole process.

    Did you find a 220V transformer (I have had no luck finding anything
    other than 110V units from the scrap piles) from a microwave or was it
    110V originally and you rewound the primary too?
     
  7. Jeff

    Jeff Guest

    Robert,

    I like the no other power supply idea, will have to give this a good
    looking over and put something together soon.

    THanks again for all the replies and advice.

    Jeff
     
  8. mike

    mike Guest

    I'm in the US. They're all 120V. The SS Relay is 240V simply 'cause I
    had it in the junk box. A microwave transformer is a lousy choice for a
    spot welder. Designed to be low coupling for the microwave application.
    That's not what you want here. But they are cheap and easily available.
    If you're gonna wind the primary, start with a different core.`.
    mike

    --
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    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
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  9. Jeff

    Jeff Guest

    MIke,

    I am in the US also. I don't really have the ambition to rewind the
    primary, and yes, the MOTs are readily available at least.

    Thanks again.
     
  10. Rich Grise

    Rich Grise Guest

    In the spot welder designs I've seen on the web that use MOTs, they take
    out the shunts. In this picture,
    http://www.neodruid.com/images/dscf0145.jpg
    you can see the shunts in the window between the windings. Toss them. (or
    put them in your "there must be a use for this crap somewhere" box) Those
    things protect the transformer and maggie by bypassing some of the flux.
    Lose them, and you have a regular transformer. Of course, you already
    know to toss the secondary and wind 3-4 turns of #2 weld cable. :)

    Good Luck!
    Rich
     
  11. Clarence_A

    Clarence_A Guest

    Top poster -- PLONK!
     
  12. mike

    mike Guest

    I agree, toss the shunts, but I'm not sure that's the whole story.
    Is the core made of the same stuff as a real transformer?
    YOu also lose some coupling with the side-by-side windings.
    I expect there are some rules of thumb about this??
    mike

    --
    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    ..
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
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    Bunch of stuff For Sale and Wanted at the link below.
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  13. mike

    mike Guest

    HI Clarence

    It's always



    nice to hear from people
    who have a contribution to the thread.

    See, you can make everybody happy if you post creatively...
    Suggest you have another beer and watch more rasslin' on TV.
    mike


    --
    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    ..
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Bunch of stuff For Sale and Wanted at the link below.
    MAKE THE OBVIOUS CHANGES TO THE LINK
    ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
     
  14. Rich Grise

    Rich Grise Guest

  15. Rich Grise

    Rich Grise Guest

    Oh, is Clarence The Plonker still around? I killfiled him months ago.

    Cheers!
    Rich
     
  16. Jeff

    Jeff Guest

    Yea, those have already been knocked out, all I have left is to figure
    out the timing method and get some rod stock for the arms, which is no
    big deal.

    Thanks for the reminder though.

    I think I am going to use 2 transformers to ensure adequate amps since
    I am doing slightly thicker metal than alot of what I see these created
    for. Need to do 2 sheets of 24 or 22 gauge sheet steel together.

    Jeff
     
  17. mike

    mike Guest


    Remember that there's no free lunch. If you expect twice the power out,
    you'll need twice the power in. Put the primaries in series and run it
    off 220 if you expect any real gain.

    If you're only doing one job, put an ad on a local newsgroup and get
    someone with a thirst for beer and a welder to help you.
    mike


    --
    Return address is VALID but some sites block emails
    with links. Delete this sig when replying.
    ..
    Wanted, PCMCIA SCSI Card for HP m820 CDRW.
    FS 500MHz Tek DSOscilloscope TDS540 Make Offer
    Wanted, 12.1" LCD for Gateway Solo 5300. Samsung LT121SU-121
    Bunch of stuff For Sale and Wanted at the link below.
    MAKE THE OBVIOUS CHANGES TO THE LINK
    ht<removethis>tp://www.geocities.com/SiliconValley/Monitor/4710/
     
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