YABC (Yet Another Bypass Capacitor) Question

Discussion in 'General Electronics Chat' started by jackw19, Jan 27, 2009.

  1. jackw19

    Thread Starter New Member

    Jan 22, 2009
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    I'm hoping someone can help me understand a couple aspects of the attached circuit. It's for a simple, low end MIG welder rated at about 120 amps at 20vdc at 20% duty cycle. Even though it's a small machine, the basic design features are similar to much larger transformer-rectifier MIG's, but without all the bells and whistles to complicate the diagram.

    BTW: VR1 is a 10 joule 68vdc varistor, R2 is a 20w 50 ohm wirewound and C1 is 46,000 uf 35vdc.

    1) First of all, what's the purpose of (what I'm sure are) ceramic capacitors C2, C3 across the diodes of the center-tapped FWB? I understand these might have some role in supressing noise coming off the diodes--but what difference would that make in this simple machine? There are no IC's anywhere, so what's to matter? Anyway, some manufacturers use them, and some don't. I've seen them in 4-diode FWB's too. What would be a good size for these? .01uf, .1uf, 1uf, ... all of the above. I haven't been able to find a circuit diagram that calls out a value.

    2) Some manufacturers also include ceramic capacitors from the output terminals (labeled work and torch in this diagram) to ground. What would these capacitors be doing. What would be reasonable values?

    3) What's the purpose of varistor VR1? These are kind of like diacs, right? At a threshold voltage they switch full on--so is this varistor to protect the big capacitor C!, or to protect the fellow welding from high voltage? A varistor in this location doesn't seem to be a common feature in MIG circuits I've seen.

    4) I do hate to unambiguously identify myself as ignorant, but what's the purpose of the big resistor R2 across the output? Maybe to limit current in the event of a dead short of the electrode to the work? Not every manufacturer uses them, but I've seen several diagrams with such resistors. On a big 250 amp machine, this resistor might be 300w 50 ohm, if it was present.

    Thanks in advance; I really appreciate the help.
     
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  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Those capacitors might not have much effect on your machine's operation, but they'll sure have an effect on anyone trying to listen to a radio within a square mile of you. Those are R.F. suppression caps!

    Eric
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    VR1 is hard to figure out. Something like a varistor can't do much across the DC output unless it is big enough to act like a one-man crowbar circuit, and pop the breakers on overvoltage. As far as I know, they don't stop conducting until you remove all voltage.
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Oops...missed the second part of the queston. VR1 is a DIAC, and it is indeed a voltage regulator....worlks like two zeners back-to-back, but they are usually designed for some serious current and/or/ voltage.

    eric
     
  5. jackw19

    Thread Starter New Member

    Jan 22, 2009
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    Thanks, gents. Appreciate your comments.

    Any ideas about what R2 is doing across the output?
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    R2 is there to pull some current and "improve" regulation. The load it provides doesn't let the filter charge up to peak, so the voltage shift under full load is not so great. Keeps the power supply nice and warm, too.
     
  7. jackw19

    Thread Starter New Member

    Jan 22, 2009
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    Ah .... this is starting to make some sense now. Better regulation. MIG welding works best with "constant voltage" power supplies that have a very flat volt-amp curve. Charting volts on the vertical axis and amps on the horizontal, the slope of a typical volt amp curve would be on the order of -1/10.

    Almost all MIGs use a constant speed wire feed, and this also contributes to good regulation. Assume the gun tip is held at a constant distance from the work. When wire stick-out is short, arc length and hence resistance is high; since voltage is roughly constant I=E/R forces current to be low. This reduces heat at the tip of the wire allowing it to extend further. Then arc length and resistance drop, current and heat increase burning back the wire and lengthening the arc.

    Just adding the constant resistance of R2 across the arc would tend to damp variability of the system, especially when arc length was shortening and current might want to increase suddenly leading to an unstable arc and spatter.

    But beenthere mentions an interaction of R2 with the capacitor filter. Does this somehow involve the time constant of R2 and the big capactior? I do know that welders say that too much capacitance leads to instability and spatter.
     
  8. Bailey45

    Member

    Oct 27, 2008
    18
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    The purpose of R2 could also be to discharge the large capacitor when you turn off the welder. The resistor would completly discharge the capacitor in approximatly 10 sec. Without this discharging a huge amount of energy would stay in the capacitor for hours while it self discharged.
     
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