Help identifying circuit

Discussion in 'General Electronics Chat' started by bteddy1, Aug 10, 2018.

  1. bteddy1

    Thread Starter New Member

    Aug 10, 2018
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    0
    Hello,
    I need a little help. In reference ti the attached circuit.

    1). The capacitor in parallel with the diode in the bridge rectifier.
    a). What is its functional name, how do I look it up on the web?
    b). What is its purpose?
    c). How do I calculate its value/specs?

    2). The diac in parallel with the resistor in parallel with the bridge rectifier.
    a). What is its functional name, how do I look it up on the web?
    b). What is its purpose?
    c). How do I calculate its value/specs?

    3). The reactor (inductor) in series with the pos(+) output.
    a). What is its functional name, how do I look it up on the web?
    b). Its purpose is to limit inrush current and maybe filter/smooth ripples?
    c). How do I calculate its value/specs?

    Thanks for the assist.


    Circuit.png
     
    Last edited: Aug 10, 2018
  2. ericgibbs

    Moderator

    Jan 29, 2010
    7,539
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    hi bteddy1,
    Welcome to AAC.
    A question for you first.
    What do you think that circuit is designed to do.?
    E
     
  3. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    This looks a bit like another circuit from the world of homework.

    The capacitors across the 4 diodes in the bridge rectifier circuit are to absorb voltage spikes if they ever exist in the circuit. The same capacitors also contribute to the ripple voltage across the load. In my biased opinion the circuit could be useful if the lower end of the varistor, (it is not a diac symbol) were instead connected to the terminal marked positive. Then the symbol taken as an inductor could instead be the coil of a DC relay. The varistor serves to clamp the inductive spike as the relay is de-energised, and that resistor would then serve to reduce the dropout time a bit..
    If you try to look up anything from this circuit on the internet you will gain no useful data, and all sorts of sale offers and links to porn sites. So I would suggest avoiding such a waste of time.
     
  4. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    Why do I get this message displayed below when I post a response??? It happens frequently now.
    The following error occurred:
    You must wait at least 30 seconds before performing this action.
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    I am going to assume that this is a copy from a textbook and not an actual circuit design.
    Note that boxes are drawn around the bridge rectifier and the diac.

    Hence the capacitors drawn are to indicate that there are inherent capacitances in a pn junction rectifier.
    Similarly there are inherent leakage in a thyristor represented by the resistor. Certain applications may be dependent on these effects and must be taken into account when doing circuit analysis, modelling and simulation.
     
  6. ericgibbs

    Moderator

    Jan 29, 2010
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    As we think it is not Homework, I will post my reply.

    Rectifier Bypass capacitors.
    They reduce HF noise caused by the reverse recovery current being switched off when the current carriers in the diodes are depleted.
    The fast switching times of this current edge produces HF noise, the capacitor acts as short to the HF noise and prevents it from causing interference with the powered system.

    https://www.electronics-tutorials.ws/power/transient-suppression.html

    https://en.wikipedia.org/wiki/DIAC

    https://www.autodesk.com/products/eagle/blog/inductor-plain-english/
     
  7. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    Yes, the capacitors across the diodes certainly do also remove that switching noise. I had forgotten about that. AND that is despite having had to fight that exact problem in an audio amplifier a while back. So really, they do both functions.
     
  8. bteddy1

    Thread Starter New Member

    Aug 10, 2018
    4
    0
    Thank You for the responses.

    No, it is not homework.
    No, it is not from a text book.
    It is from an owner's manual form the late 60's to early 70's.

    I am trying to restore the unit and need to understand what it is.
    OEM parts are not available and are not marked on parts.

    That's is why I am asking for math, formulas, equations etc...
    To come up with something as good or better.

    Thanks again
     
  9. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    OK, if it is late sixties that device is definitely a varister, not a DIAC. The purpose is for it to break down and conduct if the voltage across it is too high. AND I am wondering if the device that looks like an inductor is an inductor, or is it a relay coil, or some other kind of electromagnetic thing. It could also be a power supply for something,Because you have an owners manual there should be some indication as to what the system is, because this circuit could be for quite a few very different applications. There is an incredible amount of expertise available at this website but the majority of folks are not very good mind readers, and so we need more descriptions to let us know what the something is. THEN you can get huge amounts of correct information.
     
  10. bteddy1

    Thread Starter New Member

    Aug 10, 2018
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    Welder.

    Secondary is 100V, 200A.

    The inductor is an inductor (EI laminated core two wires)

    The inductor I think is an inrush current limiter.
     
  11. Ylli

    Active Member

    Nov 13, 2015
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    The inductor helps get the arc started. When you make and then break a circuit with a series inductor, the inductor 'kicks back' and produces a higher voltage.
     
  12. bteddy1

    Thread Starter New Member

    Aug 10, 2018
    4
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    Is it voltage or current it kicks back? I thought it was the current that inductors resisted a change in. Voltage is capacitors, right?

    How do you design for that? What is its name/ term/ configuration?
    I have seen them air core, solid core, laminated core. what are the formulas to design one.
    What are the specs to get it to do that. Low/high resistance coil, low/high inductance coil?
    I would like it to have a equal or higher duty cycle than the transformer (30%).

    All the texts I have come across are just general inductors, no special purpose applications.
    I have found air core, solid core, laminated core, ferrite (HF), and seen reference to gapped core (to prevent saturation) but no formulas.
    what/where are the formulas to design one.
     
  13. ebp

    Well-Known Member

    Feb 8, 2018
    2,332
    809
    The inductor (reactor) is for "filtering" or "smoothing." With most moderate-power low(ish) voltage power supplies these days, capacitive filtering is by far the most common, but when currents are very high inductive filtering/smoothing is often used. The nature of an inductor is to "try" to keep the current through it

    [EDIT]: I was assuming that the inductor is quite large. It it is not it may be there mostly to aid in striking the arc.

    Unlike a capacitor, you can't leave the energy stored in an inductive filter where it is when current stops (or "tries to" stop) flowing. The inductor actually tries to keep the current flowing at exactly the same magnitude if the circuit suddenly goes open and that means the voltage can go extremely high (which is great for striking an arc). This can put much more reverse voltage stress on the rectifier diodes, so something like a varistor (blue arrow) is used. Given the age of the welder, there is some chance that that is actually a selenium transient suppressor.

    If you can post some photos of the existing parts it might help people provide guidance.
     
    Last edited: Aug 10, 2018
  14. LesJones

    Well-Known Member

    Jan 8, 2017
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    MisterBill2's post #9 proves how difficult thread starters make it for us by withholding information. With the information on the context of the question it would have probably been answered in the first reply.

    Les.
     
  15. MisterBill2

    Distinguished Member

    Jan 23, 2018
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    If the device is an arc welder then the purpose of the inductor is definitely to assist with striking and maintaining an arc. If it is not burned or physically damaged then quiet probably it does not need to be replaced. If it is damaged or burned then probably it can be re-wound, which is much less expensive. The main items of concern would be the four diodes. The very good news there is that diodes have come a long way in 40 years, and the replacements will perform better than the originals. If the welder rated current is 200 amps then I suggest diodes rated for 600 amps and at least 400 volts. But before replacing them you should check and see if they are OK. The safe way to do that requires that you be able to disconnect one side of the transformer secondary from the diode bridge, so that you can substitute a much less powerful source to do the checking. If the diodes are not working at 12 volts then they probably will not be working at 100 volts either, and the test will be much less exiting.
    What test equipment do you have available?
    With a visual inspection, do any of the circuit parts seem to be physically damaged or burned, or possibly missing? That will have a big effect on the diagnostics and the recommendations. Also, what part of the world are you in, which affects what resources may be conveniently available. My responses for the next few days may be variable since I will be traveling and not always able to respond right away.

    AND, that same circuit, with one changed connection, is the same as a small relay control box used to switch on a window fan from a wall-mounted thermostat.
     
    ericgibbs likes this.
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