Spark Eroder pulse generator

Discussion in 'The Projects Forum' started by sihaam, Nov 28, 2012.

  1. sihaam

    Thread Starter New Member

    Nov 4, 2009
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    Hello All,
    I am building a small 5 amp spark eroder. It will use 50V DC. The power supply provides straight 50VDC, but I need a variable square wave pulse generator to take this 50VDC and pulse it for me. All help will be appreciated
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    Is 5A the peak current while pulsing, or is that the "average" current over time?

    What frequency do you want? Do you want to have control over the frequency and/or duty cycle (the on versus off time percentages).
     
  3. sihaam

    Thread Starter New Member

    Nov 4, 2009
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    0
    The 5A is the average.
    Spark erosion is the process whereby you use set "sizes" of sparks to cut away metal. In spark erosion, you need to "chip" the workpiece with sparks of a uniform amperage, eg 3amps x 50VDC. Every time a spark is generated, it "eats away" a precise amount of the workpiece. The time space between the sparks (the on time must be able to be varied from 1 millisec to 500 millisecs). Similarly, the off time must also be able to be varied. So I assume that the circuit will have two knobs, one to control the ON time and the other to control the off time. I am no an expert in electronics, but everyone in spark erosion tells me that the wave must be square. I assume the square means that the entire period of the "on" pulse will be eg 3 amps x 50V and when off, it is abruptly zero. In this manner we will be generating a set size of the spark.
     
    Last edited: Nov 28, 2012
  4. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    1,521
    In the pulse EDM I am working on I'm using a SG3525 chip - http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00000958.pdf It will allow pulse width adjustment from 5-95percent and allow you to change frequency too.

    It will be run from a 12V supply and the output from it will be connected to a mosfet driver to control the output mosfets. While your 50V power will work, most sink EDM's use a ~100V open circuit power supply.

    Are you going to use a stepper motor to move the ram? What are you using for the voltage detector/comparator circuit to control the work gap?

    Do you know about the Yahoo EDM groups? I am a member of two of them, Homebrew EDM and EDMHomeBuilders.
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,090
    3,027
    You will indeed need two controls, but the usual arrangement is one for frequency - the inverse of the length of one on-off cycle - and one for pulse width or on-time percentage, with 50% being equal on and off times. This is all you need to specify the same thing you just described.
     
  6. sihaam

    Thread Starter New Member

    Nov 4, 2009
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    0
    Shortbus, Actually it is a 60VDC p/supply. Most of the guys I know use this current rated p/supply: 60VDC @ 350watt. No complaints so far. I do not know anyone using 12V. I shall certainly join that group, as I am always willing to learn more. Yes I use an XY table with stepper motors, drivers and a PC. I do not bother with the voltage comparator as I simply set the timing to correspond with the erosion. The motors then step accordingly. I have the complete XY table system, but decided to now incorportae my own power supply and pulse circuit. Basically I am trying to cut costs as the "off the shelf" power supply comes with the pulse generator at a ridiculous rip-off price
     
  7. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    You only use the 12V from the SG3525 chip to control the power mosfets. The SG3525 allows bot frequency and pulse width to be done by one chip reliably, that is the main function of that chip. The 60V power supply is called the "open circuit voltage", that is the voltage used to break down the dielectric and start the spark. After the spark is established, the current starts to flow and the voltage will drop to the "machining voltage". Machining voltage in a wire EDM is usually in the range of 30 - 20 volts.

    Adding a window comparator to your circuit will greatly improve your cutting time and lessen the wire breakage. Having the comparator control the movement of the table is pretty easy to do. It can be done by, when the voltage is in the machining range, it allows the clock to trigger the stepper movement. When it is lower than the machining voltage the steppers would be stopped.

    I can give you some links to EDM if you want. Most are biased toward sinker EDM, but the basics of the power supply are the same between sinker and wire. I did sinker EDM as a die maker for about 13 years, so that's what I'm building. Got the machine built just need to finish up a spark generator and the controls.
     
    Last edited: Nov 30, 2012
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