electro etching machine

Discussion in 'The Projects Forum' started by ThunderSpark, Mar 3, 2010.

  1. ThunderSpark

    Thread Starter New Member

    Mar 2, 2010
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    0
    I am making an electro etching machine to etch flat steel surfaces.
    It will out put AC or DC

    It is powered by a
    25.2VAC, 2 amp transformer – AC output to
    Full bridge rectifier (400V - 8Amp) – DC output
    DPDT switch to choose output

    What size rheostat will I need to vary the current/voltage down to 12V, or less?
    Also will the rheostat work on both the AC and DC.
    I choose a rheostat because that seems to be what the commercial units use.
    This unit will see very limited use, may be one per week.
    I know very little about the mathmatics.
    Thank You for you help.
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
    5,201
    312
    I may be wrong.. But I think you are going to have a hard time rectifying 25.2VAC, 2A to 400VDC, 8A.

    Or are you saying your rectifier is rated for UP TO 400vdc at 8Amps?
    If not, you have some explaining to do.

    After you do that, the rest should be a cinch.

    Do you have to use the 25v ac at all?

    Could you just use a wall wart to get your 12vdc from that?
    That would cost you about five bucks.


    Do you have a schematic or what you have done so far?

    bytheway, thunder doesn't spark. ;)
     
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  3. ThunderSpark

    Thread Starter New Member

    Mar 2, 2010
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    Sorry, the rectifier is rated at 400vdc 8 amps.
    I found an online calculator for rating rheostats: Ohms Law Calculator.
    I just plugged in the voltage and amps and got the other figures.

    I already have a 25.2V transformer, just wanted to get that voltage down to 6-12 volts, so its not so hard on my etching templates. I may just go with a LM338T after transformer, and before rectifier - instead of rheostat, that would probably be cheaper.

    Is a wall wart referance to Major chain store? $5.00 for a 12 volt transformer, what is brand and amp?

    Thanks for your reply.
     
  4. jpanhalt

    AAC Fanatic!

    Jan 18, 2008
    5,671
    897
    Note that 25V AC is approximately 35V DC after full wave rectification. Then to drop that to 6 volts at just 2 A is over 50 W of dissipated heat. Any linear voltage regulator will get quite hot or need a big heatsink. Have you considered using a Variac (or other brand) to control the input to your 25V transformer? Your AC or DC could be switched at the output of the transformer.

    John

    BTW, I think keeping the isolating transformer (i.e., the 25V one) is absolutely essential for safety. A Variac does not provide isolation.
     
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  5. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    If you want to use your transformer I would use a switch mode power supply approach. Switch mode circuits are easy to get hold on. And few external components are needed.
     
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  6. retched

    AAC Fanatic!

    Dec 5, 2009
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    A wall wart is a device. Like the black box charger for cell phones. It look like a "wart" on the wall.

    Many of these plug in "wall warts" are transformers and some are regulated. These are a much safer way to power the devices you want.

    For the 12v at 2amp I would recommend a laptop power supply. These are easy to come by and are regulated.
     
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  7. ThunderSpark

    Thread Starter New Member

    Mar 2, 2010
    3
    0
    thanks to everyone for your help.
     
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