Center-tap transformer to switch Vs

Discussion in 'General Electronics Chat' started by KeithW, Mar 24, 2013.

  1. KeithW

    Thread Starter New Member

    Mar 24, 2013
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    Sorry about the terse unreadable title. Had to do it to circumvent the acknowledged forum bug.

    I'm following plans for an electro-etcher/marker, found in multiple places online. Such a device uses DC current to etch and AC current to mark. Various effects on the etching process can be achieved by controlling the voltage and current of the circuit. The basic design is to put 120V wall AC through a step-down transformer, rectify the output, and not much else. It isn't even particularly important to smooth the DC circuit (the discontinuities in rectified DC apparently have little effect on etching behavior).

    To switch the device to an electro-marker you simply bypass the rectifier entirely and work directly off the stepped-down AC.

    One thing I would like to verify, although this isn't my main question, is whether it is appropriate to connect the transformer chassis to the "wall" ground (the green ground from the wall, bundled with the 120V AC). Is that the right thing to do?

    What I would like to add is a two-volt-option switch. In high mode it use the outer taps of the transformer and in low mode it would use an outer tap and the center tap. That's pretty much it...but I have one question that I cannot for the life of me find a solid answer to. In fact, I find conflicting answers at various resources.

    In each of the two configurations, I don't know whether it is optional, required, or prohibited to attach the unused transformer tap to the wall ground (to restate, this is the green ground from the wall, bundled with the 120V AC), or alternatively to leave it floating (right term?). In high-voltage mode, thus not using the center tap, do I leave the center unconnected or do I connect it to wall ground? The followup question, for which I have found practically no information whatsoever, is the low-voltage mode. Do I leave the unused end-tap of the transformer open or connect it to wall ground?

    Thanks a bunch!
     
  2. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Are you looking at something like this?

    http://www.logiudicecustomknives.com/knifeshop/etcher/images/electro-etcher_circuit.gif
     
  3. KeithW

    Thread Starter New Member

    Mar 24, 2013
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  4. KeithW

    Thread Starter New Member

    Mar 24, 2013
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    Yes, although there are variations to found around the internet. That one uses the center tap and leaves the unused end tap open. Consider this variation, which uses twice as much voltage:

    http://chriscrawfordknives.com/#/electro-etching-unit-3/4535349992

    He uses the two end taps, but instead of leaving the center tap open, he grounds it (he doesn't even include a circuit diagram, but his step #1 grounds the center tap).

    So, as you can see, things seem a bit ambiguous to me.

    Any help is much appreciated. Thank you.
     
  5. tubeguy

    Well-Known Member

    Nov 3, 2012
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    I would build the unit in a metal box.

    Ground the transformer frame and the 120 AC supply green ground to the metal box at one point.
    Instead of grounding the center tap, ground one end of the secondary and connect that to the black binding post.

    Then you can select either the center tap, or the other end of the transformer secondary with your added switch.

    Edit: The schematic I posted shows a 400v rated bridge rectifier.
    You only need a rectifier with a rating of 100 volts if connecting it to a 24 volt secondary.
     
    Last edited: Mar 24, 2013
  6. KeithW

    Thread Starter New Member

    Mar 24, 2013
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    What is the "black binding post" in this design? Presumably that end still attaches to one of the AC line of the rectifier even if it is grounded, right? Otherwise, where do the two rectifier inputs come from?

    Also, hmmm, what if I'm well underway ;) and have used a plastic enclosure in accordance with the numerous similar etcher designs, tutorials, and walkthroughs available online? Are you saying the switch I want to add can't be wired without a metal box...or are you just saying you prefer metal boxes for certain kinds of devices and for some reason I don't personally understand yet, you see this device as a candidate?

    Surely the metal box isn't a requirement for the circuit to actually work properly, right?
     
  7. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Yes, looking at the schematic I posted, you will see that one side of the transformer secondary connects to one of the ~ AC connections on the bridge rectifier and connects through one side of the DPDT switch to the black binding post. That side of the secondary should be grounded.

    Then connect your selector switch to select either the center tap or the other side of the secondary, then to the other ~ AC connection on the bridge rectifier and then to the DPDT switch and RED output as shown.

    I see the schematic has the Rectifier DC + and - reversed. + should go to DPDT switch red side , - to DPDT switch black side

    I just prefer metal boxes when 120vac line voltage is involved for additional safety. ;)
     
    Last edited: Mar 24, 2013
  8. KeithW

    Thread Starter New Member

    Mar 24, 2013
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    Oh, sorry. I didn't realize that was a "binding post". That's the banana jack of course, to which the etching leads attach. Thanks for the terminology lesson.

    I follow you. Cool.

    Hmmm, I see that. That's interesting. That does seem wrong doesn't it.
     
  9. tubeguy

    Well-Known Member

    Nov 3, 2012
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    Actually, I need to correct myself.
    I said binding post, but to be accurate, a banana jack is not necessarily also a binding post and vice-versa. Some binding posts accept banana plugs, some do not. :)
     
  10. KeithW

    Thread Starter New Member

    Mar 24, 2013
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    I know this is a silly (and incredibly naive) question, but what does grounding one of the transformer's secondary end-taps AND an AC rectifier input AND a binding post to ground "do"? What does is the distinction between an AC circuit with no ground and an AC circuit with a ground? It seems like such a fundamental difference that I wouldn't expect a circuit to be viable both ways, yet it appears to be at least an option in this case...since the original plans obviously "work", even if they are suboptimal in some way to your more expert eye.

    I guess I can google this on my own too...which I will now do. :)

    Thanks.
     
  11. tubeguy

    Well-Known Member

    Nov 3, 2012
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    No questions are silly - borrowing a quote "no one is born knowing this stuff"
    You won't learn any thing without asking questions. :p

    Grounding one end of the secondary keeps the secondary from "floating"
    at an unknown voltage level. If you are engraving knives or something that itself is not connected to another grounded AC supply this is a good approach.

    If you were connecting this circuit to another chassis which was also grounded through the wall AC supply. then you might not ground the secondary on this unit, but instead use the fact that this secondary would be grounded through the connection to the other piece of equipment.

    I hope this makes some sense... or Google "ground loop" ;)
     
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