Multiple CT transformer grounding, powered breadboard

Discussion in 'The Projects Forum' started by RockyBlackburn, Nov 1, 2010.

  1. RockyBlackburn

    Thread Starter New Member

    Sep 11, 2006
    7
    1
    I'm looking to build a powered bread board similar to the one here http://www.globalspecialties.com/pb505.php. And it seems that I've got stumped right off the bat. At the moment I'm looking at putting three transformers inside. Now the problem. I want to put all the rectifiers on one pcb and wondering if I could tie all the center taps onto the same ground plane or not? If so, should it also be tied to the enclosure?

    The idea is to have one transformer Hammond 185C12 for powering internal gizmos. Another transformer Hammond 185D24 for my fixed voltages then another transformer Hammond 182S15 for high amp variables.

    All these transformers will most likely be run off of 220V on the primary and will be set up for series on the secondary. Also, all non rectified voltage will be available to tie points on exterior of enclosure.

    Just thought of another question about transformers that I had the other day. My original thought was to use a large 48V transformer and hook it up parallel to give 24V @20.82A (Hammond 182S24) then use a full bridge to give + and - supply options. I'v decided not to go this route but am still curious about how would I reference ground? I kind of like the other route better because it gives me more AC options outside the enclosure.

    I'm thinking about either aluminum tread plate or stainless for the bottom and sides. Some sort of grilling for front and back and lexan on top. It will probably weigh a ton!

    Well anyway here is the regulated supplies I'm thinking about so far:
    Off of the large 30V transformer, 1 variable 10A and 2 variable 5A.
    Off the 24V all fixed 5V 12V and 15V.
    And a variable current source off of one, the other or both.
    These will also have negative counterparts just in case.

    I'm also thinking function generators, logic switches, and of course logic indicators. I like the idea of having a logic 0 and 1 indicated with 2 led's. not just one led on or off. Anyway it is about time for me to hit the sack, so I'v uploaded some screen shots of what little I have so far. I know the text is unreadable on the first bitmap, and there is nothing is showing a fuse (yet) but I'm about to fall asleep here, so that will have to wait till another day.

    Thanks.
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    I'm wondering why you think you need so much current capacity for a breadboard?

    Keep in mind that with so much current available, when you make a wiring mistake (it'll happen sooner or later) and create a direct short, a high-power supply will melt whatever is the weakest link, which may very well be your breadboard. Having lots of power available isn't always a good thing.

    Even your banana jacks/terminal posts will be a problem; many commonly available posts are limited to 10A, some go up to 15A. Do you really want a supply that's capable of melting your jacks/terminals?

    A high-power linear supply using conventional transformer(s) will be heavy, expensive, and generate lots of waste heat. You will probably need (a) large fan(s) to keep the supply from melting down. The fans themselves will generate noise, both audible and electrical, which further complicates the design.

    Instead of building such a hot, heavy, expensive and power-hungry supply, you might consider using something like this:
    http://www.mpja.com/prodinfo.asp?number=18455+PS
    +5v @3A, +12v @ 2A, -12v @ 0.5A will take care of most, if not all of a hobbyists' needs, and at under 10 bucks is doggone cheap. Being a switching supply, the outputs will have a bit of noise on them - however, you can add another filter stage on each output and the power rails should then be nice and quiet. It'll be efficient, and thus not require much if anything in the way of cooling except air circulation.

    Your material selection for the sides/bottom will be somewhat difficult to work with, the stainless in particular. They're also conductive, which is not necessarily a good thing. You might consider using ABS or PVC plastic sheeting instead; they're insulators, saw like butter and glues together with cements available from any hardware store.
     
    Last edited: Nov 2, 2010
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  3. wayneh

    Expert

    Sep 9, 2010
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    I'm a little befuddled by all the details but the answer to the first quoted question is YES. The output secondary from a transformer is isolated from the primary. You should see little or no current flowing from one "ground" (center tap) to another.

    I think if you have the option, I would tie enclosure ground to true ground, for safety's sake. Output ground doesn't have to be tied to the enclosure.
     
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  4. n1ist

    Active Member

    Mar 8, 2009
    171
    16
    I would keep each supply separate and floating, having its own neg, gnd, pos outputs. That way, you can wire them in series or use them for 2x the voltage if you need to.

    Your schematic doesn't show the primary side wiring for the transformers; don't forget the proper switch and fuse. As for the enclosure, make sure it can handle the weight of the transformers. As mentioned, tie any metal parts to safety (power cord) ground.
    /mike
     
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  5. wayneh

    Expert

    Sep 9, 2010
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    Just to expand on Sarge's points, you might consider pulling a power supply out of an old computer. You can find them for free, maybe even get paid to take one away. They all have nice 5, +12 and -12v supplies that are really handy for a workbench. I prefer the smaller, quiet ones without fans. I'm currently using one from and old Mac SE. Modular, built-in switch and fuse, overheat and over-current protection, everything you'd want. Except those whopping currents.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    I have an old Compaq ATX form factor 250W supply that I converted to a bench supply. It has plenty of power output for most jobs; it's really overkill for breadboarding.

    You can easily convert both ATX and ATXplus12 supplies to bench supplies.
     
  7. RockyBlackburn

    Thread Starter New Member

    Sep 11, 2006
    7
    1
    Sorry about the long road. I kind of thought that they could be tied together, but was not sure. Thanks for the safety warning. It will most certainly be bonded to earth! I'm not to much into electro-shock therapy myself.

    n1ist, thanks for the concern about the lack of documented fuses or switches on the primary side, they will be in the there. Things such as those may not make there way into my preliminary drawings, as I progress, they will find there way in. As for being able to handle the weight, metal is my thing.

    wayneh and SgtWookie, using ATX power supplies is a very great suggestion, and in my case, a free one! I have several laying around. I think I'll convert one. But I still plan on building my own supply(s) for this project. I need to do it that way for the experience and knowledge.

    This will mostly be a sort of modular project, with a nice large enclosure and lots of room inside for adding functionality as I go along. So using linear supplies, which I am comfortable with will only be a starting point.

    So, I'll try using just the 30V ct to begin with, I know it's still over the top, but it's a start. Low amp fuses on terminal posts for breadboarding and higher amp for the ones used off board.

    I do appreciate all of the time and effort all of you put in on helping people like myself. Thank you all.

    I'm going back to the drawing board.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Well, try to lessen your ambitions about the output current.

    It's possible to make high current output linear supplies, but they need HUGE heat sinks and fans to keep them from melting down, and they generate lots of waste heat.

    It's reasonably easy to make low-current-output linear supplies using LM317 and LM337 adjustable positive and negative regulators, but even with low current output they need heat sinks; if you get them anywhere near their rated current output with much of a voltage differential between IN and OUT, they need LARGE heat sinks.
     
  9. wayneh

    Expert

    Sep 9, 2010
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    I would humbly submit that the experience and knowledge you will gain is that you didn't need to over-build in the first place. A power supply with nothing to power.

    The essence of design is to deliver an end result with minimized resources. It's normal to have overkill components in a design, because that often reduces the biggest resource of all - time spent designing. But you shouldn't set out to intentionally over-build. If you've got specific high-current projects in your future, fine, design for those. But I'll bet exceedingly few of the projects you'll see on this forum ever go above, say, 5A or so.

    You can gain tons of experience and knowledge by starting with that ATX supply you have and only stepping up once you've found a specific need.
     
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