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12vac to 12 vdc - blackhole

Discussion in 'General Electronics Discussion' started by netxtown, Mar 22, 2015.

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  1. netxtown

    netxtown

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    Mar 22, 2015
    Of course it occurred to me to use a 12vac outdoor "garden light" transformer to power up some LED strips (3528's, 5 meter, 300 LED). And so the search began for a rectifier/regulator circuit that would pass 5-6 amps. I have scanned high - and I have scanned low - in fact I believe I may have found the very end of the internet....and nowhere am I finding valid (aka informed) discussion on tapping these transformers to run some strips.

    Now add to it that I read through the LED resource info - and I was good right up to the part where the content went upside down with amp reg instead of voltage reg. Now I'm off in the ditch.

    SO... using a full wave bridge (6a/200v diodes) with a 1000uf/25v smoothing cap....what needs to happen from there to the led strip? Anything? Amp regulation?

    FWIW - the LED count will vary from project to project. The plain white strips (as in NOT RGB) are being used on the frames of xmas yard ornaments. Some use 3-4 meters whereas others are using more. I get it I can distribute power at varying intervals/cuts along the strips. All told though, I don't see any project going over 60-65 watts. My intent was to build a small circuit, pot it, then tie wrap it to the ornament with a short run of cabling to the existing yard light fixture (parallel tap).

    (As it turns out, nearly every single one of the yard ornaments will sit within 6 feet of a regular low voltage yard lite (some incandescent most are xenon) - so it just makes sense to use what is already right there! The xfrmr has got plenty to spare (300w rated output / <100w being used)

    What the heck am I missing?? I tend to ask questions rather than shoot first (blow up $10 led reels).
     
    Last edited: Mar 22, 2015
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Rectified and filtered 12VAC will give you around 18VDC. This is far too much for normal LED strips. You need a 12V regulator. This may dissipate quite a bit of power depending on what you use so it may get quite hot.
     
  3. netxtown

    netxtown

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    Mar 22, 2015
    And that's what I also thought. I can heat sink it - but the LED resource discussion on regulating current has thrown me for a loop.

    I looked at: http://www.circuitstoday.com/12-v-high-current-regulator and also http://schematiccircuit.com/schematic-7812-power-supply-high-current/12-v-35a-power-supply/

    ....and it appears either will meet the need - albeit the latter with some reduction in the number of TIP2955's. If I understand the first circuit correctly, the zener will hold voltage right at 11.8v - while the TO3 can will allow the amperage. In the second circuit, the 7812 holds the voltage while the TIPs handle the amperage.

    As for heat sink - the intent was to stuff all of this into a 4-5" piece of aluminum square tube (1.25" outside, 1" inside), pig tail the input and out, pot it, and be done. I can certainly mount heat sinks to the exterior of the tubing. I would think the aluminum tube + a heat sink should be sufficient. However, with the 7812 embedded in epoxy - not sure how that will effect things.

    I know I am at least halfway to solving this...I think.
     
  4. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    The led strips have current limiting resistors to allow them to be operated from 12V.

    You should probably look at switch mode voltage regulators available as pre built modules. These will operate much cooler than a linear regulator.
     
    Last edited: Mar 23, 2015
  5. netxtown

    netxtown

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    Mar 22, 2015
    Thanks for your replies..but a buck converter is exactly what I don't want to do.
     
  6. davenn

    davenn Moderator

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    why ? its the best and most efficient option

    Dave
     
  7. netxtown

    netxtown

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    Mar 22, 2015
    Can you point me to one made in the US?
     
  8. davenn

    davenn Moderator

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    Sep 5, 2009
    not offhand, I have no idea who in the USA would make them
    you cant even buy the parts for the small cost of a few $$ that they cost on eBay

    and you didn't answer my question !!
     
  9. netxtown

    netxtown

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    Mar 22, 2015
    Davenn - Ok....I concede I had/have a bit of an ax to grind with the import stuff. I've been hung out to dry a few times. Currently, I am now almost 4 weeks waiting for a tool that should have been here 3 weeks ago. Anyways...I digress.

    So, I slapped the chip off my shoulder....and started the search from a different perspective. I can put a cheap bridge on the output of the xfmr - which should give me about 16-17 vdc out. From there, I've been searching for a buck DC/DC with variable input (12-18vdc or wider) with 12vdc 6-8 amps out (or variable as well), and waterproof. I can find plenty at 3amps or 5amps....but anything over that seems the pricing shoots through the roof.

    But! (there's always a but!) I had an epiphany that just might provide some options. So, I'll bounce this around.

    The ornaments I design vary in size...thus the amount of LED strip varies from about 4 meters upwards to 20 meters. I am wondering the feasibility of a single heavy duty bridge - and then use multiplesmaller bucks. No reason I can't cut the strips to create 'sections'.I'm thinking the easiest way to eat an elephant is one bite at a time.

    I found this:
    http://www.ebay.com/itm/Waterproof-...322?pt=LH_DefaultDomain_0&hash=item1e98bdc482

    The input is pretty narrow.

    (PS - thanks for the kick in the ***.)
     
    Tha fios agaibh likes this.
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    A very good idea. There are numerous very cheap buck converter modules available that claim to be able to do 3A. If you use them with a length of LED strip that draws no more than maybe 1A to 1.5A they should be under little stress. I would also advise (for the longevity of the LED ropelight) that you run them at 11.5V -- or at least err on the side of the voltage being under 12V rather than over.

    I guess the issue is that they're not waterproof, but I would take "waterproof" with a large grain of salt. http://www.ebay.com/itm/291362318499
     
    Arouse1973 and davenn like this.
  11. davenn

    davenn Moderator

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    Last edited: Mar 23, 2015
  12. netxtown

    netxtown

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    Mar 22, 2015
    A good discussion - but it's a solution that isn't. The bucks certainly make sense from a cost perspective - yet they fail to meet the requirements of the operating environment. And, from here, I guess I am back at square one.
     
  13. davenn

    davenn Moderator

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    Specifically ??
     
  14. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I'd say it's "waterproof".
     
  15. davenn

    davenn Moderator

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    well the chances are he's going to have to pot whatever PSU he buys aye ;)
     
  16. netxtown

    netxtown

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    Mar 22, 2015
    Actually - that's it. I'm assuming the bucks will overheat once potted. Also, the output adjust would have to be relocated so that it will be accessible after potting...which means it is no longer waterproof as water can enter via the adjust. What ever happened to that 'one size fits all' thing?? LOL!

    The difficulty is that we can be tech heads all day and all night. Rest of the world wants plug n play. So, how do you explain to a 'customer' they need a VOM to adjust the output to exactly 11.5 volts for best performance? I'll be afraid of the instruction manual! In at least 6 languages! And all folded up to less than an inch square!

    I can take a bridge rectifier, add a smoothing cap...feed that into a buck, adjust output..and be done. But! (there's always a but!) Their landscape transformer doesn't put out exactly the same as my bench unit - and now it has to be adjusted on site. And I don't do home installs. Ugh.
     
    Last edited: Mar 25, 2015
  17. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    25,448
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    Adjust to 11.5 volts before potting. Epoxy a metal block to the chip before potting. Have a tapped hole in the block matching to a hole in your aluminium tube. Some thermal grease on the inside and a little silicone on the outside will ensure water doesn't get in through the hole.

    edit: there are also potting compounds that you can poke screwdrivers through, or you can leave a hole in the potting and fill it up with silicone after adjustment. The silicone can be removed if you want to adjust it again later.
     
  18. netxtown

    netxtown

    8
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    Mar 22, 2015
    Steve - that is exactly what I envisioned. I'm gonna pick up a 10 pack of the bucks and start experimenting.

    Question about bucks. They 'advertise' them as "Step Down". To me, step down infers pre-determined incremental steps downward. Or, are they designed so long as the input is higher (say about 18v) that the output can be variably adjusted downward??

    Also, I'm looking at some LM338's that put out a good deal more amperage - and would require heat sinking - and mounting the heatsink external to the aluminium tube...
     
    Last edited by a moderator: Mar 25, 2015
  19. BobK

    BobK

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    Jan 5, 2010
    A buck converter can convert down to any voltage lower than the input. There may be a dropout voltage, i.e. it may need say 1V more than the output to operate, while some can operate all the way up to the input.

    Bob
     
  20. davenn

    davenn Moderator

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    Sep 5, 2009
    if you look at those converters you will see a blue ( usually blue) adjustment pot on them
    that is to adj your output voltage

    that's going to get bigger and bulkier and then you mite as well go for that second option I gave you which is much more efficient and doesn't generate anywhere as much heat
     
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