Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Out door led lighting

  1. Jan 13, 2013 #1
    Hi, i recently installed 24 miniture lamp posts around my yard. These lamp posts are using 12 volt push in twist (car backup or brake bulbs). I powered the 24 bulbs with a large 12 volt transformer which is controlled by a switch inside the house. Here in lies the problem not thinking i orderd on
    ebay 30, 12volt led bulbs. Realizing after the purchase, they are dc. The bulbs are rated .37 watts. My delemma is this. The wireing is underground
    (romex) and of of course each lamp has an exposed wire going to the socket. Can i place a single diode in the line feeding the each bulb which will cause a 60hz pulse? Will that be noticeable on the led? If not what size diode should i get? Other option is a full wave rectifier which will cause a lot more work. First accessing the hidden transformer box and then checking each light indivualy as the lights are not polorized. The connectors are snapped onto a bus in any orientation. That bus runs around the yard under each lamp. If a full wave rectifier is recommended,
    (which i will build) what size diodes would i need to support lets say 12 watts. 24 x .37 = 8.88 watts. 12 watts will give me a little wiggle room.
    Thank you in advance for any help.....bob
  2. jcsd
  3. Jan 13, 2013 #2


    User Avatar
    Science Advisor
    Gold Member

    There isn't enough information to make a very accurate answer but a near enough answer would be easy.
    The LED rating is for a DC supply so you can work out the steady current from P=VI. A rectified AC will only be acting for half the time and the peak voltage is root 2 times the nominal RMS value. This doesn't mean that the peak current will be root2 times the RMS current because LEDs are not linear and the source resistance may be relevant but diodes are cheap so you may as well get one for each LED that's rated at 1A and then you won't need to worry.
    You may find that the flicker is bad, with the LEDs only operating over half a cycle and you could reduce the effect of flicker by connecting half of the LEDs and diodes 'the other way round' to the supply so one half will be lit for half the cycle and the other half will be lit for the other half of the cycle. Conversely, why not just feed the lot from one hefty bridge rectifier and get twice as much light from your array as all diodes will be lit during both halves of the cycle? The rectifier will be a fraction of the cost of all your LEDs I think. And why build a rectifier bridge yourself when you can buy them?
  4. Jan 13, 2013 #3


    User Avatar
    Science Advisor
    Homework Helper

    If each buib is 0.37 watts, 30 bulbs is about 11 watts. So for a 12 volt supply you only need 1 amp to light all of them.

    I wouldn't bother to "build" anything. Just replace your 12V transformer with a 12V DC power supply. You should be able to buy a "wall wart" 12V 2A supply (50% safety margin!) for a few dollars. You won't be able to "build it yourself" any cheaper, even if you ignore the cost of your time doing it.

    Even a regulated supply (which you don't really need) like this one is only $10... http://www.mini-box.com/24w-12v-2A-AC-DC-Power-Adapter

    If you ran the wiring around your garden without bothering about the polarity, sure you will probably have to swap the wires round for some of the DC lamps, but that shouldn't be a huge task.
  5. Jan 14, 2013 #4


    User Avatar
    Science Advisor
    Gold Member

    I can appreciate his concern with getting it right but this is a more trivial problem than the OP may realise. Recognising this is largely a matter of experience (knowing what you can get away with) and one soon gets a feel for the orders of magnitude of current, power and volts that are involved with projects.
    One important thing, which the OP may have not considered, is that a fuse should be placed in circuit between the power supply and the cable going underground. If a short circuit fault should develop out in the garden the fuse will protect the supply from overheating due to feeding into a short for an extended period of time.
  6. Jan 14, 2013 #5
    Thank you all for your replies. I like the idea of a new power supply however i still like the idea of building my own. Thank you for the fuse recommendation i will do that. Full wave rectifier is easy enough to build.
    Do you think if i used four 1 amp diodes that would do the trick?
    Thank you all for your replies so far. Bob
  7. Jan 14, 2013 #6


    User Avatar
    Science Advisor
    Gold Member

    As you say, you need about 11W (and with a bridge rectifier you will get that), which is about 1A. You mean a Bridge Rectifier, I think. A 'full wave rectifier' is a configuration with two diodes and a centre tapped transformer - at least, it always was when I was a lad and people had to use valve (thermionic) rectifiers to power their valve equipment.
    I would look at prices and go for higher current diodes if they're not too pricey. Also, is your transformer beefy enough? (Just checking) You don't want the PSU to let you down in the middle of the night.

    A fuse is particularly useful when a workman could put a spade into the ground and short the conductors without knowing. It could even be you because you are bound to forget the routing by next digging season.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Similar Discussions: Out door led lighting
  1. Light out of phase? (Replies: 2)