Need some help

Discussion in 'General Electronics Chat' started by lurch, Apr 30, 2008.

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  1. lurch

    Thread Starter New Member

    Apr 29, 2008
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    Hi, first post and new to designing curcuits.

    What I would like to do is try to design or buy one or two bridge rectifiers to convert 12v ac to 12v dc.

    The purpose of this is to custom make a led tail light/brake light for my snowmoblie. The complete curcuit must fit inside a 7/8" square aluminum tube that the tail light will flush mount into.

    From what I've been reading a rectifier will turn 12v ac into 16.8v dc unless a resistor is used to correct this?

    What I need to have happen when the curcuit is complete is to have the tail light on when the motor is running and have the brake light brighten when I apply the brake. I have three wires that go to the stock lighting system now. Hot (12v ac), ground and hot (12v ac) when brake is pressed.

    Do I need to have two rectifiers for this (one to each 12v ac source) or can one be used for both hots?

    If I use a string of 16 1.5v led lights will it light with what I'm guessing is the correct voltage of 16.8v dc. Can I then connect the second hot to the same string and have them turn brighter when the brake is applied? The string of leds would be in parallel. The reason for do this is to have them under powers when motor is run and overpowered when the brake is pressed. Will overpowering the led string by 33% cause the string to blow out right away or just shorten the life of the string?

    Or would it be better to run two stings of 8 leds, one for tail light and one for brake light and just stager the lights?

    Thanks
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Have you purchased the LEDs already, or is that still up in the air?

    You might consider Luxeon Superflux Red-Orange LEDs - that depends upon how much current you have available, of course.
    I don't know if you are planning on this setup being street-legal or not. But, the DOT requirement for brakelights isn't a pure red, it's a red-orange.

    The Luxeon Superflux are available in a wide-angle view, which is really what you want. Standard "super bright" LEDs tend to have a rather narrow focus of perhaps 20° to 45°, which is too narrow for a brakelight. You would want more than 60°.

    I picked up some of these:
    HPWT-MH00-F4000, TS AllnGaP Red Orange, 70°, 70mA, 221mW, Vf(min) 2.19, (typ) 2.6, (max) 3.03
    I Purchased them in an auction on Ebay pretty cheaply. These things are BRIGHT! :eek: They're rectangular critters with four legs, roughly 0.3"x0.3" with the pins on 0.2" centers.
    As the specs say, the typical forward voltage (Vf) is around 2.6. Your mileage may vary. It's always a good idea to test them.

    Radio Shack sells a full-wave rectifier bridge that's good for up to 1.4A @ 100PIV for $1.59:
    http://www.radioshack.com/product/i...&cp=2032058.2032230.2032269&parentPage=family
    Or, you could pick up one of their rectifier diode assortments and build one yourself, and have leftovers for more projects or spares. But having one that's already assembled cuts out some hassles.

    Regulation is the next thing. LEDs need their current regulated. The more current they get, the brighter they are until you burn them out by exceeding their specifications.

    It's not hard to build a current regulation circuit with some resistors, a single standard red LED, and inexpensive transistors.
     
  3. lurch

    Thread Starter New Member

    Apr 29, 2008
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    Thanks for the reply, I guess I'll be buying the rectifier and building the led string.

    Is what I posted going to be the dcv coming out of the rectifier? ( 12v ac = 16.8v dc)

    This is for a snowmobile that will be used for back country mountain riding so I think as long as it's as bright or brighter than the stock one I should be good. I'm going to cover the leds with a red lens. Would white leds work or would I be better off the going with the super bright reds?

    Any one have a answer about the underpowering and overpowering of the leds to make the lights dim then brighter when the brake is pressed?

    Would I need to install a capasitor to clean up the dcv for the leds to operate correctly?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    If you use the little bridge rectifier I mentioned, it will be rippled DCV. You could use an electrolytic capacitor with a rating of at least 30V to smooth out the ripples.

    I suggest that you go with LEDs that are already red-orange; in case the lens gets damaged or falls off the lights will still be the proper color. The particular LEDs I recommended will also have the correct viewing angle in the absence of a lens.

    In order to get the tail/brake change of light intensity, you'll need to rectify both sources of voltage. The running taillight will get it's current from the normal running light circuit. When the brake circuit kicks in, it can turn on a transistor in the current regulator circuit to boost the current supply to the LEDs.

    Each "string" of LEDs will need it's own current regulator circuit. How many LEDs you can run in a series string depends upon the Vf of the LEDs. Each current regulator circuit will require a pair of transistors, a standard red LED, and three resistors. I'll draw it up later on.
     
  5. lurch

    Thread Starter New Member

    Apr 29, 2008
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    I purchased the rectifiers you recommened and was looking at the capacitors. They have ones rated for 35v but the microfarad rating were different on them. Does it matter which one I use? In my line of work the microfarad is made common so all our footages will remain the same no matter the guage of wire we measure. Does a lager microfarad rating mean the capacitor will run more efficient?
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    How many uF's you'll need depends upon how many LED's (actually, how many strings of LEDs) you're planning on using.

    If you go with the Luxeon LEDs I suggested, you can light up to four LEDs in a single string.

    Figure on needing roughly 33uF to 50uF per string. It can all be in one capacitor, if it'll fit on a board in your 7/8" square tube. The capacitor "smooths" out the "bumps" in the voltages from the rectified AC sine wave.
     
    Last edited by a moderator: Dec 27, 2013
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