Running a 12v DC LED off of a 12vAC circuit.

Discussion in 'General Electronics Chat' started by Evan Rander, Jul 29, 2015.

  1. Evan Rander

    Thread Starter New Member

    Jul 29, 2015
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    Good Evening everyone!


    I'm trying to run an LED light bar off of my 12vAC system, Just wondering what type of rectifier i will need. A little lost when it comes to the lingo.

    I've found some 2 amp 800 volt rectifiers for .65 a piece. But am not sure if this will work, the light i'm running is only 20w and 1amp. I'm assuming they're saying the most you can run through it is 2amps, at a max voltage of 800volts? So, in this case that theoretically should work, yes? Here is the link to the rectifier. http://www.allelectronics.com/make-a-store/item/fwb-28/2-amp-800-volt-bridge-rectifier/1.html

    Also, i'm wanting to run a Capacitor after the rectifier to supply the LED with the cleanest power for longevity purposes. 1500uF capacitor should be decent? Based on 1000uF per Amp? If this is right out of left field. Please feel free to correct me! Thanks so much

    Evan.
     
  2. pwdixon

    Member

    Oct 11, 2012
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    You can't just put a diode and a capacitor in the way and get 12Vdc. Even a bridge rectifier won't give 12dc, you'll end up with something like 15Vdc. You need to know what the spec for the LED is in terms of what it can tolerate as a dc input and what ripple it can tolerate. More details of the LED and what the power source is would help.
     
  3. Evan Rander

    Thread Starter New Member

    Jul 29, 2015
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    The power source will be my snowmobile 12vAC stator. The LED is capable of handling anywhere from 9-36Volts DC.

    I'd love to give you more information about the stator, however specs seem to be vague. Usually, on the snowmobile they'll power 2 60watt h4 bulbs. handle bar grip heaters, gauges, and i know that other people have been running 5 amp oil pumps off of them as well.
     
  4. Evan Rander

    Thread Starter New Member

    Jul 29, 2015
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    Even tho the input is 12vAC you can't put in a full wave bridge rectifier to a capacitor to the light bar? (4 5watt Cree LEDs) and expect the light bar to receive 12vDC?
     
  5. pwdixon

    Member

    Oct 11, 2012
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    Sounds like your original plan of rectifiers (in a bridge I assume) and a cap will probably do the job ok, you might not even need the cap, I'd try it. We don't have a lot of use for snowmobiles in the uk so I've got no idea what effect it will have on that, probably no effect if it can drive large lamps and oil pumps normally.
     
  6. pwdixon

    Member

    Oct 11, 2012
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    Jumped messages, but I'm not sure what you are asking.
     
  7. Evan Rander

    Thread Starter New Member

    Jul 29, 2015
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    I was just clarifying. I should have erased it. All good. Thanks for the replies. So the 2amp 800volt bridge rectifier i posted the link to should be okay for the 20watt 1amp light?

    Thanks again.
     
  8. pwdixon

    Member

    Oct 11, 2012
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    It should be but it might get quite warm, I'd be inclined to use a higher current SIL device and bolt it to a heatsink (casing) to dump the heat. A 10A device won't be much more money and would give a more reliable solution.
     
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  9. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    Because of how original equipment (running lights, gauges, grip heaters) are mounted and wired, is one end of the stator effectively connected to the snowmobile frame (grounded)?

    Are you sure that there is not already a potted bridge rectifier that turns AC to DC to power some of the original equipment?

    Does the light bar have two wires going into it, say red and black, where it expects 12Vdc to be connected between red and black? Is the black wire also connected to the metal housing that the light bar is mounted inside of such that the black wire is connected to the frame of the machine when the housing is bolted to the frame?

    Do you have the factory wiring diagram of how the machine is wired before you start mucking about with it?
     
    Last edited: Jul 29, 2015
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  10. Evan Rander

    Thread Starter New Member

    Jul 29, 2015
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    Mike ML, as my snowmobile doesn't have a battery, i believe the whole snowmobile is AC, Yes the light bar has red and black and it expects 12vDC, the ground is only connected to the led chip boards, not the body of the light. I do not have a factory wiring diagram. I am making a kit for the snowmobile to be plug and play. I don't want customers mucking around in a wiring harness trying to find the DC (if there is any). I want the customer to hook my light bar directly up to the stock headlight location where it then runs through the bridged rectifier, capacitor and powers the light flawlessly.

    PwDixon, i will look into getting something that will handle a lot more power. Thanks for your advice.
     
  11. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    I expect that the DC voltage across the filter capacitor (between the bridge rectifier and the LED lightbar) will fluctuate from a few V to over 17V as a function of engine rpm. This fluctuation is hugely greater than what it would be in an automotive application where there is a battery and alternator (12.6 to 14.5V)

    So, what provides current regulation for the LEDs inside the light bar? Is this a pre-made assembly that you are purchasing ready-made. If so, what are its specs regarding the allowed range of input voltages?
     
  12. Evan Rander

    Thread Starter New Member

    Jul 29, 2015
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    Sorry it took me so long to get back here,

    Yes, thats correct. the light bar is already a prebuilt unit. Its tolerances are from 9-36VDC.
     
  13. MikeML

    AAC Fanatic!

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    Then you shouldn't need anything but a full-wave bridge rectifier and filter capacitor.
     
  14. tom_s

    Member

    Jun 27, 2014
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    +1 above

    this snowmobile no have a 2way or radio on it?
     
  15. Evan Rander

    Thread Starter New Member

    Jul 29, 2015
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    Awesome, thanks for your replies.

    What size would you recommend for the capacitor? I've heard 1500uF per amp? is that fairly accurate?
     
  16. Evan Rander

    Thread Starter New Member

    Jul 29, 2015
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    No, they don't. Basic electronics onboard are the gauges, handle heaters and headlights.
     
  17. MikeML

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    I would have to know the ripple frequency.
    How many poles are on the alternator?
    How is it geared with respect to engine rpms?
    What is the engine idle rpm?
     
  18. tom_s

    Member

    Jun 27, 2014
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    driving along in the snow with nowhere to port the phone music. if anyone does it, remember it was my idea first :)

    personally i'd try it with the bridge and 1000uf/25v pending space you have
     
  19. David Knight

    New Member

    Aug 4, 2015
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    Capacitance depends on line frequency and load current. The capacitance needs to be big enough to hold the voltage high enough when the diodes in the bridge rectifier are not conducting. A quick LTSpice simulation would give you a good idea of how much capacitance you need.

    Other important figures of merit for the filter capacitor are temperature rating, ripple current rating, and rated lifetime. You'll be using aluminum electrolytic capacitors, which dry out with heat. If you're storing your snowmobile in a cool place, this may not be a problem. If it's in a warm shed, the capacitor life may be shorter. There are polymer aluminum electrolytic capacitors that don't dry out. They cost more, but can handle more current, and have longer life. I like to overspecify the capacitors in my designs to improve longevity.

    I like the comment above to use a 10A bridge rectifier attached to a heatsink. Bridge rectifiers into capacitive loads have high peak currents and do get hot. An 800V Bridge rectifier is not necessary. Diodes with higher voltage ratings tend to have higher forward voltage drop, making the device more lossy. Since you're working with low voltage, you could build your own bridge rectifier with 10A 40V Schottky diodes. Schottky diodes have low forward drop and will be more efficient than regular diodes, which allows for the use of a smaller heatsink.

    What do you guys think about SMT vs through hole? I think that a snowmobile may be a high vibration environment. I think that through hole soldering would be more resistant to vibration that SMT.
     
  20. David Knight

    New Member

    Aug 4, 2015
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    What's the part number for your LED light bar?
     
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