AC to DC hi low switch for snowmobile led headlight

Discussion in 'Analog & Mixed-Signal Design' started by Pedroz, Aug 20, 2016.

  1. Pedroz

    Thread Starter New Member

    Aug 20, 2016
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    image.jpeg
    Hey I just have a question about electrical circuit. This is for a headlight switch that controls high and low beam on a snowmobile. The system uses 12VAC to power and switches the high and low beam. The ground is always used and the button switches between high and low beam wires. I need to convert this 12AC to 12VDC in order to run an LED headlight.

    I tried the diagram below and what happens is that DC current is produced but it is produced to both the high and low wires at double a voltage at the same time (24 VDC). Not 12VDC to either high/ground or low/ground.

    When I just had one rectifier to the highbeam it worked fine at 12 V DC but I wanted to be able to have a high and low beam. Do you know if I need to diode or something to make 12 V DC at the high and ground or 12 V DC at the low and ground?
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    A basic problem with converting to DC is that one terminal of the alternator is almost certainly grounded to the vehicle chassis and so is one terminal of each lamp. Consider this circuit :-
    AC-DC-problem.JPG
    For AC, the high and low beam non-grounded terminal would be connected to A1 and A2 respectively. The lamp holder would provide the ground connection.
    For DC, using a full-wave rectifier, each lamp would connect between B and C. Note that C is no longer at ground potential, so the usual lamp holder would short out D4.
    Will all your LED lamps have holders which are isolated from chassis ground?
    Isn't Hi/Lo switching on a LED headlight normally done using PWM (pulse-width-modulation)?
     
  3. Pedroz

    Thread Starter New Member

    Aug 20, 2016
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    Hi. Thanks for your reply. I should have said that I bought a harness that plugs into the existing head light plug and switches to high and low with the existing switch.
    The problem is that this light relies on DC power when my snowmobile produces AC power (no battery).
     
    Last edited by a moderator: Aug 22, 2016
  4. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Can you post a link to the harness spec? I see a power input plug on the right, but no sign of how you connect the Hi/Lo switch?
     
  5. Pedroz

    Thread Starter New Member

    Aug 20, 2016
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    This the harness I built with the 2 rectifiers and I picture of how the wiring to the bulb/harness works.
     
    Last edited by a moderator: Aug 22, 2016
  6. inwo

    Well-Known Member

    Nov 7, 2013
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    Try 1/2 wave.
    A diode in each wire. H and L.
    Might end up with too much voltage full wave.
     
  7. Pedroz

    Thread Starter New Member

    Aug 20, 2016
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    Ok thanks. Would that be the same as keeping my existing setup with the 2 rectifiers and putting 4 diodes going to the rectifiers?
     
    Last edited by a moderator: Aug 22, 2016
  8. inwo

    Well-Known Member

    Nov 7, 2013
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    No, just to diodes.
    Shouldn't hurt to try. Use standard headlamp to test.
    Use a single diode from each bridge.
    ie. either ac to +.
    snowled.png
     
  9. Pedroz

    Thread Starter New Member

    Aug 20, 2016
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    Thanks I will give that a try.
     
    Last edited by a moderator: Aug 22, 2016
  10. ScottWang

    Moderator

    Aug 23, 2012
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    I already merged some of your two separated posts to be one with other member's replied and your new post.
    When you click the 'reply' button then you can see the left side and right have 'QUOTE', you just write down your new reply on the next line as below.

    '['QUOTE="Pedroz, post: 1032313, member: 369455" ']'Thanks I will give that a try.'['/QUOTE']'
    You just write down your new reply from this line ......
     
  11. ian field

    Distinguished Member

    Oct 27, 2012
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    You can get "corn cob" LED H4 replacements, but they need DC, and I'm not sure if you can get 24V ones.

    It would be easier to sort out the AC before it reaches the dip switch.

    The standard corn cob H4 is only 7W, so it might be worth looking for a battery you can take the supply from. Using the AC feed means rectifiers, smoothing capacitors (or a battery) and regulators.

    The old Brit motorcycles used a 100W 15V Zener - if you need 24V; just hook 2 in series - but each needs a finned heat sink in the airflow.
     
  12. Pedroz

    Thread Starter New Member

    Aug 20, 2016
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    The led I got works from 8 - 24VDC. I will see if I can find a wiring schematic to see about getting DC to the switch. Thanks
     
  13. ian field

    Distinguished Member

    Oct 27, 2012
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    If its the AC lighting system typical of small motorcycles - there may not be much in the way of regulation! Off load it could probably deliver a dangerous shock at optimum RPM. Any regulation is likely to be crude - many motorcycles just switch in a shunt resistor when the main beam is off.

    Given the example of a "corn cob" LED H4 taking only 7W - you're going to have to dump some power to regulate the voltage.

    Simplest is to clamp the output from the bridge rectifier with a 100W 15V Zener - but probably not cheap. The 8 - 24V range opens options, you might be able to get a salvage power Zener without getting too spendy.
     
  14. Pedroz

    Thread Starter New Member

    Aug 20, 2016
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    I tried a diode and the high and low lead at the bulb harness and I only got 6V output.

    3 wires go to the headlight switch, so not sure how I will rectify the power to DC before it got there.

    I guess I will just have to live with either high or low like I had it earlier (non-switchable).
    I appreciate all the help and suggestions out there! Thanks
     
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