A few simple questions.

Discussion in 'General Electronics Chat' started by dustiin, Jan 13, 2010.

  1. dustiin

    Thread Starter Member

    Jan 13, 2010
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    Hello. This is my first post and from what I've been reading this a very active forum with lots of information. However using the search results didn't turn me to any answers I wanted so I decided to make a post.

    To start off I am working on this dirtbike. It's all AC power there's no battery. I'm looking to hook a LED tailight up to it. But I've read somewhere that LED's run on DC power and read somewhere about a guy adding a LED taillight to his AC system on his bike and the LED taillight running fine on the AC.

    So the question is will the LED take the AC? The LED taillight uses 12v at 1.5amps. The bike is putting out 130w at the stator at 12v and 10.8amps. (correct?

    If the LED tailight won't take AC power I planned on getting a rectifier but at my electronics class we have plenty of resistors, capacitors, diodes, etc. From what I've gathered up I can hook 4 diodes up to make a bridge rectifier, true? and will this drop the voltage any?
     
  2. retched

    AAC Fanatic!

    Dec 5, 2009
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    The diodes will drop your voltage .7v per diode. That isnt too much of a problem because the LEDs can run off of way less. even if you get to 5v you could still run a bunch of LEDs with enough current. Which you should have.

    If you have an old alternator sitting around, you can pull the rectifier out of it and you will be golden.

    Make sure of your current limiting resistors for the LEDs. And when you check your power output from the rectifier, make sure It doesnt change to much at higher RPMs. If there is a big change, you will need a regulator to keep things happy.
     
  3. dustiin

    Thread Starter Member

    Jan 13, 2010
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    How and what kind of diodes should I wire together to make my bridge rectifier to get it to convert AC to DC? I have no old alternators sitting around.

    I have no resistors for these LEDs. Didn't know I have to wire a resister for a LED.

    I will check for the higher current at higher rpms. Will get a regulator if they're big.
     
  4. flat5

    Active Member

    Nov 13, 2008
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  5. dustiin

    Thread Starter Member

    Jan 13, 2010
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    Thanks flat. I'll use a
    MB151MB151 100V 15A Bridge Sound about right? Lines on the bike don't exceed 12v or 10amps.

    The LED taillight I plan to use is off eBay and doesn't have any details. I sent a question hoping for an answer soon to tell me the current draw and any other details. Seems like I won't have to put any details since it seems well built.

    Will this rectifier drop down the voltage or amps? I'll buy this whenever I get home.
    Thanks again.
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Don't buy things on E-bay if there are no details.
    Maybe the tail-light is designed to be connected directly to 12V without current-limiting resistors. Maybe it has a heatsink built-in.
    Maybe the alternator on your bike has a voltage regulator.
    Who knows?
     
  7. dustiin

    Thread Starter Member

    Jan 13, 2010
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    The item was very detailed but never told the amps. Asked the seller and he said it is 1 amp. Also my dirtbike does have a 12v voltage regulator.

    Don't think it has a any heatsink or resistors built in because it goes on just like my tail light is now just it's LED and looking at pictures shows no resistors or heatsinks.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    12V at 1A is 12W which will fry LEDs in an enclosed housing without a heatsink.
    It must have something (resistors or a current regulator) to limit the current into the LEDs.
     
  9. dustiin

    Thread Starter Member

    Jan 13, 2010
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  10. Mike33

    AAC Fanatic!

    Feb 4, 2005
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    You could call the number given in the Ebay ad and ask them some questions? Like, how is this unit normally attached, etc...
     
  11. dustiin

    Thread Starter Member

    Jan 13, 2010
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    Well, it seems pretty easy. The 3 holes just bolt onto the rear fender (after you've drilled holes) and then the other 3 wires connect to the ground, hot, and brake switch.

    But onto the question I've been asking does the rectifier lower any voltage?
     
  12. retched

    AAC Fanatic!

    Dec 5, 2009
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    yes. ~.7v is standard and a 1.4v drop for bridge rectifiers (using silicone)

    info on voltage drop with diodes:
    http://www.satcure-focus.com/tutor/page3.htm
     
  13. dustiin

    Thread Starter Member

    Jan 13, 2010
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    So it should run on 10.6volts just fine right? Interesting page on diodes.
     
  14. retched

    AAC Fanatic!

    Dec 5, 2009
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    Even though your bike has a 12v system it acutally (most likely) produces around 14v so you will still have 12v after diode drop
     
  15. dustiin

    Thread Starter Member

    Jan 13, 2010
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    Went out and checked. Yupp it's 13.7volts. For some reason it wouldn't measure amps. Thanks for all the information. I guess I'll post back if I have any problems with this rectifier or just any questions of curiosity.

    Thanks everyone.
     
  16. SgtWookie

    Expert

    Jul 17, 2007
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    Ouch - it won't measure Amperes because you just blew the fuse in your meter.

    You cannot measure high current directly without damaging your meter in some way.

    You should measure the voltage across a known resistance value. That will tell you the current flowing through the resistor.

    If you have a precision 1 Ohm non-inductive resistor rated for 100W, you are golden to measure current in any circuit under 50V. You can read the voltage across the resistor, and since I=E/R, the voltage across the resistor equals the Amperes flowing through the resistor.
     
  17. dustiin

    Thread Starter Member

    Jan 13, 2010
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    Well, The stator outputs 130w. 130/12 = 10.8amps. Took the fuse out it says it's rated 15 amps and it's not blown. Measured a random battery and it gave me back an accurate reading. Don't quite understand what you're saying by figuring it out but I see how using a known resistance value can lead me to amps. But unfortunately, I'm not that knowledgeable about resistors and such.

    But since the stator outputs 130w that I'm sure of /12 = 10.8amps.

    Shouldn't just knowing that be enough to tell me how many amps is flowing?
     
  18. Audioguru

    New Member

    Dec 20, 2007
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    You are shorting the alternator and battery with your meter. They are not designed to feed a short, they are designed to feed a load resistance.

    A current meter measures the current flowing through the load when it is connected in series with the load. The load current is not always the max current that is available. It could be zero, a small current or a high current.

    You still don't know if the LED tail-light already has a rectifier bridge and resistors inside. If it already has a rectifier bridge then adding a second one might dim the LEDs.
     
  19. dustiin

    Thread Starter Member

    Jan 13, 2010
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    There is no battery on the bike but I see what you're saying.

    The tail light probably doesn't have an rectifier bridge because it's designed for most dirtbikes and most of them have a battery on them. The resistors are probably already built in since they didn't just maybe throw it al together and hope it works. I'm sure they've tested it with a lot of variables.
     
  20. dustiin

    Thread Starter Member

    Jan 13, 2010
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    Also I sent a message to the seller and it is already taken care of he also says there is no rectifier bridge as it is designed to be ran on DC systems. So it seems like it's ready to be bolted on and hooked to a rectifier. Yay.
     
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