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Help with LEDs and AC power

Discussion in 'LEDs and Optoelectronics' started by CarlosT, Jan 23, 2015.

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  1. CarlosT

    CarlosT

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    Jan 23, 2015
    Hello I am making advertising lightboxes that are powered by a motorbike alternator AC like the headlights and tail lights. The bikes are typically old and in a bad state (i live in Cambodia). I use a bridge rectifier to switch AC to DC. Which works fine to power the 5630 LED strip lights (about 1.5 meters - 2 amps) WHEN the bike is moving. The problem is when its idling the LEDs have a horrible flicker. I first tried adding a 10000uf capacitor and that didn't help then i added a voltage regulator to keep 12v but that also didn't help. Finally I made a circuit with a bridge rectifier, a 16v 5f super capacitor bank and a voltage regulator. This works like a dream to kill the flickering however it makes the bike's headlight very dim.

    The problem as I understand (bearing in mind i know very little about electronics) is that the super cap is drawing all the available power 4.4amps by my multimeter. But the LEDs only need about 1.8 amps. How can I make the circuit pull only the power it needs. I thought once the cap was full it would pull less amps but that does not seem to be the case it just sits on 4.4amps. Is there some kind of current limiter I can add to the circuit? I considered using a bridge rated at 2amps but i suppose that would just burn out?
     
  2. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Doesn't the bike have a battery? You could add a series resistor but you will still need to supply a certain amount of current for the LEDs plus some current to charge the capacitor back up to replace the energy lost due to the fluctuating voltage. You may need to turn off the lights initially to allow the capacitor to charge up otherwise the voltage to the LEDs could be too low and charging could take quite some time if the resistor value is too high.

    Edit: Thinking about it, I am not sure but do you think the capacitor is going to reduce the current of the system? The only way to do that is to add a resistor. The capacitor will only smooth out current demand while the system is fluctuating. You will still have whatever current you are driving the LEDs with plus the charging current of the capacitor. So the energy the capacitor supplies will have to be replaced otherwise the voltage will just keep on falling.

    Adam
     
    Last edited: Jan 23, 2015
  3. duke37

    duke37

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    Jan 9, 2011
    It is over 60 years since I rode a motorbike.
    The alternators are made with a large amount of leakage inductance so that they are more like a current source than a voltage source. At low frequencies the effect of the inductance is low so the output does not vary a lot with speed. It does mean that the voltage is strongly dependant on the current drawn.

    Output ripple with a rectifier can be calculated from I = C * dV/dT
    Guessing dT = 0.1 sec I = 4 and dV =1 , then C = 4F which agrees with your experiment.
    Your 10mF capacitor gives a calculated ripple of 40V which is absurd, it is doing very little.

    You say that the leds only need 1.8A, but what current are they taking? A resistor is series with the leds will drop the current and leave more for the lights.
     
  4. CarlosT

    CarlosT

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    Jan 23, 2015
    Thanks for the quick replies guys.

    @Arouse1973 i didn't explain the full problem. These lightboxes sit on the top of tuktuks that is like taxi over here, its a carriage pulled by a motorbike/scooter usually an old clapped out dailim or honda dream. Connecting anything to the battery is not an option as the drivers tend to fiddle with it and disconnect it as they worry about their battery going flat and just reconnect in when they come for an inspection and payment. Then I get the blame from the advertiser that their signs are not on at night. In an attempt to rectify this problem i am trying to connect the lights directly to the tail light of the tuktuk itself.

    I am liking the idea of a resistor in series. At present the caps take about 3 seconds to fire up. I wonder what effect that would have. I actually thought the resistor would just use up the excess 2A I am not using and not help the situation but then again I don't even really know what a resistor is.

    @duke37 - Thanks for clarifying that I am indeed using the correct capacitor (even if it has taken a large chunk out of my profit). Please excuse my ignorance about electronics but I believe the answer to your question is that the LEDs are taking the full 1.8A they require from the capacitor bank and they even stay nicely lit for about 5 seconds after turning off the headlights and then slowly fade for another 20 seconds. But when I put a multimeter on the capacitor connection its drawing 4+ amps. I don't really understand how the capacitor can continue to draw such a high current even when its seemingly filled up. I expected it would have a rush when they first turn the headlights on and then settle down as it fills up.

    Can someone please explain which resistor I would need and where in the circuit it should be placed?
     
  5. CarlosT

    CarlosT

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    Jan 23, 2015
    Just to reiterate the problem I am facing is that the headlights/tail lights are going very dim even at full throttle when the capacitor is in the circuit. The LEDs are at full brightness. When I remove the capacitor the headlights brightness is fine but the LEDs flicker violently when the bike is at a low idle.
     
  6. Gryd3

    Gryd3

    4,098
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    Jun 25, 2014
    Could you possibly submit a drawing of how you are connecting these components?

    Alternator -> Bridge Rectifier.
    Capacitor is placed in parallel to the Bridge Rectifier output.
    LED circuit is connected to the Bridge Rectifier Output.
     
  7. CarlosT

    CarlosT

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    Jan 23, 2015
    Here is a very dodgy photo from my phone camera...

    Have I got the capacitor parallel bit wrong? It seems to me I have connected it directly to the bridge rectifier.
     

    Attached Files:

  8. CarlosT

    CarlosT

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    Jan 23, 2015
    Here is a lame drawing that may help clarify the circuit...
     

    Attached Files:

  9. Gryd3

    Gryd3

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    Jun 25, 2014
    Looks wired right.. I wonder why such a high current draw.
    I was thinking perhaps it would be simpler/cheaper to tap into the main positive power line from the battery for the light. Use a small relay attached to the tail / license plate light to turn on the relay (and thus the lights) only when the bike is running.
    Depending on how they tamper with the bike... they could simply disconnect one of the wires to this new method of yours as well.
     
  10. CarlosT

    CarlosT

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    Jan 23, 2015
    I agree, except that if its not wired to their battery they don't think its draining their battery, also sometimes they have to disconnect their bike so they ask for a plug which is just begging to be disconnected. But I may end up having to go down that path which makes installations a lot trickier. When you are rolling out dozens of these things you want a quick fix. But the capacitor is certainly an expensive route and it looks like if i need a 6 Ohm resistor its going to be even worse.

    Is this possible?
    A small motorbike battery in the unit which is charged by the alternator via the tail lights which is triggered to connect to the LEDs by a relay also powered by the tail lights?
     
  11. duke37

    duke37

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    Jan 9, 2011
    Without the specification on the various components, it is difficult to recommend any solution.
    The power supply is the part which could do various things, if it produces a fixed 12V output with different input voltages, it may take more or less current than the output current.

    I could not find a clear description of the led strip but it looks like there are leds with series resistors with many of these branches connected in parallel.

    I would try a 1Ω 10W resistor in series with the led feed or shorten the strip.
     
  12. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    If you disconect the LED light do the bike lights look ok with the cap connected? If so then its just the impedance of the cap. When the supply dips and the cap takes over it will need to be charged up again. If you dont have a limitting resistor the the capacitor could draw a very large current. This happens in pulses dependant on the input. If this is happening often then this could make the lights look dim.
    Adam
     
  13. CarlosT

    CarlosT

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    Jan 23, 2015
    @duke37 here links the components

    LED STRIP (using 1.3m)

    SUPER CAP

    VOLTAGE REGULATOR

    and the bridge rectifier is a SV4B 20 94 can't seem to find a link for this exact one, I bought it in a shop.

    The thing is the LEDs don't seem to be the problem i can run 4m of them without the headlights dimming. In fact with the bridge and the 10mF capacitor it even works on a bike that has a high idle. The problem is I have to account for bikes with a low idle which is where the super cap came in. But as soon as I add that the headlights dim by half.
     
  14. CarlosT

    CarlosT

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    Jan 23, 2015
    @Arouse1973 I will try that tomorrow, I hadn't thought of that.
     
  15. CarlosT

    CarlosT

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    Jan 23, 2015
    @Arouse1973 Yes without the LEDs the supercap still makes the headlights dim.
     
  16. CarlosT

    CarlosT

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    Jan 23, 2015
    If I was going to put a load resistor on it. Am I correct in thinking I would need a 25W 6 Ohm resistor like -> this one <-
     
  17. CarlosT

    CarlosT

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    Jan 23, 2015
    Can anyone see a problem with the attached solution?

    It uses a bridge rectifier, 2 relays and the smallest motorbike battery i can find. In the illustration the grey wires activate the switch between the brown and the red wires and the yellow wires are ground. I have tested this on a wall socket 12v power supply and a motorbike battery and it works as intended (i think). The idea is that the alternator will both power the the LEDs and charge the battery and when the alternator is not putting out enough current the LEDs will pull from the battery.

    This only lights up and charges when the motorcycle lights are on as its attached to the tail light leads.
     

    Attached Files:

  18. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Why don't you have it charging all the time. So by the end of the day you will have a fully charged battery.
    Adam
     
  19. CarlosT

    CarlosT

    14
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    Jan 23, 2015
    Because I want something that does not need to be connected to the motorbike in any way if possible. This is only connected to the carriage via its tail lights. It makes the installation a great deal simpler.
     
  20. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Ok I understand.
    Adam
     
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