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newbie question

Discussion in 'General Electronics Discussion' started by uncletomcobblyandall, Feb 11, 2014.

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  1. uncletomcobblyandall


    Feb 11, 2014
    Hi all
    I am pretty much a novice with electronics, I have played around with basic electronic kits and know the basics of what a diode or a resistor does etc etc, but I am not an expert by any means. My dilemna is this: I have twin headlights on my motorcycle, one being high beam, one being low beam, and they are never on together, when i turn high beam on the low beam light goes off. What I want to do is wire it so that the low beam stays on when the high beam on. I want to do it without using a relay and having to run more wires back to the battery or fuse box under the seat. I know it can be done with a big enough diode so that when the high beam goes on power will go through the diode and power the low beam circuit as well, but when the high beam is switched off and the low beam circuit comes on again the diode should prevent the current from the low beam circuit flowing back through the high beam circuit. The headlights are both 55 watt and 12 volt so by my calculations (remembering I am a novice) I need to have a diode that is rated at at least 5.5amps. The highest rated 12volt diode i can find is 3amp. So my question is this, can I successfully use a higher voltage diode that is rated at 6amps? I know that diodes have a starting current that if the current is beneath that amount it won't cross the semiconductor and there will be no current flow. If I use say a 120volt 6amp diode, will this work? And will there be too much voltage drop through that big a diode? Or am I better off going the relay route to achieve the desired outcone? Any and all advice will be greatly appreciated. Thanks.

  2. shumifan50


    Jan 16, 2014
    My first concern would be the wiring to your headlamp as it is intended to power one 55watt bulb, not two. The current could overheat the wires to the headlight when both are on.
    The second concern would be the charging system of the mcycle. It might not be able to cope with that current drain causing the battery to discharge(assuming you have a battery). My bikes in the past have had decidedly light-weight 'alternators' or dynamos requiring very little additional drain to fail. Tourers are not as bad as sports bikes in this respect.
  3. uncletomcobblyandall


    Feb 11, 2014
    Thanks for your reply shumifan
    I ride a harley cruiser with a heavy duty charging system designed for running driving lights etc which I no longet have on the bike. You might be right though about the wiring not being able to handle the current, so I think I might just go the new circuit way with a relay to feed the low beam circuit when I turn high beam on. I guess what I really need to know, if not for this project, at least to put in the memory banks for future projects, is, can you use a higher voltage diode on a circuit successfully? I realise there would be an issue if it was reversed and you tried to use a lower voltage diode on a higher voltage circuit, but not sure if a diode needs the full voltage it was designed for to operate properly.

  4. shumifan50


    Jan 16, 2014
    AFAIK it will conduct as you expect, but I also suspect that it will get fairly hot and might require cooling. But here I am also out of my depth (I am a software guy really). The voltage ratings I saw are reverse max voltages. The wattage rating is more important.

    It will be nice if Steve or BobK or any of the other helpful guys can explain all the values and their meanings in the specs of a diode.
  5. duke37


    Jan 9, 2011
    The diode voltage can be as high as you like. 100v would be fine.
    The diode current should be bigger than the load current, in your case I would go for a 10A diode.

    A silicon diode will drop about 1A (look up the specification) so will dissipate 5.5W and will need cooling such as bolting into a heat sink. Some diodes have insulated packages and some live packages, choose one to suit you. An isulated TO220 package would be suitable if you can find a flat surface to mount it.
    A Schottky diode will drop less voltage and so produce less heat. Schottky diodes have a lower voltage limit but a 50V diode should be available.

    The lamp driven through the diode will be less bright than when driven directly
  6. Arouse1973

    Arouse1973 Adam

    Dec 18, 2013
    The working voltage of the diode you are talking about is the reverse breakdown voltage as already mentioned. The forward voltage drop of a diode is a function of both current and temperature which increases with increased current and reduces with increased junction temperature.

    Wattage is not generally a concern as this has already been considered, because the forward voltage drop at maximum current produces the maximum wattage, you Don't see this mentioned much in the data sheet of the NXPS20H100C power diode. If you go outside this then you are outside the current spec of the diode. Wattage comes in handy for working out heat sinks.

    NXP do power schottky diodes rated at 10 A and 100V. You also have to remember that leakage current goes up with temperature also, but this might not be an issue.

    This might be relevant?
    I don't know much about motor bike alternators but for the vehicle directive a test is done called the load dump. This simulates the battery becoming disconnected while traveling along at high speed. A pulse will be produced by the alternator while the regulator is beginning to regulate this higher voltage. this can be over 100V and could possible damage components not rated for this surge. So what ever circuit you use make sure this is considered in your new application.

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