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Which Diodes do I need for this application?

  1. Oct 12, 2009 #1
    I am adding a function to the rear Turn Signals on my motorcycle (12vDC negative ground system). Turn Signals have 1157 incandescent bulbs - they have two filaments and three wires: ground, hot to running light, and hot to flasher.

    I want my Turn Signals to ALSO act as Brake Lights. So all I have to do is add a wire from the Brake Light to the hot flasher wire of my Turn Signal.

    Except, of course, that there are two Turn Signals. So if I connect wires to both, my Turn Signals will no longer operate independently (because they are both connected to the Brake Light).

    So I figure a Diode will prevent current from one Turn Signal passing across the brake light to the other Turn Signal. Of course, I will need two diodes - one on each Turn Signal.

    So I need a Diode that will live happily in a 12vDC circuit (actual voltage varies from 12 to 15). The 1157 incandescent bulbs are 28 watts or (approximately) 2.33 amps. So I want a diode that will allow 2.33+ amps to flow through one way and prevent current in a 12-15 vDC circuit from flowing the other way.

    I have found this diode at Radio Shack for $1.49 for two:
    3A Barrel Diodes, Model: 276-1141 | Catalog #: 276-1141
    Diodes contain 200A surge. Peak Inverse Voltage (PIV) is 50.
    Package of 2.

    It says nothing about 12vDC, but I think it means 3 amps can pass thru with no problem. And that the diode will not let current pass the other direction up to 50 volts of pressure. Am I reading this correctly??

    Next is a tricky question mixing theory and practice. It seems that many years ago the auto industry wired cars under the mistaken believe that electrons move from Positive to Negative. So they designed cars as Negative Ground Systems.

    Of course, we now know that electrons really flow from Negative to Positive - however - the auto industry never changed their system, because it works. Motorcycles are the same.

    So if I solder a Diode into a Negative Ground System - which way do I point it???
  2. jcsd
  3. Oct 12, 2009 #2
  4. Oct 13, 2009 #3


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    (Don't worry if you don't get a reply straight away.)

    Yes, you could do that. I suspect that any front turn lights would also come on when you hit the brakes. This would add to the current.

    You would have the diodes between the brake light switch and each lamp with the cathode end (usually a ring around one end of the diode) connected to the lamp.

    3 Amp diodes are possibly a bit small for this application. I think I would be looking for 5 Amp diodes. More if the front lights also come on. The 50 Volt peak inverse rating is plenty for this application.

    You could consider using 4 diodes to also apply power to the running lights if these are not powered during the day. This would give you some extra brightness during the day.

    At these currents, there will be about a 1 volt drop across the diodes, so the lamps will be slightly dimmer than normal.

    Before you jump in, though, there is a little test that needs to be done. The flasher control will have 12 volts applied to its output, like this and we need to check if it is safe to do that. We need to check the resistance to ground from the output of the flasher. If it is open circuit, there should be no problem. If it isn't, you might have to consider a separate brake light.
    Last edited: Oct 13, 2009
  5. Oct 13, 2009 #4
    Thanks, vk6kro, now I am confused on a higher level - the definition of progress.

    I forgot about the front Turn Signals - on most bikes they are not also running lights. But I converted mine so they are both running lights and flasher ((so I have 4 running and 4 flashers)).

    Anyway that adds another 2+ amps to each diode when the Turn Signals act as Brake Lights. So the total would be approx 4.5 amps per diode. Is that too close for a 5 amp diode??

    I never even thought about 'back pressure' on the Turn Signal Flasher. It is a Thermal relay (not electronic - although I could change this relay) and it is directly grounded to the battery (as versus grounded thru the bike frame).

    I am not sure what you mean by "resistance to ground from the output of the flasher"
    The output of the Flasher Relay goes to front and rear Turn Signals on one side of the bike or 2 bulbs 1157 with 28 watts each flash. The bulbs are grounded directly to the battery (as versus thru the frame).

    ??? as a mechanical relay - when it is open, it has ""infinite"" resistance to ground because the physical switch is open ??? is this what you mean ???
  6. Oct 13, 2009 #5


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    I have seen flashing lights on cars run faster if one of the lights burns out, so the output was directly connected to the operating mechanism of the flasher and there is a chance that putting 12 volts on the output could cause damage.
    I can picture a bimetallic strip bending into a right angle and staying like that.

    If this was possible, you would need two diodes for each lamp. One coming from the brake switch and one from the flasher switch.
    An easy way of doing this would be to get two 30 amp bridge rectifiers and only use two of the four diodes in each one.

    You probably know that lamps draw a big current when you turn them on, so you have to allow for this with the diode ratings. Diodes are cheap.

    What happens if you are waiting to turn and you have the brakes on? Your turn lights would be on all the time instead of flashing.

    In my country, brake lights have to be red. What happened to the original brake light on your bike?

    I have also seen bikes with a row of red brake lights behind the back seat. You can get LED substitute lamps that would be good for this. They look pretty impressive.
  7. Oct 13, 2009 #6
    In the USA brake lights do have to be red. I replaced all 4 of the original Turn Signals on my bike with larger, double-sided Turn Signals. The backs are red and the fronts are amber. It is legal to have Turn Signals be red.

    I have already installed a set of red LEDs as additional brake lights. And they are very effective.

    However, when I take long camping trips my bike is loaded with lots of gear and it is difficult to see the regular brake light and one set of my supplemental LEDs are completely covered.

    So, if I could get my Turn Signals to also be supplemental brake lights that would help a lot.
  8. Oct 13, 2009 #7
    Thanks to you, I THINK I have a diagram that will work.

    I have seen posts on this website with wiring diagrams - how do I do that ?
    Then I could post what I think will work and you could check it.

    In words: four 5 amp diodes, one each on wires from brake switch to each turn signal, and one each on wires from flasher relay to each turn signal.

    Thus the current arriving at the bulb (from either source) could not back track to either the brake switch or flasher relay.

    These bulbs only draw 28 watts (less than 2.5 amps) so shouldn't a 5 amp diode be enough?

    (There is very little room to add electrics inside motorcycles.)
  9. Oct 14, 2009 #8


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    See attached diagram.

    Something like that would probably be OK. There will be a slight dimming of the rear flasher lights, though.

    The front flasher lights would be unaffected.

    Attached Files:

  10. Oct 14, 2009 #9


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    Just any FYI, positive ground systems were the norm long before negative ground. Any 6 volt system I've ever dealt with used positive ground. Some 12 volt systems even used positive ground for a while after the switch. Most of the time electronics are designed with the negative being the 'common' or 'ground' whichever terminology you prefer to use and it certainly isn't because they didn't know which way the current flows.
  11. Oct 14, 2009 #10
    Excellent - our diagrams are topologically the same although mine sure doesn't look as good as yours.

    Back to the diodes I have to buy, if you please:

    ? the voltage on a diode is the Maximum it can prevent from flowing 'backwards' ??
    so, in my case, anything over 15 volts will work for my application - right ??
    so the 50v diodes I have found are OK - not overkill - does it increase the voltage drop ?

    ? the amps on a diode is the Maximum it will allow to pass 'forward' ??
    so, in my case, anything over 2.5 amps will work -- right ??
    how much 'fat' should I allow for surge & etc -- is 5 amps, total, enough ?

    ? and even though the electrons are actually flowing from Negative to Positive ? I point the diodes when I solder them in toward the Negative ? the little color band toward the Negative?

    again, thanks a lot, Bob
  12. Oct 14, 2009 #11


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    Who is Bob? :) My real name is Mike.

    The reverse voltage rating of a diode does not change its forward voltage drop. This is pretty standard for silicon diodes.

    Don't forget, though, that these diodes will get hot. They will be dissipating 2-3 watts each.

    I suggested using bridge rectifiers. These come in a square block about an inch square and they can be mounted on metal to get rid of heat. You would need two of them.

    Don't worry about electron flow. Diodes conduct if their anode is more positive than their cathode.
  13. Oct 14, 2009 #12
    Very good to know - the slight dimming of my light is (I assume) from the voltage drop of the diode. So if I find a diode with 100volt PIV or 50volt PIV they will both work the same in my application.
  14. Oct 14, 2009 #13
    I assume the heat generated is from the voltage drop across the diode - when the diode is conducting current.

    If I understand your wiring diagram correctly - the diodes will only conduct current when the light is either flashing or braking. Both are short lived events. So I am just going to ignore the heat dissipated.
  15. Oct 14, 2009 #14
    I have found (so far) 3 diodes that meet my basic specs of 3+ amps and 15+ volts.
    However, I have no idea what the differences between them are:

    Schottkey diode
    10 amp, 100 PIV for 85cents each

    Rectifier diode
    6 amp, 200 volts at 90cents for two

    High-Voltage UltraFast Rectifier
    15A, 600V to 200AC at 1 dollar each

    As a shot in the dark - I would go with the Schottkey.
    Will that work??

    I would really prefer which ever diode is the smallest as there is very little room for installation.
  16. Oct 15, 2009 #15


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    Any of those diodes would probably be OK.
    I like Schottky diodes so I'd probably use those, but they would be a bit of overkill.
    You might get a lower voltage drop with Schottkys, which would be good.

    Diodes have very small mass and can overheat rapidly. They can be destroyed like that, so it is worth considering how hot they will get. If they get too hot to touch, they are probably at risk of destruction.

    Diode size influences heat dissipation so small diodes will get hotter for the same power dissipation.
  17. Oct 15, 2009 #16
    I assume when a diode dies, that it won't conduct current in either direction ??

    These diodes will be located under my passenger seat with lots of other wires and connectors. I would typically wrap them heavily with electricians tape for insulation. However, with the heat dissipation issue, perhaps I should wrap just the leads and leave the diode itself exposed to whatever air is circulating?? ((this assumes, of course, that the diode's exterior shell is non-conducting??))
  18. Oct 15, 2009 #17
    I found a Schottkey diode for 85 cents that is Dual packaged
    It is 10 amps, 100 volts.
    It has 3 leads: 2 IN and 1 OUT
    I assume that means 10amp diode on EACH lead IN??

    This will make wiring easier and smaller (I think).

    As only 1 diode would be working at any given time, this could help heat dissipation ??

    Well, here my lack of EE education raises an interesting question.

    When the Flasher is ON it will allow current into the Bulb - lets call it 2 amps for simplicity.
    When the Brake is ON it will allow current into the Bulb - it would also be 2 amps because that is what the 1157 bulb will draw. So only one diode will be dissipating heat at once.

    However, what if the Brake is ON while the Flasher is ON?? I assume the 1157 bulb will only draw 2 amps no matter how much current is available?? So 1 amp will come from each?? The Brake (being constant) will fulfill all the bulb's current draw, so the Flasher will not send any more current to the bulb? What would actually happen??
  19. Oct 15, 2009 #18


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    Diodes usually fail in the shorted condition. Maybe it depends on what caused the failure.
    Either way, it is bad news and you don't want it to happen.

    You could use those dual package Schottkys if they have two anodes and a common cathode.
    They are probably like the ones in PC power supplies and are intended to mount on a heatsink.
    They would look like a small power transistor.

    If Silicon diodes are in parallel, the one with the lowest turn-on voltage will conduct first. At heavier currents, the voltage across the conducting diode will increase and eventually the other diode will start conducting. Schottkys however, don't have such a sharp turn-on voltage as normal Silicon diodes, so I'd guess they would tend to share the load.

    Either way, the lamps would still only get their normal current or a little bit less.

    I would never wrap up a diode in anything unless it was drawing a trivial current. It makes a big difference to the heat dissipation.
  20. Oct 16, 2009 #19
    Yeah, they have a metal tab with a mounting hole.

    So my plan is to attach the wire from the Flasher Relay to one anode and the wire from the Brake Switch to the other anode. Then simply run the wire from the Bulb to the common cathode.

    They are 10 amp diodes (in a dual package) and I will only be running 2.5 amps through each one and only when my Turn Signals are Flashing or Braking - so I am expecting they won't overheat.

    You have been a great help - so thanks a lot.
    I will order parts this weekend and let you know when they get installed.
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