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Thread: Establishing rear dynamo light power requirements

  1. #1

    Default Establishing rear dynamo light power requirements

    Hi, I hope somebody can help here.

    I want to power my rear dynamo light which is an Exposure Red Eye (http://www.exposurelights.com/cycle-...eye-long-cable) off a front Dynamo light other than the Exposure Revo. The Exposure Red Eye and Exposure Revo are both designed to work together.

    The new front dynamo light output is 9V to the rear light. However the Exposure Red Eye I believe requires less than that. How can I establish the max input voltage and max current requirement of the rear light in order to buy a suitable resistor, if required, to drop the voltage from 9V to whatever it is that the Red Eye requires? At this stage I'm only assuming the voltage needs to be dropped. I think the LED is a Seoul P4 and to be honest I'm not sure whether it's red or white.

    This new dynamo light however doesn't have a super capacitor to the rear light so I'll then need to figure out how to provide a standlight to the Red Eye because normally the Revo carries out that function. So what would be the best way of establishing what super capacitor it is that I need to buy? Presumably that's all I'll need i.e. just a resistor and a super capacitor?

  2. #2
    Flashaholic* znomit's Avatar
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    Default Re: Establishing rear dynamo light power requirements

    Why not use a B&M secula instead?
    http://en.bumm.de/produkte/dynamo-ru...ht/secula.html

  3. #3

    Default Re: Establishing rear dynamo light power requirements

    Quote Originally Posted by znomit View Post
    First is that I have a Red Eye in my possession and secondly because having seen those B&M Seculas they are not water tight and not as robust as the Red Eye. I will be using it off-road in all conditions so a trustworthy light is of great importance to me.
    The manufacturers of these rear lights don't provide the maximum voltage rating for these anywhere that I can find. Being sure that my new front light with it's 9v output won't overdrive the rear lights is complete guesswork it seems.

  4. #4
    Flashaholic angerdan's Avatar
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    Default Re: Establishing rear dynamo light power requirements

    There's a 12V IP67 rear light from supernova:
    supernova-lights.com/en/m99-taillight

  5. #5

    Default Re: Establishing rear dynamo light power requirements

    Quote Originally Posted by angerdan View Post
    There's a 12V IP67 rear light from supernova:
    supernova-lights.com/en/m99-taillight
    Thanks for all these suggestions for new products. I don't want to buy a new light though. I want to do some DIY work using my existing lights and my non existent electrical talents. So the rear light I've established can handle no more than 4.25 volts. With the output of the front dynamo light operating up to 9 Volts what would be the most effective way to ensure my rear light doesn't melt? Additionally it has no stand light feature so I'd need to add a capacitor in the line to the rear light somewhere. How would I work that out?

  6. #6
    Flashaholic angerdan's Avatar
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    Lightbulb Re: Establishing rear dynamo light power requirements

    With the non existent electrical talents i would try to find some electrican who does all the soldering and connections for you.
    There are ready solutions, but then you'd have to buy additional stuff.

    In case you're only looking for an AC-DC step-down buck adapter:
    ebay.co.uk/itm/Netzteil-AC-DC-3-3V-600mA-Stromversorgung-Buck-Konverter-Step-Down-Modul-/142446627019
    Last edited by angerdan; 10-07-2017 at 03:31 PM.

  7. #7

    Default Re: Establishing rear dynamo light power requirements

    Quote Originally Posted by angerdan View Post
    With the non existent electrical talents i would try to find some electrican who does all the soldering and connections for you.
    There are ready solutions, but then you'd have to buy additional stuff.

    In case you're only looking for an AC-DC step-down buck adapter:
    www.ebay.co.uk/itm/Netzteil-AC-DC-3-3V-600mA-Stromversorgung-Buck-Konverter-Step-Down-Modul-/142446627019
    Ok I lie. Not totally non existent. Just my electrical knowledge is poor, basically the Maths side of it. Initially I soldered two 2 Watt resistors in series; a 12 ohm and a 3 ohm resistor. The result of this was the rear light was very bright and, as I cycled along unplugging the rear light from the front light, there was a noticeable difference in the brightness of the front light. So the rear light was drawing way too much power. So I had a 60 ohm 0.25W resistor which I soldered in place of the other two. The rear light is still very bright with a slight dimming of the front light as I unplug and re-plug the rear light. This has all been very trial and error. My schoolboy physics knowledge, which is likely incorrect, tells me that increasing the resistance will increase the voltage, V=I*R. So I'm concerned that if I use say a 90 ohm resistor next will I breach the 4.25V limit of the rear light. I don't have a multimeter to hand...yet.

    The objective being to have a respectably bright rear light whilst affecting the brightness of the front light as little as possible. Then there's the capacitor to act as stand light for the rear light which I'll need to incorporate.

    I can't see how that module will work when the front light's output to the rear light operates between 0V-9V depending on the speed I'm cycling.

  8. #8

    Default Re: Establishing rear dynamo light power requirements

    Hi raggatip,
    I suspect that you are seeing a problem when there really isn't one. That is, I think you need to be thinking in terms of current rather than voltage. As I understand it the dynamo is a 'current device' as are LEDs. If the connection to your new front dynamo light is in parallel rather than series, the rear light (your RedEye) will only see the voltage it's LED demands to fire up (i.e; it's forward voltage). The reason you saw your front light dim is because your rear is drawing current away from it because it has a lower Vf. I believe that you can reduce this current draw by increasing the impedance of the rear circuit with a series resistor - but others here will be able to explain this far better than I. It's been covered a number of times in other threads (I know because I've asked a similar question). What is the front light you are using BTW? I've never heard of one with an output of 9 volts. All the ones I have seen simply supply the rear light with a parallel connection 0 either before the rectifier or immediately after it. I imagine that the Revo does one or the other...
    Sam.

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