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Thread: Axa Pico 30 dynamo front lamp

  1. #1

    Default Axa Pico 30 dynamo front lamp

    Hello all.

    After about four years my Axa 30 Pico dynamo light started to become unreliable, cutting out at speed. I've since replaced it with a different light, but thought I would take a look inside to see if it could be salvaged or repurposed.

    The images can be accessed here: https://forum.cyclinguk.org/viewtopi...01260#p1301131

    It looks pretty simple, with only eight components. Would anyone hazard a circuit diagram, and maybe suggestions for how it could be 'fixed' or improved? It is surprisingly effective as a light, and I was surprised when I opened it that it had survived so long, despite apparently having zero waterproofing.

  2. #2
    Flashaholic* Steve K's Avatar
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    Default Re: Axa Pico 30 dynamo front lamp

    The photos on the CyclingUK forum are helpful, but it's not easy (or not possible) to figure out the schematic from them. You should be able to look at where the traces run in order to figure out what is connected to what.

    Even with that, you'd need a meter to figure out of any of the devices are shorted or opened, or if there is a cracked solder joint, etc.

    At a quick glance, my guess is that the large rectangular black component near the connector is a bidirectional zener diode. This is intended to clamp the voltage to a safe level. It's possible that this could be shorted out, which would prevent the headlight and taillight from working.

    Not sure what the three terminal device is that is between the zener and the bridge rectifier.

    The only remaining parts are the LED, the super cap, and three resistors. Well, there's also the switch, and that could have some problems.
    To be honest, the connector could have problems too. A meter would really help to answer these questions.

  3. #3

    Default Re: Axa Pico 30 dynamo front lamp

    One of the common problems with circuits is that the water gets in leaving sediments that short the circuit. You can try to wash the circuitry in distilled water, rubbing it with a brush. Dry out the circuit and test it with a 6V source that can be DC, in place of the dynamo. Once you determine that everything works fine, apply a conformal coating to the board. At times, when judging that the circuitry is poorly sealed, I open up a new product and apply a coating to be a step ahead of problems.

  4. #4

    Default Re: Axa Pico 30 dynamo front lamp

    Thanks all. Belatedly, I had a go at sketching out the circuit. You can see it here: https://forum.cyclinguk.org/viewtopi...07428#p1307428

    I haven't actually tested it - the light may be fine. It only intermittently cut out (on fast descents) - so not a permanently shorted zener.

    Does anyone know how the transistor works? It seems odd that it's on the AC side of the circuit. Is it there to regulate voltage or current?

  5. #5
    Flashaholic* Steve K's Avatar
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    Default Re: Axa Pico 30 dynamo front lamp

    I'm having a hard time figuring how a transistor would be used on the AC side of the bridge rectifier. There is a slim chance that it is there to act as a shunt regulator, in order to limit the charge voltage of the supercap. Of course, the LED and 3 ohm resistor should be doing a reasonably good job of that already.
    As wired, a pnp transistor wouldn't function as a shunt regulator.....

    I'll guess that the device isn't a transistor at all. The schematic doesn't show a part number, so I'll assume that you checked the pins with the diode test feature of a meter?? That is usually enough to at least indicate if there is a pn junction across the pins, but that doesn't necessarily mean that the device is a transistor.
    Hmmm..... I'm stumped!

  6. #6
    Flashaholic* Steve K's Avatar
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    Default Re: Axa Pico 30 dynamo front lamp

    a quick update... it occurred to me that the "transistor" could be a dual diode package. This is a pair of diodes that are connected and housed in a single package. An example is this part from Diodes Inc. .....
    https://www.diodes.com/assets/Datasheets/ds30232.pdf

    My guess is that this dual diode is intended to quickly charge the supercap through the 10 ohm resistor.

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