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Thread: Electronics/ Wireless World 1994/5 Dynamo backup

  1. #1

    Default Electronics/ Wireless World 1994/5 Dynamo backup

    I always liked the battery backup in Dec 1994 and repeated/clarified in Oct 95 Wireless world which was
    a voltage doubling rectifier (cascaded type (so no floating generator needed) ) clamped to 27V followed by a LM2575 step down regulator to 6V (which trickle charged 4 ni-cds for backup when stopped. The LM2575 was held off until rectifier output capacitor had reached a bit more than 20V DC.
    .....
    which appealed to me because the designer claimed that operating at the higher voltage of 30 Volt-ish (and less hub current) compensated for the solid state losses by reducing I2R losses at the dynamo coils ( a 15% efficient union 6701). He seemed to imply that his ni-cds never needed topped up.
    I never built the circuit (at the time of Halogen bulbs) because at the time I thought a switcher would not work on stripboard ) and then LEDs arrived. But when I mention it in the context of LED lamp drivers the response seems underwhelming....

  2. #2

    Default Re: Electronics/ Wireless World 1994/5 Dynamo backup

    I am reworking the LED conversion of my winter bike with S-A Dynohub and still deciding on the electronics side, so this looks very interesting.
    Is it possible to post or link the circuit, or better the whole article? Otherwise, a trip to the public library and probably a long wait.

    The circuit sounds well-adapted to bulbs, but might need some adaptation for LEDs. But even with a bulb, the voltage doubler seems odd to me.
    It implies limiting the voltage at the dynamo to 13.5V, which could be a fairly low speed. With the batteries to handle low speeds,
    a higher output would be possible without the doubler.

    I think the key idea here is the 20V threshold forcing the dynamo/buck combination into effective operation, with the battery covering the gaps.
    I doubt there is much information on how rechargeables handle such frequent, shallow charge/discharge cycles, but no reason to think it harmful.
    With LEDs there is also the possibility of eliminating the battery, at some cost in flicker. With experience from my existing LED conversion,
    I see low-speed flicker as an advantage, but it is quite low-powered.

  3. #3

    Post Re: Electronics/ Wireless World 1994/5 Dynamo backup (pdf link)

    Quote Originally Posted by gilesa View Post
    .....Is it possible to post or link the circuit, or better the whole article?
    Both magazines (Dec 1994: original circuit idea (on pdf page 45 of pdf) and and Oct 1995 containing clarifying letter (page 884 of pdf)) are available at https://www.americanradiohistory.com...d_Magazine.htm

    The circuit sounds well-adapted to bulbs, but might need some adaptation for LEDs.
    I presume you mean a raw LED rather than an approved 6V Dynamo LED lamp unit?....I think using a LED Specific switch-mode IC (with shutdown pin) in place of the LM2575 that would be a better basis for that.
    But even with a bulb, the voltage doubler seems odd to me.It implies limiting the voltage at the dynamo to 13.5V....
    But the doubler output will droop once loaded heavily....I think (hope?)holding off till 20V is to avoid very low speed flicker, to allow smoothing caps to charge up enough that will last a half cycle
    Last edited by aahhaa; 12-18-2019 at 09:56 AM. Reason: middle para corrected

  4. #4

    Default Re: Electronics/ Wireless World 1994/5 Dynamo backup

    Thank you for the link. I should be more careful what I ask for, as that archive has already been a great time waster!

    > I presume you mean a raw LED rather than an approved 6V Dynamo LED lamp unit?

    Yes. I am re-using a chrome-plated headlamp with polished metal reflector that is probably original,
    About 1952, so now "vintage" enough that I want to avoid any irreversible changes.
    Also, I think most such approved lamps have their own electronics. I would be reluctant to gut one.

    > I think using a LED Specific switch-mode IC (with shutdown pin) in place of the LM2575 that would be a better basis for that.

    If the battery is not dropped, then a constant-voltage output is still useful. I would like to lose the battery (maintenance) and
    use a capacitor standlight. Do you know of suitable ICs? The Zetex ZX*3** series will not handle the input voltage and
    everything else I found seems over-specified. Also, 20-pin SMDs are well beyond my hand-soldering skills.

    > But the doubler output will droop once loaded heavily....I think (hope?)holding off till 20V is to avoid very low speed flicker, to allow smoothing caps to charge up enough that will last a half cycle

    After seeing the circuit, I understand the doubler. The caption says something like "... full output at walking pace."
    With that goal, there is no incentive to use all available power at higher speeds. With bulbs, that would mean a switchable
    second and even third headlamp. Now there are incentives to use that power: LED characteristics and device charging.

    I was a little surprised to see only four cells, That means a dim bulb without dynamo input.

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