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I have a circuit in my bicycle where I get the variable voltage, variable frequency AC from a hub dynamo, rectify it with a bridge, regulate it with a 6V zener, and filter it with a 6800 uF capacitor.

While I am moving, the voltage gets quite stable and I can successfully power some 20 leds (around 400mA max), but as soon as I stop, lights turn off.

Now that I want to add an Arduino to the circuit (with a LM2940 LDO to step down from 6 to 5V), I need something that holds more charge - even if it is only part of the Arduino subcircuit - because otherwise the Arduino will keep resetting.

Since the capacitor is "just" something that holds charge at a given voltage range (after it's been regulated by the zener), I wonder:

Could I replace a Zener + capacitor regulator/filter in an intermitent charge/discharge circuit with a battery pack? What should I take into consideration? Would I need a dedicated power-path circuit, or is there a "pure dumb battery" way of doing it?

My goal is not even to have the leds on all the time, I actually would only want some juice to prevent Arduino resets at stop lights (2 minutes max). All the leds would be driven by Mosfets, so the Arduino path consumption should be quite low.

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    \$\begingroup\$ Have a look at super-capacitors (also called Electric Double Layer Capacitors). They can handle frequent charge/discharge cycles and are ideal for these sorts of buffering applications. \$\endgroup\$
    – Jon
    Sep 9, 2015 at 21:40
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    \$\begingroup\$ I think you may have to have two storage caps. One for the LEDs to prevent flicker and a diode ISOLATED one for the Arduino to keep it running for longer pauses. It should draw much less current so a big cap or modest supercap may be enough. \$\endgroup\$
    – KalleMP
    Sep 10, 2015 at 5:52

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Here's a quick schematic I put together of your idea. It basically uses a supercap(as Jon suggested, sizing the supercap would totally depend on the expected runtime of the device) as energy storage. The circuit might need more refinement, but should give you a rough idea of the components, and i think it is totally viable. Let me know if you need any more info on the schematic

schematic

simulate this circuit – Schematic created using CircuitLab

It basically uses as Verter module from adafruit, couple of 250F 2.5V supercaps(digikey has plenty of these) in series and a bq24640(I could not find an off the shelf module for this, but there are eval kits available for this chip on digikey)

hope this helps, good luck!

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  • \$\begingroup\$ Thanks for your sample project. I found it a bit more complex that I hoped, but this Verter thing is quite interesting! \$\endgroup\$ Sep 10, 2015 at 13:35
  • \$\begingroup\$ yes also ebay has a ton of modules intended for similar applications, or if you're up for some more fun you can build your own modules! \$\endgroup\$
    – user7994
    Sep 10, 2015 at 18:49
  • \$\begingroup\$ Well, actually I was hoping for a more simple/passive/dumb circuit, but I'm afraid that just actually swapping my cap for a battery, one for one, would expose the battery to an excessively irregular and possibly dangerous charge/recharge regimen, am I right? \$\endgroup\$ Sep 10, 2015 at 19:36
  • \$\begingroup\$ Yes, your supply would have too much ripple on it, also it is not regulated, the super cap would not only act as a backup but also 'smooth' out the ripple. Have you checked alibaba/ebay, I suspect there might be some off the shelf solution for this. \$\endgroup\$
    – user7994
    Sep 10, 2015 at 19:41
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You won't have reliable performance if you put the super caps in series like that. If the 2 caps are not equal, or if on leaks charge more than the other, the voltage won't share equally across them and one will get damaged from over voltage.

you can put 2.5 V zeners across each individually (not readily available !), or just a 1 Megohm R across each.

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