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I’m trying to design a circuit to enable my bicycle lights and also have at least 2 minutes of standing lights coming from a bicycle 6 V AC hub.

I thought about building a rectifier bridge with 4 1N5820 diodes and then have 3 sets of capacitors:
1 for straightening the voltage
1 for regulating the voltage
1 to regulate the exit voltage

But then I got the idea to have 2 super capacitors to allow me to have 2 minutes of standing lights.

Now, getting into the challenges:

  1. Can’t I regulate the voltage from the hub after the rectifier bridge with a capacitor?
  2. How do I enable the super capacitors to charge while allowing the lights to light in parallel?
  3. Do I need an NPN transistor to control the discharge of the capacitors while no power is generated by the hub?
  4. I see lots of people talking about using a MOSFET for that, but I can’t seem to understand why it’s better.

Any help would be appreciated.

enter image description here

ocrdu
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Joe
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  • _”1 to regulate the exit voltage”_ What is exit voltage? – winny Sep 12 '22 at 06:08
  • Capacitor across lights fed by bridge will do what you want. Capacitor will charge as lights operate. Capacitor will discharge into lights. Note that supercap MUST withstand max voltage from alternator. If Vcap drops from say 6V to 3V then for a 1 Farad cap E available = 0.5 x C x V^2 = 0.5 x 1 x (6^2-3^2) = 13.5 joule. If used over 2 minutes this gives power of P=e/t = 1.35/120 ~~= 100 mW. or 50 mW each front and back. With eg 150 l/W LEDs that's about 7 lumen each. That's usable but you really want more. 10 farad would be about right. – Russell McMahon Sep 15 '22 at 11:28

2 Answers2

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  1. Capacitors don't regulate voltage, so you can't regulate voltage with a capacitor.

  2. With a suitable charging circuit, similar to rechargeable batteries. Maybe a constant current circuit with a voltage limit.

  3. Maybe or maybe not, but you don't say where in the circuit the transistor would be or what would it do in the circuit.

  4. MOSFETs can be better in some circuits due to less voltage drop.

ocrdu
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Justme
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  • Hi, I thought that capacitors smooth the current, so, basically combining it with resistors, you can regulate the output. – Joe Sep 10 '22 at 10:04
  • Also, for a newbie like me, your answer didn’t really help me going forward…. But thanks for trying – Joe Sep 10 '22 at 10:05
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    Regulation and smoothing are different things. What kind of answer you had in mind then to go forward?Maybe edit the questiond to be more detailed how you want them to be answered? – Justme Sep 10 '22 at 10:44
  • My professors in uni used to say, if you don’t understand it means we didn’t explain it good. So, basically, I’m trying to say, I’m trying to solve a problem, and I find difficulties with the theory, I also uploaded a thought diagram… an answer that would make me better understand would be appreciated. – Joe Sep 10 '22 at 11:35
  • Give you an example, you said smoothing and regulation is different, I’d expect a diagram, an equation, a link or maybe just an explanation for the statement. – Joe Sep 10 '22 at 11:37
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    @Joe you posted your questions at a question and answer site, not a forum ... questions are answered as asked ... edit your post so that it clearly describes the problem you are trying to solve ... then ask a question about the problem, not about your solution to the problem ... transistor, capacitor, supercap are all your attempted solutions – jsotola Sep 10 '22 at 19:09
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I assume, you don't need the standby light function immediately. I replaced your charge diode by a resistor R1. Using this you can charge the supercapacitor slowly with low current. This additional current is not a problem for a typical alternator.
D1 is an overvoltage protection of 5.6 V for the capacitor. Using diode D2 costs ~0.4V, the standby light receives 5V only.

schematic

simulate this circuit – Schematic created using CircuitLab

If you add a boost converter between the capacitor and the light, you can squeeze much more power out of the capacitor but this needs a lot more components and enable/disable logic.

Jens
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