Following the circuit diagram, included as an image in this guide, where do I replace/add a larger capacitor for continued power while the bike is stopped?

My guess is that it should replace the capacitor between the rectifier and the voltage regulator, but I'd like some input from someone who knows. I'm planning to insert a 2200uF capacitor.

enter image description here

  • 2
    \$\begingroup\$ Well - yes - but only for a very limited definition of "continued power". You'd probably increase the time from a few milliseconds to a few tens of milliseconds. \$\endgroup\$ – brhans Mar 15 '17 at 19:40
  • \$\begingroup\$ @brhans, I'm interested in even nominal improvement. Where should the cap go? \$\endgroup\$ – radicalbiscuit Mar 15 '17 at 19:43
  • 1
    \$\begingroup\$ If you have a rough idea of the amount of current your bike is drawing during these periods where you want it to have continued power, we can figure out how long. \$\endgroup\$ – pipe Mar 15 '17 at 19:43
  • 3
    \$\begingroup\$ it has to be at input, Vin \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Mar 15 '17 at 19:53
  • 4
    \$\begingroup\$ For what it's worth, a 12v drain from a 2200uF cap (draining 17v to 5v on the input side) will run a 5 volt device on the output at 200 milliamps (1 watt) for 158 milliseconds. You need a BIG capacitor to run anything for a few seconds. \$\endgroup\$ – Bryan Boettcher Mar 15 '17 at 20:16

Your guess is right. Placing it on the "left" side of the regulator works well. You can also place it on the "right" side, but then you would need a few extra components (at least one extra diode). It's easier on the "right".

Note: I have to use stupid words like "left" and "right" because the person who drew that schematic failed to add reference designators to the components. Don't be that guy.

A few caveats:

  1. You have to make sure that the capacitor has a sufficient voltage rating. Since you're taking the AC voltage from a dynamo, it's hard to say how high the maximum voltage will ever be. I have very little experience with them. I assume that 6 V is just a "nominal" value, and the output can increase. The peak voltage will be the AC voltage times 1.4, then subtract about 1.6 for two diodes in series (your bridge). $$6 \cdot 1.4-1.6=6.8$$ Now double this (13.6 V), and buy a capacitor rated for at least this much. I suggest at least 16 V, but 25 V would feel safer.

  2. As brhans pointed out in a comment, unless you seriously beef up the capacitance, chances are that you will hardly notice the increased time. This is hard to say without knowing the expected load. The drawback of using a silly amount of capacitance (part from the obvious: cost and size), is that you will get a slower start-up time; it will take time to charge up the capacitors.

  • \$\begingroup\$ Many thanks for the answer and maths. 6V is a nominal value. I don't have my hub dynamo yet, but I read that someone measured output at up to 17V. I'm not a fast rider, but some hills are steep enough to get up to high speeds. Given your advice, I think I'll go for 50V rating. \$\endgroup\$ – radicalbiscuit Mar 15 '17 at 20:09
  • \$\begingroup\$ @radicalbiscuit Good! 35 V is probably safe enough even in the extreme events of 17 VAC input, but the cost difference will be minimal anyway. \$\endgroup\$ – pipe Mar 15 '17 at 20:18

The generator undoubtedly has a current limit, so a 'capacitor' after the diode bridge and before the regulator will work, but a battery in the same place will work better. 6V gel cell would be appropriate, and your wheels won't go dark at an intersection on a rainy night.

A battery will be damaged if it discharges too far, so a disconnect switch would be prudent.


If your generator is like the "dynamo" (sic) on my bike, as your road speed increases it tends towards a constant (AC) current device. Reason: both its EMF and its internal reactance increase in proportion to its rotation frequency. I have for several years used a stack of either 5 or 6 1.2V AA size NiMH cells in this position. They sometimes need recharging offline if I do too many miles with the lights on.

  • \$\begingroup\$ That would definitely do the trick. Just wondering, what's incorrect about "dynamo?" \$\endgroup\$ – radicalbiscuit Oct 19 '17 at 20:21
  • \$\begingroup\$ I was taught at school that the device on my bike is an "alternator". A "dynamo" inherently generates direct current (DC). \$\endgroup\$ – Dreamer Dec 22 '17 at 23:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.