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I'm building a lighting system for a bike, powered by a dynamo (typically rated 6V, 3W max). The load will include LEDs and a bit of logic (such as N555/ATTiny85 for blinking, etc.). I also plan to include some form of short-term energy storage (for red lights): either a supercapacitor or a small lithium battery.

What is an appropriate choice for rectifying the 6V AC input? Obvious choices are:

  • a full-wave diode bridge rectifier (pro: easy and cheap, exists as a single-package; con: but at a low voltage, that 1.5V loss stings a bit);
  • a Schottky diode bridge (a bit more efficient, a bit pricier, I don't believe it exists as a single package);
  • a simple MOSFET bridge, which I used in a previous iteration of this circuit, but it apparently cannot charge a capacitor (although I believe I used it successfully for that purpose in said previous circuit...), which can have a low drop but is more cumbersome;
  • a MOSFET bridge driven by a LT4320: I've never used this component, and it apparently needs at least 9V to work. Also, I've never soldered SMD yet.
  • there seems to be a dedicated chip, FDMQ8203, but that is a bit expensive (incl. shipping cost) for a chip I'm not sure to be able to solder (tiny MLP package).

Did I miss any other rectifier circuits, and is my evaluation of the pros and cons of each type of rectifier accurate?

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    \$\begingroup\$ Technically bikes use alternators. Dynamos generate DC. Dynamo is the common (mis-)terminology however. \$\endgroup\$
    – Transistor
    Commented Sep 18, 2020 at 13:20
  • \$\begingroup\$ Is it for a bike or for bikes? If it's for a product in series production I wouldn't worry too much about soldering. \$\endgroup\$
    – Lundin
    Commented Sep 18, 2020 at 14:09
  • \$\begingroup\$ It's for one-ish bike - mine, and then maybe those of my immediate family. Definitely not a series production. (Although the complete circuit, STL, etc. might end up on Instructables or the like one day, this will be somebody else's problem). \$\endgroup\$
    – Circonflexe
    Commented Sep 18, 2020 at 16:08

3 Answers 3

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If your LEDs are not in series then about 3.5 VDC will suffice for short term battery operation and maybe 4v+ when directly powered.

A Schottky diode bridge will drop about 1 volt, allowing reasonable headroom from a 6 V alternator.. 6VAC will rectify to around 7V+ under load.

A single lithium ion cell will be suitable for this application. A readily available TP4056; charger module with under voltage discharge protection should do what is needed. A fully charged battery, + Schottky diode will provide about 3.5VDC to the LED circuitry. When the alternator is operating the voltage will exceed the battery voltage and the battery will not discharge.

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A MOSFET bridge as shown won't charge a capacitor since it also allows current to flow in reverse. You can add a Schottky diode at the output, or just replace the bottom 2 MOSFETs with two schottkys.

A bike 'dynamo' acts like a constant current source -- so when lighting a lamp, the brightness doesn't change much with speed. It does this because while the voltage generated rises with speed, so does the impedance (Z) from the inductance of the winding -- the net result is that the V/Z remains quite constant. So, if your LEDs don't fully load the dynamo, its voltage (and so the rectifier voltage) will become quite high -- you will need a high BV rectifier, and if you use MOSFETs, be sure their VGS isn't exceeded.

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The question has arisen before on this site and I can't find an answer that tackles your exact question. In the meantime you may find some of the following questions and answers of interest.

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