Re: That Lowe's flashlight, again

Peter Cole wrote:
SMS wrote:

A bridge rectifier is only 81.2% efficient (maximum with zero-voltage drop diodes)

You keep repeating this, but it's not true. You misunderstand (and misquote) a reference on rectification.

I think where you're confused is that you're looking at the AC input power and the full wave rectified power, but not the DC power. Indeed, with perfect diodes, you'd have 100% efficiency, but you won't have DC yet! When you convert to DC, using capacitors to filter out the ripple, you are at 81.2% (assuming perfect diodes and capacitors).

If you're able to run the LED directly from the the rectified AC power from the dynamo, without over-voltaging it, then you could get by
(put two LEDs in series, as you want to keep them under 4.5 volts).

In reality, the almost all LED lights run off pure DC. Many flashlights use DC-DC converters between the LED and the power source so they can run off of a variety of different voltages.


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Relevant Pages

  • Re: That Lowes flashlight, again
    ... I think where you're confused is that you're looking at the AC input power and the full wave rectified power, ... Indeed, with perfect diodes, you'd have 100% efficiency, but you won't have DC yet! ... The ideal diode has a discontinuity in its resistance, changing from infinite to zero when it passes through the forward voltage threshold. ... The only reason to stack 2 or more LEDs in a generator driven application is to improve the efficiency slightly by matching the generator load more closely to its designed load and making the real rectification power a smaller fraction of the load power. ...
    (rec.bicycles.tech)
  • Re: That Lowes flashlight, again
    ... misquote) a reference on rectification. ... power and the full wave rectified power, ... with perfect diodes, you'd have 100% efficiency, but you won't have DC ... You are mixing up voltage and power. ...
    (rec.bicycles.tech)
  • Re: That Lowes flashlight, again
    ... I think where you're confused is that you're looking at the AC input power and the full wave rectified power, ... Indeed, with perfect diodes, you'd have 100% efficiency, but you won't have DC yet! ... LEDs are constant voltage devices. ... The only reason to stack 2 or more LEDs in a generator driven application is to improve the efficiency slightly by matching the generator load more closely to its designed load and making the real rectification power a smaller fraction of the load power. ...
    (rec.bicycles.tech)
  • Re: That Lowes flashlight, again
    ... power and the full wave rectified power, ... with perfect diodes, you'd have 100% efficiency, but you won't have DC ... (put two LEDs in series, as you want to keep them under 4.5 volts). ... run off of a variety of different voltages. ...
    (rec.bicycles.tech)
  • Re: That Lowes flashlight, again
    ... misquote) a reference on rectification. ... power and the full wave rectified power, ... with perfect diodes, you'd have 100% efficiency, but you won't have DC ... Ideally, since you need a stand-light anyway, you'd be charging a small SLA battery with the rectified and filtered output from the dynamo, and the DC-DC converter would drive the LED. ...
    (rec.bicycles.tech)