On Fri, 9 Apr 2021 18:02:40 -0700, sms
wrote:
On 4/8/2021 5:43 PM, Jeff Liebermann wrote:
snip
Pathetic? The idea doesn't go far enough. That was the dynamo I was
thinking of when I suggested that it be used on disk wheels. The idea
was to move the dynamo along the radius of the disk wheel to adjust
the dynamo's rotational speed and thus obtain a constant output output
voltage at any speed.
A CVT inside the dynamo would be a better design, whether a rim, tire,
or hub. But of course it's really unnecessary to have a constant voltage
out.
I was looking for simplicity. A CVT (continuously variable
transmission) is not a simple device to build into a small dynamo
housing.
There are already dynamo powered lights with a USB port for charging
devices. If someone is really going to spend $300 then they'd be better
off buying a dynamo hub wheel and one of those lights, or a battery
powered light that can be charged and/or operated from a dynamo.
For the latter, I put together a Google Doc document that explains how
to do it
https://docs.google.com/document/d/1UVk_yelr5UxrcKqDtGgsHTBVa3eWoUdpqDKhbZSHO_E
Very nice but I screwed up.
The DC output voltage from a bridge is NOT:
DC_out = 1.414 * AC_Vrms_input
More correctly, the DC output is:
DC_out = 0.9 * AC_Vrms_input
For the calcs, see:
https://www.electronics-tutorials.ws/diode/diode_6.html
For such a low voltage dynamo, the Schottky diode voltage drop should
also be included in the calculations.
Another problem is you're ignoring the original problem(s) and solving
one that was not even mentioned (USB devices).
Jay arbitrarily decided that an 800 lumen light would be nice if it
could be powered by a dynamo. Without questioning the decision to
specify such a high power light, I went into what is necessary to
power an 800 lumen light solely on dynamo power. It was generally
assumed that building a battery operated 800 lumen light, where the
battery is charged by the dynamo, is possible. I later estimated the
percentage of power produced by the typical cyclists at 15 mph using a
commodity 3 watt dynamo light, and a higher power light suitable for
delivering 800 lumens. While there was an increase in drag, it was
within the abilities of most cyclists. All that was needed was a
dynamo that delivered both higher output power at higher efficiency.
I then arbitrarily decided that removing one of the key features of
most bicycle dynamos would offer better efficiency at high RPM. The
early dynamo power lights ran incandescent lamps. These lamps were
very sensitive to over-voltage. Even a little over their rated
maximum power, and the filament would fuse open. The resulting dynamo
included an inductor (saturable reactor) which would reduce the
voltage to the incandescent lamp at high RPM, where the voltage was
highest. This formed a crude and cheap voltage/current regulator and
is responsible for the "flat" voltage and power curve of the dynamo.
What I proposed was to remove the inductors and allow the dynamo to
produce more output power at high RPM. However, Frank offered a good
question. Does this also improve efficiency at high RPM. I assumed
that it does, but I'm not sure and won't know without some testing.
Another item that was mentioned was operating the headlight at low
bicycle speeds, where the dynamo is unable to deliver sufficient
output. This can be in various ways such as a CVT, PWM (pulse width
modulation) light dimmer to reduce the load, switch to fewer LED's,
etc. Of course, a battery/dynamo hybrid will solve the problem as
long as the battery can be recharged, but for now, batteries are not
part of the dynamo problem.
My best guess(tm) is that a higher power dynamo can only work if the
losses are reduced. That means, much higher voltages, no saturable
reactor regulators, very wide input range boost converter on the
input, high efficiency buck converter to drive the LEDs, and high RPM
for high operating frequencies (about 50KHz) so that ferrites can be
used instead of powdered iron. In other words, start from scratch and
design something that will drive a modern LED headlight, not an
adaptation of something designed to drive an ancient incandescent
light bulb.
--
Jeff Liebermann
PO Box 272 http://www.LearnByDestroying.com
Ben Lomond CA 95005-0272
Skype: JeffLiebermann AE6KS 831-336-2558