The number of LEDs is still negotiable, but I imagine I want anywhere
from 12 to 32, if the power supply can handle it.
---
You've got to make up your mind what you want to do first. That is,
if you want a certain number of LEDs, then the power supply _has_ to
be designed to handle that number. On the other hand, if your power
supply decision is fixed, then the number of LEDs you can have will
also be fixed. For example, if we look at a super-bright red LED
with a forward voltage (Vf) of 1.7V at a forward current (If) of
20mA, then each of those LEDs will consume:
P = If * Vf = 0.02A * 1.7V = 0.034W = 34 milliwatts
and the array will consume 36 times that, or about 1.22 watts.
That's not an awful lot of power, and probably either of your motors
could generate that.
The nice thing about the LED array is that it can be arranged to
"fit" whatever your motor can put out by wiring the LEDs in series,
in parallel, or in series-parallel.
That is, if you wired all 36 LEDs in series their Vf's would add,
but their If's would stay the same, so the power supply would have
to be able to put out 36 * 1.7V = 61.2V at 20mA.
On the other hand, if you wired all the LEDs in parallel their Vf's
would add, but their Vf's would stay the same (well... kinda. More
on that later.) so the supply would have to be able to supply
36 * 20ma = 720mA at something greater than 1.7V. Greater, because
the Vf's of the LED's won't all be the same, so the current into
each LED will have to be limited by a resistor or a current
regulator of some sort in order to keep the LEDs with low Vf's from
being current hogs and committing suicide.
Let's take a look at a preliminary design using your stepper motor
(alternator) with an output of +/- 40V, assuming that that's what
you'll be able to get under "normal" cycling conditions, whatever
that means.
Let's also make the assumption that it's a 1° stepper and that at
+/- 40V out the output frequency is about 720 Hz.
Next, we'll full-wave rectify it using some fairly fast diodes in a
bridge, smooth it, and we'll wind up with:
ACIN>---+--------+
| |
[1N5817] [1N5817]
|A |K
| +--------+-->39VDC
| | |+
| | [BFC]
| | |
+--------|--------+-->0VDC
| |
|A |K
[1N5817] [1N5817]
| |
ACIN>---+--------+
Since 36 LEDs will drop about 61.2V (let's say 60V) 18 will drop
30V, so if we make two series strings and connect them in parallel,
we'll have a load that'll draw 40 mA and drop 30V. Since we have
39V available and, presumably, can draw more than 40mA from the
alternator, we have more than enough to run through a current
regulator and even some left over to charge the BFC, and the circuit
will look like this:
ACIN>---+--------+
| |
[1N5817] [1N5817]
|A |K
| +--------+---------+---------+
| | |+ | |
| | [BFC] [LED1] [LED19]
| | | | |
+--------|--------+ . .
| | | . .
|A |K | . .
[1N5817] [1N5817] | [LED18] [LED36]
| | | | |
ACIN>---+--------+ | [IREG1] [IREG2]
| | |
+---------+---------+
The problem, of course, is going to be that at some speed the
alternator's voltage is going to drop to the point where it will no
longer be able to run the LEDs. If you're pedalling along
'normally' and all of a sudden have to come to a stop, then the BFC
will have to supply all the current and it'll be able to do that
for:
C dV 1F * (39V - 30V)
t = ------ = ----------------- = 225s = 3min 45s
I 0.04A
Not bad.
Of course, the BFC will start charging again once you get under way,
and the amount of time it takes it to get to 39V is going to depend
on how much current your alternator can put out. But... as long as
the voltage across the cap rises to what the current regulator needs
for headroom, the LEDs will light. Which brings up another thing.
If the LEDs are going to flash, adjusting their duty cycle will
lengthen the time the charge will last. For example, 100ms on and
900ms off will increase the discharge time to about a half hour
(assuming a perfect BFC)
Anyway, I think the next step would be for you to either get or
build a full-wave bridge and measure what kinds of voltages and
currents you can get out of the alternators and motors (generators)
at various RPMs.