I grabbed out some stepper motors and a couple of magnet DC motors and hooked them to the scope.
With the stepper motors, I read a flat line until I hooked up an LED. I don't understand why.
Otherwise I observed what I thought was a pretty nice looking sine wave. I presume a bridge rectifier would flip the bottom waves to the top, is this correct?
I don't know which choice of motor makes more sense. If I turned the stepper motor pretty fast by hand, I saw it range +/- 40v. The other style motor went to +12 or so by hand and about +30v if turned by a high speed drill (engaged by a large attachment, so the motor spun much faster than the drill)
I don't know which type to use.
With voltage being subject to such variation, I don't see how I can reliably limit the proper current to LEDs. Is there another means available beyond a resistor/LED pair? What do you do if voltage is not a static variable?
--- 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:
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:
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.
--- I think the magnet on the spoke gives a single flash each time the magnet passes the coil, so if you can live with that that would be the simpler way to go.
---
--- One regulator will suffice for each series string
Neither do I. I have observed the voltage output of a moving, unloaded step motor lots of times.
Yes. If each winding has just two leads (a 4 wire motor) then you can use two bridge rectifiers to combine their outputs into a single DC output. If they have 6 leads (each winding center tapped), you can also use a pair of diodes for each phase to combine the outputs into a single DC supply.
The best choice may get down to which one is a more efficient generator or which one is easiest to mount, though the stepper has the advantage of no brushes to wear out. The rectifiers on the stepper will also prevent energy going back into the motor when you slow down, but a single diode on the brush DC motor will provide the same effect for it.
The most effective solution may be a switching regulator (that converts power in to power out with little loss) that accepts a wide range of input voltage and regulates an output current that feeds all LEDs in series or a regulated output voltage that feeds all the LEDs in parallel (or small seriesed groups in parallel), with a small ballast resistor in series with each one (or group).
But I doubt you will be ready to design such a regulator till you have quite a bit more experience.
If the current is regulated, and has enough voltage behind it to drive all LEDs in series, then no additional resistor is needed. The only purpose for the resistor is to stabilize the current and the current regulator does that.
The number of LEDs is still negotiable, but I imagine I want anywhere from 12 to 32, if the power supply can handle it.
As for the type, I have only a few assorted LEDs right now so I will probably have to order some. I'd like some of the really bright ones like flashlights use, but if those are expensive, I think some medium size bright red ones show up the best.
As David kindly posted a link below, that is nearly what I want to do.
In addition I want to explore those 1 farad backup memory capacitors to keep providing power when I'm stopped. I want to make it very very cheaply.
I'm open to either a motor (which seems easier) or a magnet on the wheel (if it has an advantage).
I will have to learn about current regulators. If I make one, do I still have to put a resistor with each LED or does one regulator suffice for all?
Something I was wondering about with a similar problem (using dimmable luxeon LEDs for room lighting) is whether you can get away with using just a single current regulator circuit for several strings if you put a small resistor in series with each string.
Intuitively, I would think that the resistors would help balance out any manufacturing variations in the LEDS so that the current would go more equally between the two strings, but I don't know if this is right - e.g. is there a runaway burn out scenario here where one of the strings gets hotter and then starts hogging the current more and gets even hotter?
--
http://www.niftybits.ukfsn.org/
remove \'n-u-l-l\' to email me. html mail or attachments will go in the spam
bin unless notified with [html] or [attachment] in the subject line.
This is one of the choices I described, and is certainly practical with an appropriate value of resistor. They can be much lower resistance than when they must limit the current from an unregulated voltage. 10 ohms per LED in the string work. At 20 mA, that wastes
0.2 volts per LED in the string. Something like 11% of the power.
--
http://www.niftybits.ukfsn.org/
remove \'n-u-l-l\' to email me. html mail or attachments will go in the spam
bin unless notified with [html] or [attachment] in the subject line.
Because the voltage is going to vary over a wide range as you speed up and slow down etc, I would be thinking of using a NiCd or such to power the leds (handy when youve stopped too) and using the steppers thru a regulated charging circuit to recharge the NiCd(s)
David - just play> Part II of "Generator for LEDs"
typically this means starting with DC at a voltage above what is needed and limiting the current by means of an electronic circuit that measures the current and takes action to correct it.
there's a vast range of brilliance some LEDs are 1000 times more luminous but only cost ten times as much snd want less than three times the power if their dimmer cousins.
You'll do much better buying a few high intensity LEDs than buying many cheap ones.
they aren't very cheap.
you'll need one regulator for each series group of LEDs, if you have multiple groups you may be able to use some resistors to balance the current and still only use one regulator.
Ok. The particular stepper motor of interest is PM42L-048. I found a PDF that says it is a 7.5 deg/turn 24v motor and as I understand is rated for 600mA. Is this information meaningful to me as a generator?
I can't find good data sheets on the other motors I got.
I think I have a bridge rectifier that I pulled from a dead power supply or something. Any reason this won't suffice?
My other challenge is how to attach to the shaft of my motor. It has a gear on the shaft and I can't fathom a way to make it have a 2 to 2.5" wheel on it to engage the bike tire, or else interface with the chain like a sprocket.
My initial experiments with this method were not encouraging.
--
No, that\'s the rated current for the bipolar chopper version of the
motor to put out the torque it\'s rated for. The only way you\'re
going to find out if the motor you have will work for you is to try
it. Determine how fast it\'ll be spinning when it\'s on your bike,
spin it up through the range with an electric drill, and load it
down electrically to to find out how much power you can get out of
it.
yes, it means don't try to get more than 600mA out of it also voltages far exceeding 24V aren't a good idea. (I'd draw the line at
100V)
7.5 degrees means 48 poles in the stator, these motors are normally two phase, so 24 poles in the rotor, - 12 cycles of AC per revolution to each phase.
(most bicycle generators have a 4 pole rotor) so this motor should produce usable output and around 1/12 the speed so you could use a bigger wheel on it to reduce drag....
I can't see the gear, can you put a tyre on it? can it be removed and replaced with something else?
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.