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Motor generator

Discussion in 'Electronic Basics' started by Ryan, Oct 23, 2005.

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  1. Ryan

    Ryan Guest

    Part II of "Generator for LEDs"

    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?

    Thank you.
  2. John Fields

    John Fields Guest

  3. David

    David Guest

  4. Ryan

    Ryan Guest

    With voltage being subject to such variation, I don't see how I can

    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

    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?
  5. John Fields

    John Fields Guest

    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:

    | |
    [1N5817] [1N5817]
    |A |K
    | +--------+-->39VDC
    | | |+
    | | [BFC]
    | | |
    | |
    |A |K
    [1N5817] [1N5817]
    | |

    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:

    | |
    [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

    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.
  6. 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.
  7. Ryan wrote:
    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.
  8. Jasen Betts

    Jasen Betts Guest

    neither do I, I can only succest operator error.
    use current regulation.

    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.

  9. Jasen Betts

    Jasen Betts Guest

    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.
  10. Andy Baxter

    Andy Baxter Guest

    John Fields said:

    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.

    | | |
    | | |
    | [LED] [LED]
    +-----------+----+ | |
    | Current source | [LED] [LED]
    +-----------+----+ | |
    | [...] [...] more leds.
    | | |
    | [R] [R]
    | | |

    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?
  11. 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.
  12. Andy Baxter

    Andy Baxter Guest

    John Popelish said:
    That's what I was thinking - thanks.
  13. quietguy

    quietguy Guest

    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 playing with ideas
  14. Ryan

    Ryan Guest

    You've got to make up your mind what you want to do first. That is,
    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.

    Thank You.

  15. Jasen Betts

    Jasen Betts Guest

    the resistors have to be greater than the negative-resistance effects of

  16. Jasen Betts

    Jasen Betts Guest

    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

    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

    (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?
    i'd be worried about buckled rims...

  17. Jasen Betts

    Jasen Betts Guest

    the stepper motor has two outputs, for best results you should use two
    bridge rectifiers.
    a toothed pulley? did this motor come out of an inkjet? :)
    make one using epoxy putty :)

  18. John Fields

    John Fields Guest

    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
    Doesn't matter. Your approach has to be experimental with any of
    the motors, so the procedure is the same: Try it!
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