Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Constant Current Source for high current

  1. Mar 29, 2017 #1
    Hi,

    I am building a charger for my bicycle front and tail light that uses dynamo to charge them while cycling.
    However, depending on my speed, current is fed differently to lights and they start to flicker. Therefore, I was wandering if there is a way to make a constant current source which outputs around 1A regardless of my dynamo circulation speed?
     
  2. jcsd
  3. Mar 30, 2017 #2
    Use your dynamo to charge a battery of the correct voltage and run your lights off the battery.
     
  4. Mar 30, 2017 #3
    I think you will like the result of just adding a capacitor
     
  5. Apr 5, 2017 #4
    I don't think a capacitor will work because the dynamo gives AC.
    To charge a battery is a good idea but will require a rectifier to obtain DC. For battery charging purposes, a smoothing capacitor will increase the charging voltage. Also, remember that a rectifier drops some voltage - maybe 0.7 volt.
     
  6. Apr 5, 2017 #5
    A dynamo is a DC generator
    If it was AC it would be an alternator
    A capacitor is a very easy fix for his light. I have done this myself when I was a kid. I tore it out of a smashed tv
     
  7. Apr 6, 2017 #6
    I agree with you that "dynamo" means a DC generator, but I have always found that the ones used on bicycles are AC.
     
  8. Apr 6, 2017 #7
  9. Apr 6, 2017 #8

    Nidum

    User Avatar
    Science Advisor
    Gold Member

    The lamp only needs to have a roughly constant voltage supply .

    The output voltage of most cycle dynamo's is usually too low at slow speed and too high at high speed .

    Can't do much at the lower end except by using a storage battery system but at the high end you can just clip the voltage .

    You can do this with a simple circuit employing Zenner diodes .
     
  10. Apr 6, 2017 #9
    My only doubt is that the full wave rectifier involves two diode drops of 0.7 volt each. I think if I wanted to try charging a little battery, I would try a voltage doubler rectifier circuit.
     
  11. Apr 6, 2017 #10

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    For Electrical Engineers that's true but everyone with a bicycle uses a 'Dynamo" which has a rotating magnet and coils in some arrangement.
    I remember reading (perhaps an A level exercise?) that the series inductance of the stator coil of a (bicycle) dynamo gives a certain amount of regulation of output Volts by adding reactance in series which increases with frequency. It would need to be matched to the bulb being used. This was in the days of filament lamps, of course.
    Nowadays, there is plenty of power to spare for driving LED lamps so efficiency wouldn't be as much of a problem. Driving a filament bulb of only a few Watts used to be hard work, I remember. A very low capacity rechargeable battery would be more than adequate. The only trouble would be in making a safe (AC to DC, of course) charging circuit for a Lion (say) battery that would not compromise the battery life nor blow it up.
    Try a cycling forum for some ideas, perhaps.
     
  12. Apr 6, 2017 #11
    You might be amused that as a boy of about 15, I tried to use my bicycle dynamo to light the filament on a receiving tube (type 955) for a VHF receiver. It failed, not enough power.
     
  13. Apr 7, 2017 #12
    You can get silly levels of power from a bike dynamo if you put an 8 inch three blade hobby prop on the end of it and hold it out a car window on the highway.
     
  14. Apr 7, 2017 #13

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    And you are putting "silly levels of Power" in, compared with what a cyclist achieves - even on a good day.
    I wonder how long a bicycle dynamo would work for, under those conditions, before its bearings or windings packed up.
     
  15. Apr 7, 2017 #14
  16. Apr 7, 2017 #15

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Of course, somebody was paying for that power in the form of extra fuel to push the car along the road against the extra load.
    Also, I would want my dynamo to work of a lot longer than just a few hours. But I'm stingy like that. :wink:
     
  17. Apr 8, 2017 #16

    jim hardy

    User Avatar
    Science Advisor
    Gold Member
    2016 Award

    Wikipedia says
    https://en.wikipedia.org/wiki/Dynamo

    Indeed , what you describe has the name "Synchronous Impedance" and is characteristic of an alternator. Current in the armature coils makes MMF that opposes MMF from field, preventing further voltage increase. It's called "Armature Reaction" .
    Big machines put a number on it called "Short Circuit Ratio" which is simply how much current the machine delivers into a short circuit with rated excitation.That much armature current cancels all the field MMF.
    Likewise for a little permanent magnet machine, there is some value of armature current that'll exactly cancel all the permanent magnet field MMF leaving terminal volts at zero.
    A little less current will leave some terminal volts to drive a lamp load.
    By balancing strength of the rotating permanent magnet field against number of turns in armature the designer can arrive at fairly constant current output over a reasonable range of speed.

    If OP changes his load from incandescent to LED and adds rectifiers he'll change the operating point away from what designers chose, , probably to significantly higher RPM . He might have to spill current to hold volts down.

    would be an interesting experiment.
     
  18. Jun 20, 2017 #17
    Use a single diode for a half wave rectifier and a low value resistor to charge the battery. The battery itself will act as a zener to keep the voltage relatively constant.
     
  19. Jun 20, 2017 #18

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    Yep. Far more able to maintain its own volts than a regular Zener.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Constant Current Source for high current
  1. Constant current source? (Replies: 45)

Loading...