Extremely low voltage without reducing current

Discussion in 'The Projects Forum' started by mpk1123, Jun 9, 2011.

  1. mpk1123

    Thread Starter New Member

    Jun 9, 2011
    3
    0
    hi everyone,I'm a beginner in electronics and have been learning my way through forums.
    I have a 5v DC 0.6 A power supply .
    I need to use this to get a voltage somewhere in the range of 0.5 - 1 V which i intend to use to run a 5.9V dc motor at extremely low speeds.
    The dc motor I have is scavenged from an optical pickup of a cd player which i broke up from my old cd player and i dont know much of its specifications except that it is a 5.9 V dc motor .Also I tried varying the voltage using a lab power supply unit and i observed that the motor speed increases on increasing the voltage.I hope this helps to determine the type of the motor.

    I tried using 100 k pot connected to the centre tap of a voltage divider to reduce the voltage.But the current i got was too low to run the motor.
    I got a current of 0.5 mA at 0.6 V.As i said,this wasnt enough to run the motor.
    I need atleast 100 mA of current to be on the safer side.The motor usually starts working at a current of .05 A.Also another requirement i have is that i need a very fine handle to vary the voltage, something that would give a resolution of .01 or .001 V.

    Thanks in advance
     
  2. Dyslexicbloke

    Active Member

    Sep 4, 2010
    420
    19
    Thats not a good way to run a motor ... But ...
    A voltage regulator will do wht you want, look at LM317 which is easy to use and basically indistructable, unless you try realy hard.

    A motor generates voltage when it spinns, its called back EMF (Electro Motive Force).
    As it speeds up it generates more voltage and therfore draws less current which is why it dosnt just keep getting fasrter ....
    Slow it down with a load, making it do more work, and the back EMF drops so it draws more current in responce to the load but it is also turning slower.

    When the relationship is linear it is very wide because the "Torque", turning force, is directly related to the power which in turn is Volts X Amps.
    Low voltage makes the whole system sloppy and slow to respond because the amps cant increese dramatically in responce to torque loading

    To fix the problem you need to use High voltage, and therfoer potential amperage, for a short period of time.
    The average power is still the same but now you have a big voltage so a small change in load will result in a big change in amps, there are more volts to multiply by, and therfoer power.

    Look up PWM (Pulse Width Modulation)

    Immagen you are faced with pushing a car .... and your strength is limmited.
    If you push gently and continually it probably wont move ...
    Give it a good shove for a short time and your off, then inertia takes hold and it will roll for a while.

    Over time you will expend the same amount of energy whatever you do but when moving a big load with a small amount of energy bursts of significant effort will usualy win out over minimal sustained pressure.

    Al
     
  3. russpatterson

    Member

    Feb 1, 2010
    351
    16
    Agreed. You want a PWM based motor controller. It's pretty simple once you get the PWM signal. I use a PIC micro to generate the PWM. You can do it with a 555 but IMO that's more work. In the upper right of this schematic http://2.bp.blogspot.com/-fcShHZt2i2Q/TYTvzgT5tsI/AAAAAAAAAGQ/CIQEfRTRF5g/s1600/2-12-11%2BSolar%2BBoard%2BSchematic.png

    You'll find a basic motor controller. It's just a MOSFET, resistor and diode to handle the EMF of the motor. You decrease the duty cycle of your PWM to slow the motor down (effectively running it at a lower voltage than your supply). This is a much more efficient method than a linear regulator but for the low current you're talking about a linear regulator would work to.
     
  4. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    Don't you really mean that you need to adjust speed? Reducing voltage will accomplish that to a degree, but as you've heard, PWM is much better. With voltage control, you'll have almost no torque at low voltage, and there will be a limit to how slow you can go. Probably a voltage that will spin the motor, but not start it once it has stopped.
    If you'd gotten enough current, it probably would have ruined your pot. Their meant to control low currents (information, really), not enough to run a motor (power).

    You won't regret learning about PWM - it's a great tool to have in your skills toolbox.
     
  5. Dyslexicbloke

    Active Member

    Sep 4, 2010
    420
    19
    Since your just starting out you might considder building some kits ...
    Velleman, amoungst others, do a huge range of stuff from simple relays to home automation and PC based control.
    The K8004 DC Pulse Width Modulator from Velleman would be a nice Kit to play with and would do exactly what you want with your motor right out of the box ... well after you build it that is.

    You get a full circuit diagram and an explanation of how it works which you can augment by looking up the datasheets for the componants.
    http://www.alldatasheet.com

    Kits are available all over the place but this UK stockest apparantly carries the full range ...
    http://www.esr.co.uk/velleman/products/index_kit.htm

    Also check out
    CPC http://cpc.farnell.com/ (Part of Farnell)
    Maplin http://www.maplin.co.uk
    Rapid http://www.rapidonline.com/
    RS http://uk.rs-online.com/web/
    Digi-Key http://uk.digikey.com/
    and once you know what to search for there is always FleeBay :)

    Lastly .... Get a breadboard (protoboard), a reasonable multimeter, preferably with a capacitance setting and a cheep hand held or PC based oscilloscope so you can see what is realy going on.

    Then just play and learn
    Have fun ...
    Al
     
    Last edited: Jun 10, 2011
  6. mpk1123

    Thread Starter New Member

    Jun 9, 2011
    3
    0
    Thanks a lot guys :). PWM really seems to be working.
    I havent yet build the pwm circuit,but got lucky as i asked a professor in my university that if i could use the arbitrary function generator in his lab and he agreed. i then hooked up the motor to an arbitrary function generator and supplied a square wave input to the motor. The motor moves just as smoothly and as slow as was required.
    Now, i'm trying to use a pulsed output waveform and am thinking of playng around with the duty cycle and frequency to optimize the motor speed.Then i'm planning to go about building the circuit.
    But before that, i need to optimize the frequency and amplitude. But i'm clueless here.Can you please suggest how the frequency and amplitude affects speed.One problem was that the motor was running in fits and starts,but surely very slowly as i wanted.should i increase duty cycle from 50% or reduce it to get the motor to run continuously ? Also, since the motor has to move a variable weight,i am curious to know how does the frequency depend upon the "load or weight" carried by the motor.
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    I'm certainly no expert but the idea of PWM is to send a frequency high enough that the pulsing is not "seen". I guess you'd say you're making an analog device do integrating of the digital signal, the power supply. For instance flashing an LED at any frequency above about 100Hz ensures that you won't perceive each individual flash. Same thing with a motor - it should continue spinning at essentially the same speed between pulses. As I understand it, you'll retain torque with this approach, even as rpm drops off.
     
  8. mpk1123

    Thread Starter New Member

    Jun 9, 2011
    3
    0
    but,what issues would be there if i run it at very high frequencies,say abt 10khz. Because i did that and couldnt get the motor to move. but what about the duty cycle. how does it affect the torque?

    Thanks in advance.
     
  9. John P

    AAC Fanatic!

    Oct 14, 2008
    1,634
    224
    You may not be able to achieve what you really want here. The problem is that at low speeds, a d.c. motor may be affected too much by non-ideal behavior, and you just won't get it to move smoothly. What happens is that there is both static and dynamic friction--static friction being the kind where you shove hard against something that you want to slide, and then it breaks away and moves with significant momentum. Getting a very slow smooth speed is quite difficult.

    This problem has been extensively studied by people trying to control model trains. What they do is use "pulse power", which is typically a slow PWM drive, at 30 or 40 Hertz. The idea there is that if the motor stalls, each incoming pulse will jar it loose. PWM at a high frequency won't have this effect. There are also some clever circuits around which use velocity feedback from the motor in order to modulate the driving waveform, but that wouldn't be a beginner project.
     
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