556 PWM help

Discussion in 'The Projects Forum' started by tootalled, Jul 28, 2011.

  1. tootalled

    Thread Starter New Member

    Jul 28, 2011
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    I'm looking for help on what is probably a very simple project. I have a very nice little DC motor that runs at about 2 or so rpm. I need it to run a lot slower, on the order of a few revs/hour. I bought a pwm on ebay that slows it down but not quite enough. So I'm thinking I can use a 556 timer to work one half as a pwm and the other half to cycle wait periods of up to a second or two. I don't care about continuous rotation it just needs to end up with the right rotations after an hour. This may have already been done, I just looking for something simple I can build, I'm no electronics person.

    tootalled
     
  2. pyrokid73

    New Member

    Jul 28, 2011
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    From what have said so far, it sounds like you are needing something for a constant revs/hour, is this correct?
     
  3. iONic

    AAC Fanatic!

    Nov 16, 2007
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    What is the duty cycle range of the PWM circuit you bought. Do you have a link or any specs?
    Perhaps a 556 with variable freq and duty cycle would work.
     
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    You can do it with an interruptor and a cam.

    The cam opens contacts that stop the motor in that position (say at 12 o'clock position).

    Then you pulse power to the motor (a relay or transistor in parallel with the open cam contacts), and the motor starts turning, then it will turn one full rev and stop again when the cam opens the contacts.

    That way you will get exactly 1 revolution for every input pulse. You can use that to drive a heavy clock movement etc from a little electrical pulse every second (say from a tiny clock movement).

    Of course if you need less than one rev per second you just use pulses that are less frequent. The motor will always do exactly one rev for every input pulse.
     
  5. tootalled

    Thread Starter New Member

    Jul 28, 2011
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  6. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Specs for existing PWM circuit:

    DC Motor Speed Control HHO / PWM 12V 2A 20W

    Technical information

    - Require Power Supply 12VDC
    - PMW by IC 555
    - Max load 20W ( 2A ) , suitable for small motor 20W or below.
    - Duty cycle adjust 0 - 92.5%

    Website:
    A dual 555 PWM circuit

    Circuit:
    [​IMG]
     
    Last edited: Jul 29, 2011
  7. gopalyajur

    Active Member

    Jan 3, 2010
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    One basic question, why is the motor tied to the emitter and not collector?
    All the circuits in the above mentioned website has motor tied to emitter instead of collector.
     
    Last edited: Jul 30, 2011
  8. SgtWookie

    Expert

    Jul 17, 2007
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    It is a poorly designed circuit.
    The motor should have been between the collector and Vs, along with the diode; the 1N4148 diode shown is not adequate for the suggested load current range, the TIP31C has a 3A absolute maximum continuous Ic rating, so the driver should not be expected to sink more than 1.5A if the 1k base resistor is decreased to allow 150mA base current. The approximate formula for calculating the base resistor for this particular driver would be:
    Rbase ~= (Vs - 2) / (Ic / 10)
    where:
    Vs = the supply voltage
    2 = the 555 timer max output is Vs-1.3v, and Vbe will be around 0.7v, so a total of ~2v
    Ic = the desired collector current; ie motor current
    /10 = When used as a saturated switch, gain is assumed to be 10.

    Even that is rather optimistic.
     
    gopalyajur likes this.
  9. iONic

    AAC Fanatic!

    Nov 16, 2007
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    Thanks for clarifying the specific design flaws of the circuit.
     
  10. gopalyajur

    Active Member

    Jan 3, 2010
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    One more question. When can one use emitter to drive a relay or motor?
     
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