How to control 2 motors with a 555 astable??

Discussion in 'The Projects Forum' started by Nara Shikamaru, Dec 3, 2007.

  1. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    Hi everybody!!.....i have a little problem here....in the output of a 555 astable a need to divide that output into 2 lines...and i need to do a driver so i can match two 12VDC 3/4 Hp motors to those lines and each must work with T1= 3seg and T2=3seg......how can i do that??......here ill send you the schematic to where its the basic idea of what must be done....the astable part is omited....thanx!!
     
  2. thingmaker3

    Retired Moderator

    May 16, 2005
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    Instead of tediously adjusting the 555 for 1/6Hz and 50% duty cycle, it might be easier to feed the 555 output at 1/3Hz (any duty cycle) to a J/K flip-flop. You could then control one driver with the Q output and one with the Q-not output.
     
  3. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    Hey everybody!!....i have come to this design....i can configure the astable to the period output i want and the motors will work alternately....check what i have done...will it wotk??
     
  4. thingmaker3

    Retired Moderator

    May 16, 2005
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    You might have trouble driving Q1 directly from the 555.
     
  5. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    Yes...i kind of thought so....i knew i looked kind of weird that...but i couldnt find a better way to do it....whats your idea?...i could i make some sort of coupling there?
     
  6. Ron H

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    Apr 14, 2005
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    I don't see why it should be a problem. If you were trying to do PWM, maybe, but the 555 can source and sink 200mA, so it should work just fine.
     
  7. thingmaker3

    Retired Moderator

    May 16, 2005
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    12 V / 22 Ohms = 545 mA
    Ciss for the MOSFET is around 1.3 nF, so TC would be just under 29 nSec

    Can the average LM555 source that much for very small periods of time? If so, then the circuit could work as drawn.
     
  8. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    What is "Ciss" for the Mosfet??....and how is that of 1.3nF.....and Time of Charge would be under 29 nSec???....
     
  9. Ron H

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    Apr 14, 2005
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    The 22 ohm resistor does not determine the current, because the 555 can't source that much current. The 555's typical rise and fall times are 100nS - plenty fast for switching a MOSFET with a motor as its load.
     
  10. thingmaker3

    Retired Moderator

    May 16, 2005
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    Nara:

    I apologize for my ambiguity.

    "Ciss" is the input capacitance of the MOSFET, listed as about 1300pF according to the IRF740 datasheet.

    "TC" is "time constant" - the time required to charge to 63.2% of full charge
    http://www.tpub.com/neets/book2/3d.htm

    22 * 0.0000000013 = about 0.000000029

    Ron:
    Current is sourced by the 555 when charging the gate of the MOSFET, no matter what the switching frequency is or what the drain current is. This is why 555s can't directly drive MOSFETs at high frequency.
     
  11. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    One thing I missed here is that the motor is a 3/4 HP, 12V unit. Assuming full load and reasonable efficiency, the current will be about 60 amps. IRF740 has about 0.5 ohm ON resistance, and will withstand 400 volts. You need a MOSFET with a few milliohms Rds(on), and even then you will need a heatsink. You don't need 400V capability.
     
  12. Ron H

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    Apr 14, 2005
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    But this isn't high frequency.
     
  13. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    First of all....i want to apologize with you guys....i misdraw the schematic...the 555 its configure as an Astable...and the output frecuency is 100mHz...i mean..the period is 10Sec...thats because when its On for 5 Secs it will turn the first motor...then the next Off 5Secs it will turn on the other motor...and because its Astable it will alternate continuosly the two motors....
     
  14. Ron H

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    Apr 14, 2005
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    When I look at your schematic, I see a low frequency astable. I don't see a need for you to apologize.
     
  15. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    really??...its just that im new at electronics and generally astable has two resistors to do the RC Time constant...and usually its not feedback from the output...anyways i need to calculate the resistor for a 10 Secs period....by the way...thanx for everything and for your understanding!!...greetings from Venezuela...
     
  16. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    That circuit is an easy way to get an approximate 50% duty cycle. A CMOS 555 in that circuit will theoretically have a duty cycle of (almost) exactly 50%. The bipolar 555 circuit will be a little off 50%, due to the fact that the output does not swing all the way to the positive rail, but a CMOS 555 is not a good choice for driving a big (low Rds(on)) MOSFET, because they don't have nearly the output drive capability of the bipolar unit.
    As Thingmaker3 mentioned, if you really need precise 50% duty cycle, you can run the 555 at twice the frequency, and run the output of that through a toggle flip-flop (J-K or D). But then you would certainly need a driver for your MOSFET.
     
  17. Nara Shikamaru

    Thread Starter Active Member

    Nov 24, 2007
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    Hey Ronh...and everybody....when i draw ths schematic that i sent...i "misdraw it"....but then ronh tells me that thae circuit in fact is an Astable....and i simulated it on the Proteus and it works awsome....so...the point is...did i had a stroke of luck?...please...explain to me why it works as an Astable without the other resistor..and why the 330k resistor its feedbacked from the output??...usually the Astable format is this one......well...thanx for everything!!
     
  18. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    If you understand how the "conventional" astable works, then you should understand the one you erroneously drew. Google "555 tutorial" and you will find numerous resources explaining the 555 chip.
     
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