TS556 hysteretic oscillator

Discussion in 'The Projects Forum' started by ahto555, Jun 1, 2012.

  1. ahto555

    Thread Starter Member

    Mar 12, 2009
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    I have 1/2 TS556 left and would like to make adjustable frequency oscillator 40...75 Hz. There is a catch of course - duty cycle 50% (+- 10%), but most important duty cycle should stay stable (few percent error max) during frequency adjustment over full range. Is this goal achivable with hysteretic circuit?

    Another question is about small signal npn transistors. For most small signal npn transistors base-emitter saturation voltage is about 0,6-0,8V. Does npn transistors with lower (close to 0,1V) base-emitter saturation voltages exists (Vce min 10V)?
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    Last things first. To drive a BJT you need about 1/10 the current through the BE that you are using for the CE. It is a general rule of thumb that works. Overall, MOSFETs are much better.

    Have you read LEDs, 555s, Flashers, and Light Chasers? Chapter 10 covers transistors as drivers.

    I have also written an article on hysteretic oscillators. For a true 50% duty cycle use a CMOS 555, or you can go with other configurations that are very very close (as in close enough).

    555 Hysteretic Oscillator

    Ask, and I will swamp you with information. ;)
     
  3. crutschow

    Expert

    Mar 14, 2008
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    One way to guarantee exactly 50% duty-cycle is to operate the oscillator at twice the desired frequency and run the output through a divide-by-two flip-flop circuit. That way the duty-cycle of the oscillator has no effect on the output duty-cycle (which is always 50%).
     
  4. Wendy

    Moderator

    Mar 24, 2008
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    Yeah, but it isn't needed, unless you require major precision. Besides the drive on a CMOS 555 still beats a typical CMOS chippie.

    There are several 555 configurations that can get within 0.2% of a 50% duty cycle. Do you really need those extra components?
     
  5. crutschow

    Expert

    Mar 14, 2008
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    Without tweaking any of the component values or using an additional pot when the frequency is changed?
     
  6. ahto555

    Thread Starter Member

    Mar 12, 2009
    11
    0
    TS556 is dual CMOS timer IC.
    I have read your article and forum topic about 555 hysteretic oscillator. Lets say f=0,7/RC which means, if C=1uF then at 40Hz R should be 17,5k and at 75Hz 9,3k. So putting 9,1k resistor series with 10k pot will give whole frequency coverage. The question remains how stable duty cycle will be within 40-75Hz range?
    I need circuit to measure vibration pump (also ULKA pump 24VAC, 50Hz) pressure and free flowing volume at different frequencies.
    Second half of TS556 is used as boost circuit driver. I would like to limit inductor peak current at 25A. If normal small signal npn transistor with Vbe saturation voltage 0,65V is used then P=UI 0,65Vx25A we end up with current sense resistor power dissapation 16,25W which is way too much. 0,1Vx25A would give much more acceptable 2,5W sense resistor power dissapation.
     
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