Optimizing astable multivibrator for minimum power consumption

Discussion in 'The Projects Forum' started by aeftimia, Sep 16, 2011.

  1. aeftimia

    Thread Starter New Member

    Sep 16, 2011
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  2. Wendy

    Moderator

    Mar 24, 2008
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    First of all, I would loose the transistors. If you must have them raise the resistors as high as possible, with the collector resistors being 1/10 the value of the base resistors. The base formula's for frequency still apply, you adjust the values of the capacitors to match.

    Having said that, have you considered CMOS gates? A CMOS inverting Schmitt Trigger would be very low current, and have good drive characteristics.

    If you haven't studies Hysteric Oscillators before here is a good example.

    555 Hysteretic Oscillator

    You could even use a CMOS 555 oscillator for good results.
     
  3. aeftimia

    Thread Starter New Member

    Sep 16, 2011
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    Thank you for your reply. I suppose getting rid of the transistors would mean using an integrated circuit--which is perfectly logical (pun!)--however I am trying to make due with just transistors, capacitors and resistors right now. I am trying to get a feel for how these things work without buying a black box with 6 pins--at least until I understand the circuitry well enough that these ICs are no longer black boxes to me.

    PS: Why should the ratio between the collector resistors and the base resistors be 1/10? That seems rather arbitrary. Could that be optimized?
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    The 1/10 rule involves transistor saturation characteristics. In this circuit, the transistors are being used as saturated switches. For most transistors, the base current necessary to saturate the transistor is 1/10 the desired collector current.

    By the way, "saturated" means that a further increase of the base current will not result in a corresponding further reduction in Vce (voltage on the collector with respect to the emitter).

    There are some high-gain transistors that will saturate pretty well with less than the 1/10 rule. However, they are the exception and not the rule. Those transistors are also usually quite a bit more expensive than standard versions.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Bear in mind that if your supply voltage is higher than the transistor base-emitter breakdown voltage, the oscillation frequency will be higher than the calculated frequency, and the transistors may eventually be damaaged.
     
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