Relaxation Oscillator or Astable Multivibrator?

Discussion in 'General Electronics Chat' started by Wendy, Nov 1, 2008.

  1. Wendy

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    Is this a relaxation oscillator or a astable multivibrator?

    [​IMG]

    Wikipedia claims it is a relaxation oscillator. I've always thought of it as a multivibrator.

    http://en.wikipedia.org/wiki/Relaxation_oscillator

    It matters because I'm doing research for the AAC book. I'd really like to get it right the first time. If you have an informed opinion please let me know, and if possible cite sources. Thanks.
     
  2. bertus

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  3. Wendy

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    Yeah, they are also calling the normal 555 oscillators relaxation oscillators too, and I've always heard them described as astable. An astable uses RC charge/discharge too, so what is the difference?
     
  4. bertus

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  5. AlexR

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    I've always thought that a multivibrator must consist of at least two active devices and there must be some form of cross-coupling between the two sections, whereas a relaxation oscillator consists of just a single active device and a timing circuit. This puts the 555 into the relaxation oscillator camp.
     
  6. Ron H

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  7. Wendy

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    I understand what you guys are saying, but I'm not the only one who has a problem here. Check out these pages of the AAC eBook...

    http://www.allaboutcircuits.com/vol_6/chpt_6/7.html

    http://www.allaboutcircuits.com/vol_6/chpt_6/8.html

    You'll note in both the 555 is referred to as an astable multivibrator. It could be argued that it does have a flip flop internally also.

    Even looking at the data sheet there are references to monostable and astable multivibrator modes.

    http://www.datasheetarchive.com/img/pdf_datasheet.gif
     
  8. Wendy

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    I found this in the second link you provided. It is referred to as an astable multivibrator...

    http://www.elektrorevue.cz/clanky/02005/english.htm

    Matter of fact, the second link had no examples of 555 relaxation oscillators (or schmitt trigger designs), they were all referred to as multivibrators.

    I'm inclined to go with the multivibrator camp myself, I don't think the definitions are as clear cut as is believed.
     
    Last edited: Nov 4, 2008
  9. Dave

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    Multivibrator is a catch-all term, encompassing monostable, bistable, and astable modes of operation.

    A relaxation oscillator functions in a specific way, that is a capacitor is charged from a current source then discharged at some threshold down to a new threshold when the capacitor starts changing again. In essence this satisfies the core functionality of the astable multivibrator in that it oscillates from one quasi-stable state to another.

    Traditional texts talk in terms of oscillators, even with regards to 555s, whereas the more modern texts tend to refer to these circuits in terms of multivibrators.

    I think if the circuit meets the strict criteria of a relaxation oscillator it is fair to refer to the circuit by this name, however if it merely satisfies the core requirements of a multivibrator it should be referred to in the generic terms. I like the classification described by AlexR; although I have never seen it referred to in this way, it makes sense and provides a clear distinction.

    Dave
     
  10. Wendy

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    An IC is (usually) by definition more than one device cross couples (so is an op amp). So is any oscillator made from a 555 a relaxation oscillator, multivibrator, or both?
     
  11. Dave

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    This is where the definition is loose. If we go by AlexR's definition, at what point do we make the distinction to say right component X is 1 component.

    I would say it is a multivibrator, however if it meets the conditions for a (relaxation) oscillator - which is well defined - then it can also be referred to as such. I am of the opinion that the term "multivibrator" is a generic term.

    Dave
     
  12. AlexR

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    The term multivibrator is used somewhat loosely these days to refer to virtually any form of relaxation oscillator but originally it had a very specific meaning.

    One of my old text books (Pulse Fundamentals John M. Doyle) defines a multivibrator as follows:

    "A type of relaxation oscillator of a two-stage resistance-coupled amplifier with the output of each stage regeneratively coupled to the other."

    If you accept the above definition then a 555 is a relaxation oscillator but it is not a multivibrator.
     
  13. Wendy

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    I have a feeling the definition has changed, the 555 is what, 30 years old, and it's data sheet refers to it as a monostable and astable. The other thing is, it is a true monostable without using anything resembling the classic monostable circuitry.

    With the classic astable they are not resistance coupled, but capacitively coupled. You want to see an odd design (one I had to look twice at) check this one out...

    http://www.allaboutcircuits.com/vol_6/chpt_5/18.html

    I think the key to any multivibrator is they are firmly digital in nature, while a relaxation oscillator isn't (but inclusive of the digital models). This would include 555's nicely. So a relaxation oscillator is the wider definition while the multivibrator is the tighter.
     
  14. studiot

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    Definitions, Definitions, Definitions.

    Why is everybody so obsessed by this?

    As with most definitions there is overlap, grey areas or whatever. The boundaries are not clear and sharp.

    You do not need two distinct devices to construct a multivibrator, or even active devices.

    An MV can be constructed with a single op amp by suitably feeding back the output to the input, thus providing the 'cross coupling'.

    Alternatively you can construct an MV from two self latching relays (which are not active devices). Vibrator power supplies for valve car radios were once constructed in this way.

    Either way you have to have feedback or cross coupling to promote the multivibrator action.

    Turning to relaxation oscillators, you can construct a relaxation oscillator from a single active or passive device.

    A simple example would be the original neon christmas tree flashing lights, which comprised a resistor in series with a neon. there are no active devices in this circuit.
     
  15. beenthere

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    It is a more complex circuit than, say, a neon relaxation oscillator, but the determining factor is the charge on a capacitor. The mechanism for "relaxing" that charge is not the same as the neon's conduction, but the principle is the same.

    The circuit complexity makes it look as if more is going on, but using the voltage level on that external capacitor makes it a relaxation oscillator rather than a multivibrator.
     
  16. studiot

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    What is a more complex circuit?

    A neon is a form of capacitor, with measurable capacitance. The neon flasher works by charging up this capacitance through the resistor, until such time as the voltage across the neon reaches the striking voltage (about 70 volts) and the neon (capacitor) self discharges.

    To me this is the archetypal relaxation oscillator.
     
  17. Ron H

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    A neon relaxation oscillator requires an external resistor and capacitor.
     
  18. Wendy

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    One one hand we have data sheets of the device (555), one encyclopedia (educypedia), the AAC eBook describing it as a multivibrator.

    We have another encyclopedia (Wikipedia) that disagrees, older references that disagree, and pretty well split between educated folks.

    It does matter, we're working on a piece of reference material here. This debate has some teeth to it.

    I keep coming back to the fact that all single device relaxation oscillators (which does not describe an IC) do not come out with square waves. For the most part they are negative resistance devices that at some point in their cycle "relaxes", discharging the cap in an uncontrolled fashion.

    Whether it is a 555 or a op amp wired as a schmitt trigger this is a gate. If you check the AAC book this is its fundimental definition of an astable multivibrator.

    I have no problem with it being a subset of relaxation type oscillator, but a lot of very educated types (including the chip manufacturers) class it as an astable multivibrator also. Personally I've always thought of it as a multivibrator, long before I brought this up. It was the articles in Wikipedia that got me thinking.
     
    Last edited: Nov 4, 2008
  19. Dave

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    Traditionally, texts did not mention in any great detail the concept of multivibrators; for example Horowitz and Hill (1989) almost ubiquitously refer to the subject under the topic of oscillator circuits.

    Dave
     
  20. SgtWookie

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    I'll go with AlexR's definition.

    Although a 555 can be used to replace a multivibrator, I don't suppose that it really IS a multivibrator - unless it's wired up that way. ;)

    When wired up in the classic astable mode using pin 7 (discharge), it's a relaxation oscillator.

    When the timing capacitor charging/discharging comes from pin 3 (output), then it's acting more like a multivibrator.

    But however you want to hash it out, it's a very versatile circuit that can be used in monostable, astable, PWM and many other modes.

    It's a timer with afterburners. ;)
     
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