Astable Multivibrator Capacitors

Discussion in 'The Projects Forum' started by bingoskid, Jul 11, 2010.

  1. bingoskid

    Thread Starter New Member

    May 22, 2010
    Hi there,
    I've recently started pursuing electronics as a hobby, but I'm having some trouble with the classic astable multivibrator circuit.

    I think I understand how the circuit works, but I'm confused by the capacitors C1 and C2 as people seem to be saying that it's ok to use electrolytic capacitors as long as they are polarized as shown in the diagram.

    Referring to the diagram, if Q1 is conducting (and the circuit has been running for several cycles), the voltage on the left plate of C1 will be about 0.1V (because Q1 is saturated). The voltage on the right plate of C1 will initially be -4.3V and will rise to around 0.6V before the Q2 base starts conducting and the Q1 base stops conducting, etc.

    My problem with this is that there is a time when the right hand (negative) plate of capacitor C1 is at 0.6V and the left hand (positive) plate of C1 is at 0.1V. I think that the capacitor is then reverse biased.

    As I mentioned, I'm new to the field, but I've seen endless warnings about reverse biasing electrolytic capacitors. My question is as follows:
    Is it ok to reverse bias an electrolytic capacitor as long as the voltage across it is low, or should you really not be using electrolytic capacitors for this circuit?

    I don't have much experience with circuits like this yet and would greatly appreciate some advice.

    Thanks, Ben
  2. hgmjr

    Retired Moderator

    Jan 28, 2005
    While electrolytics will work in these circuits, predicting the frequency at which the oscillator operates will be somewhat of a challenge since electrolytics do not behave like true capacitors when they are reverse biased. I would stay with non-polarized capacitors. If you need lower frequency oscillators than you can obtain with the larger non-polarized caps you can consider using logic to divide down the frequency.

  3. Wendy


    Mar 24, 2008
    There is always the two capacitors back to back to make an unpolarized version.

    xox likes this.
  4. bingoskid

    Thread Starter New Member

    May 22, 2010
    Thanks to both of you.

    I didn't know you could put two electrolytic capacitors back to back and make an unpolarized version.

    I will also do some more research into how electrolytics behave when reverse biased.
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    I did a little research, and what I found is that aluminum electrolytics can typically withstand 1V reverse bias with no harmful effects.
  6. hondabones

    Senior Member

    Sep 29, 2009
    This is a classic circuit for me. I'm studying electronics in school now and my 9 year old son is always challenging me to build different circuits and make lights blink and stuff. This by far is one of the easier ways to do it and I use electrolytic caps and have never had any problems. Of course depending on what you want to do it is always good to do the research that you are doing. FYI I have the (-) side going to the base of each transistor, as do you you I see (duh) anyway good observation I will now be inspired to further investigate.

    Also I seen the title of the post was astable multivibrator I assumed I would see a 555 timer. My question is are they similar? or, How are they similar?

  7. Wendy


    Mar 24, 2008
    Actually they are two examples of a general circuit. There are many more examples of astable multivibrators out there, just like bistables and monostables.

    A 555 is an odd duck, it also makes a dandy schmitt trigger, even though the classic circuit uses an op amp with positive feedback. From the outside there is little difference, but they do it in entirely different ways.

    I have to admit I don't know why the back to back electrolytic works, but I know it does.