bjt astable multivibrator oscillating frequency?

Discussion in 'The Projects Forum' started by imbaine13, Jan 21, 2014.

  1. imbaine13

    Thread Starter Member

    Oct 6, 2013
    62
    0
    Hello guys,

    I was putting together a bjt astable multivibrator with the following values for R2=R3=15,000Ω and C1=C2=1μF. I used the popular f=1/1.4RC to arrive at a frequency of 50Hz, unfortunately, there was no way I could tell that the cicuit was indeed working (I don't have an oscilloscope), so i thought I'd verify the formula visibly with a much lower freq; I substituted the capacitors with 470μF ones, kept the resistors and run the circuit. I'm confused because the calculations gave a periodic time of 9.87seconds, yet the the circuit was running at ≈ 6.8 seconds!! (periodic time):confused: That's a really huge margin, I'm not sure what I'm doing wrong because I have verified my calculations and redid the circuit with the same results.

    Is this supposed to happen??
    Also, all the circuits from the internet meant to oscillate at 50Hz, by calculation, oscillate at 32Hz??
    WHY???
    I'm confused?
    Appreciate your time everyone.

    Have a good one.
    BJT astable.JPG
     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    The frequency will be effected by the supply voltage. Some electrolytic capacitors typically have a tolerance of +100% -10% of nominal value.
     
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  3. imbaine13

    Thread Starter Member

    Oct 6, 2013
    62
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    Thanks Mike, The supply is 6 volts, but I have seen several circuits with varying supply voltages, yet similar capacitor and resistor values and are all meant to run at the same frequency. So is it the capacitor tolerance, or the supply voltage that changes things?
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,420
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    How do you know that the formula you used is the correct one?
    I would say if you're within the same order of magnitude you're doing well.
    There are too many variables that can affect the frequency. You have to use a frequency counter or oscilloscope in order to verify the frequency.
     
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  5. crutschow

    Expert

    Mar 14, 2008
    12,986
    3,224
    The capacitor tolerance is likely the largest variable. Do you have other capacitors you can try? Do you have a meter that can measure capacitance?
     
  6. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi im.
    An increase in supply voltage will give a slight increase in frequency.

    The problem with high value electrolytic caps is their leakage.

    The discharge time of the cap [ 470uF] is reduced due to this leakage, so the frequency will increase.
    E
     
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  7. MrChips

    Moderator

    Oct 2, 2009
    12,420
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    Many of the electrolytic capacitors I have in stock read 20% over the nominal value.
     
  8. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    A quick simulation shows the effect of leakage [ resistance].

    Electrolytic caps are a poor choice for trying to calculate accurate timing.
     
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  9. imbaine13

    Thread Starter Member

    Oct 6, 2013
    62
    0
    Thanks crutschow. Yes, i do have other capacitors which I tried and got roughly the same results. I do have a meter that can measure capacitence, and the values are really close (467μF for the 470μF capacitors) which makes me doubt the capacitors are the problem, unless a difference of 3μF can cause these variations.
    I also noticed that sometimes, the circuit is not self starting. I have to unplug a transistor and place back in to start the oscillating. I paired different transistors together with no luck. Might anyone know the cause of this?
     
  10. imbaine13

    Thread Starter Member

    Oct 6, 2013
    62
    0
    Would low value electrolytic ones perform better?
     
  11. ericgibbs

    Senior Member

    Jan 29, 2010
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    380
    hi im,
    If you use lower value electrolytic caps obviously the timing resistor value will have to be increased if you need to keep the same frequency.

    A common way when using electrolytic's is to make the timing resistor a variable potentiometer in series with a fixed timing resistor.
    In this way you can 'tweak' the variable to give the required frequency and so compensate for wide +/- capacitor tolerances and to some extent cancel out the leakage.

    E.
     
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