Cannot find components from online circuit diagram

Discussion in 'General Electronics Chat' started by gfyhv1922, Jul 23, 2016.

  1. gfyhv1922

    Thread Starter New Member

    Jul 2, 2016
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    Hi!

    I want to build this circuit: http://everycircuit.com/circuit/6368680139554816/astable-multivibrator
    But I can't find the 50uF capacitor online.

    I have two questions:
    1. Why can't I find a 50uF? And if they don't exist, what should I be using instead?
    2. Which transistor is the diagram referring to? I have 2N3904, are these suitable?
    Thanks in advance!
     
  2. benta

    Member

    Dec 7, 2015
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    I'm too challenged to use a web site that only works with "Chrome". I only see a microscopic schematic.

    Apart from that:

    1: 47 uF is a standard value available everywhere.
    2: 2N3904 is OK for this circuit.
     
  3. SLK001

    Well-Known Member

    Nov 29, 2011
    810
    224
    50μF capacitors do exist, but there is no reason to use one here. Use the 47μF that benta suggests, or a 56μF, or any capacitor in that range. The circuit will flash one LED, then the other, kind of like a railroad crossing signal.
     
  4. gfyhv1922

    Thread Starter New Member

    Jul 2, 2016
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    Hi!

    I can only find 47uF of the polarised kind, while the schematic specifies the un-polarised type. Can I interchange these type of capacitors and it will still work? Or am I right in thinking that I need a ceramic 47uF capacitor that is unpolarised?

    unpolarised meaning the small rounded capacitors
    polarised meaning the cylindrical shaped capacitors
     
  5. crutschow

    Expert

    Mar 14, 2008
    12,987
    3,226
    Sometimes in online circuits, the author will use a round number for the resistor, inductor, or capacitor component value when the value is not critical, even though that particular value is not readily available in an actual device.
    In that case you just use the nearest available value, which here would be a 47μF or 56μF capacitor
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Why do you think it needs an unpolarized cap?
    If you look at the simulation, it shows the capacitor polarity.
     
  7. paulktreg

    Distinguished Member

    Jun 2, 2008
    611
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    A 47uF 10V electrolytic capacitor would be suitable for this circuit. This is a polarised capacitor so just be sure you connect it the right way round as indicated in the circuit diagram by the + and - symbols.

    Unpolarised capacitors can be used for this circuit but usually of lower value when requiring a higher frequency.
     
  8. Papabravo

    Expert

    Feb 24, 2006
    10,136
    1,786
    It is hard to make a non-polarized capacitor with a value as large as 50 μF. Manufacturers of capacitors have to choose which values they make and that is why some possible values are not available. The ones they make are the ones people buy and vice versa. I guess in your ideal world there would be a uniform distribution of available component values instead of the reality that not all possible values are used and available. I suppose you could throw a temper tantrum, jump up and down, scream and shout. It might make you feel better, but it won't change anything.
     
  9. crutschow

    Expert

    Mar 14, 2008
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    There is actually a method to the madness of part values.
    Capacitor values typically follow the EIA E12 (10% tolerance) or E24 (5% tolerance) logarithmic part value sequence as shown here (1% resistors follow the E96 values).
    It keeps the value difference between parts approximately the same percentage (≈ double the tolerance), for each value, so that there is tolerance overlap for each value from the two adjacent values.
     
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  10. EM Fields

    Member

    Jun 8, 2016
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    50μF capacitors exist, but there's no need to use one in that circuit since you can decrease the value of the timing caps and increase the value of the timing resistors - within limits - to wind up with the flash frequency you want. There is a reverse-voltage transient across the caps, so in order to avoid any problems you could use ceramic caps. 10μF +/10% at 16 volts are cheap and are easy to find.

    I've attached a schematic and a plot with 10μF caps and 51k resistors as an example, as well as the LTspice circuit list so you can play with the circuit if you want to.
     
    Last edited: Jul 24, 2016
  11. gfyhv1922

    Thread Starter New Member

    Jul 2, 2016
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    Hi everyone,

    Great info and thanks for the responses!
     
  12. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Use two 100 uF caps in series. This makes a 50uF of capacitance. When you put the two together in series, keep the positive ends of these polarized caps together and you get a non-polarized cap.

    Here is an example from an old audio amplifier input stage, two 22uF polarized electrolytic caps are used to make a non-polarized 10uF input.


    image.jpg
     
    Last edited: Jul 24, 2016
  13. gfyhv1922

    Thread Starter New Member

    Jul 2, 2016
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    Interesting, i didn't know that two caps inverted made a non polarised cap. Are there any side effects to this that I should be aware of?
     
  14. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    The overall voltage rating of the pair is double the voltage rating of the lowest rated cap in that pair.
     
  15. EM Fields

    Member

    Jun 8, 2016
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    How do you figure that?
     
  16. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    He was told to use two caps of equal capacitance. Therefore, voltage is equally divided.
     
  17. RichardO

    Well-Known Member

    May 4, 2013
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    I have always assumed that the reverse biased cap is nearly a DC short. That is what keeps it from exploding. Have I been wrong all this time?
     
  18. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I dont know, just an assumption on ny part, I only play an engineer on the internet.
     
  19. RichardO

    Well-Known Member

    May 4, 2013
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    Well! It's off to the lab I go. I'll get back to you. :D
     
  20. EM Fields

    Member

    Jun 8, 2016
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    True enough, but for electrolytic capacitors in series opposition, why would the voltage rating for the pair be twice the voltage of the voltage rating of the weakest cap?
     
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