Which Cap Type

Discussion in 'General Electronics Chat' started by DC_Kid, May 6, 2014.

  1. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    706
    11
    for long accurate life of a astable 555 in the 1-200Hz range, which type of caps are best suited or the job? electrolytic, tantalum, other ?? my values will be 1u and 68u. operating temps may be -10 to 110F, but would normally be in that 70-80F "room temp" range.
     
  2. Wendy

    Moderator

    Mar 24, 2008
    20,993
    2,731
    Tantalum are better than electrolytic, but with timers and oscillators both tend to drift over the long term. It may not matter for your application, but then again, it might. Ceramic types (as in 1uF) are a bit more stable. Any RC circuit can have this problem.

    By drift I'm figuring 10% give or take.

    Oh, BTW, when tantalum are used as power supply filters they can go up like match heads if they fail. They are basically better caps all around than electrolytic, so they get used anyhow.
     
  3. MrChips

    Moderator

    Oct 2, 2009
    14,278
    4,195
    For stable timing on a 555 circuit you want to stay away from all electrolytics, aluminum and tantalum.

    You want to choose mica or plastic (poly---). Since you will have difficulty finding the capacitor in large values ( > 1μF) go with what is available and use a divider circuit to divide down from a high frequency to a low frequency.
     
  4. #12

    Expert

    Nov 30, 2010
    17,831
    9,165
  5. #12

    Expert

    Nov 30, 2010
    17,831
    9,165
  6. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,309
    And polyester!

    They are badly temperature dependent and as the 555 has been running for a minute the freq will change as the cap warms up.

    Actually it's hard to get a stable freq out of a 555 even on a good day. :)
     
  7. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    706
    11
    Last edited: May 7, 2014
  8. Veracohr

    Well-Known Member

    Jan 3, 2011
    600
    86
    Yes, they are electrolytic capacitors, they just use a different kind of electrolyte than the ones called "aluminum electrolytic".
     
  9. #12

    Expert

    Nov 30, 2010
    17,831
    9,165
    I don't know. MrChips said, "poly" so I got prices for anything that started with, "poly". Poly-propolene, poly-ethylene, poly-mer, whatever is available.
     
  10. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    706
    11
    just the insulator, like mica. ceramic in 1u is fine, the 68u is the hard one. thre is a 68u ceramic but its like $3 ! i am looking for the 10c/ea at qty 100, etc.
    my application has adjustable frequency, so accuracy for freq is not so important, but looking for the parts that will last a "lifetime", etc.
     
  11. #12

    Expert

    Nov 30, 2010
    17,831
    9,165
    I guess that puts you in the ceramic department, but you still haven't said what voltage you need.
     
  12. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    706
    11
    oh, 5V dc is the operating voltage from a 7805 reg.
    a 6.3v cap is a tad close. 10 or higher would make me feel better, etc.
     
  13. Wendy

    Moderator

    Mar 24, 2008
    20,993
    2,731
    For an RC oscillator, the 555 is the best of breed. It tries to compensate for power supply variation and several other things. The problem is the other components, mostly the capacitors.

    RC oscillators are used for everything from triggering SCRs on ovens to old oscopes and old TV sets. They work well, so it is a mistake to reject them because of drift of their external components. The trick is to be aware of their limitations, and know if it really matters. Most cases it doesn't.

    10% drift (which is a worst case number) for 200Hz is 180Hz to 220Hz.

    You have to keep things in perspective.
     
  14. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    706
    11
    10% is acceptable..... tolerance is one metric, longevity of the item is the other for me. good electrolytics should last a long time, cheap ones not so long, etc. depending on cost i may need to use the forbidden types, etc.
     
  15. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    7,906
    1,789
    Smaller value caps such as the NPO dielectric are best for long term and temperature stability. But using a '555 limits you in in cap value selections, with long gimes demanding long caps.

    If you skip over the '555 and look at chips such as the CD4536 you get an RC oscilator driver coupled to a long chain divider. That's the best of both worlds, you get to use small accurate caps and divide down the output to what you need.

    That particular chip is not the easiest to understand but is capable of doing lots of tricks.
     
    #12 likes this.
  16. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    706
    11
    hmmm, 4536 is ~50cent/1pc vs ~150cent/1pc 7556

    well, i already have my schematic done w/ 7556 and proto parts should arrive today.
     
  17. #12

    Expert

    Nov 30, 2010
    17,831
    9,165
    I've been using the CD4017and CD4020.
    They can't count as far as the CD4536, but they worked for what I needed at that moment.
     
  18. R!f@@

    AAC Fanatic!

    Apr 2, 2009
    9,616
    1,082
    Not a good reply but a PIC will be far easier, accurate and no cap issues that is if you can program.
    Am I wrong guys.
    I got the urge to post due to the state I am in.
     
  19. DC_Kid

    Thread Starter Distinguished Member

    Feb 25, 2008
    706
    11
    sure, an option. not so sure its "easier". i have done some picaxe stuff in the past.
     
  20. #12

    Expert

    Nov 30, 2010
    17,831
    9,165
    When analog design is the right thing to do, it's excellent, but when a microprocessor can do the job, it's usually a lot smaller and cheaper. Choose your poison.
     
Loading...