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  • EEVblog #33 2of2 – Capacitor Tutorial (Ceramics and impedance)

    Posted on September 26th, 2009 EEVblog 22 comments

    A blog so big it needed two parts.
    This time it’s all about ceramic capacitors, and that mysterious things of putting different vale capacitors in parallel. Not to mention microphonics.

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    • http://wa0uwh.blogspot.com/ Eldon Brown

      Dave – another good video blog – I always enjoy them!

      For your web site you should have a “favicon.ico” – that is the little icon that shows up to the left of your URL on a browser – yours is blank at present :-(

      It is a bit of a Geek-thing, but it is just a little icon file that you put next to your home or index.html page that is read by the browser. Your web master should know what is needed.

      Eldon

    • Laurent Coustet

      ‘Love the end :-)

      Thanks for thoses tips :-)

    • Brian Hoskins

      One interesting (and in my opinion important) thing to mention about the class II ceramic capacitors (i.e. the multi layer ones) is the problem of “aging”. I got caught out by this once, because an ICT system I designed was chucking out a high percentage of product due to some capacitor measurements coming in outside of specification. The capacitors were X7R devices, and removing them from the board and taking measurements confirmed that they were indeed outside of tolerance.
      That part of the story worked out fine, but I then decided to investigate the reels of components we have in stock and, to my astonishment, almost every single reel we had (I’m talking thousands of components here) seemed to read out of spec. Curiously, the measurements I made were always *below* the stated tolerance.

      My immediate reaction was to complain at the supplier, but a colleague pointed me in the direction of an article about the “Aging” effect on multi-layer ceramic caps. The aging effect, as I’m sure you’re aware, causes the capacitance of the device to decrease with time. This effect can actually be reversed by raising the temperature of the device above a certain threshold (approx 150 deg c I think) and with this in mind it is expected that any SMT or hand soldering process would reset the aging effect and bring the capacitors back within specification. So, whilst we did indeed seem to have a tolerance issue with some of the capacitors we were fitting, it’s not as simple as measuring a sample from the rest of the reels to confirm the suitability of the existing stock. Due to the aging effect, and depending on how long the devices have been sitting on the shelf, the measurement might not be valid.
      I did and experiment on this and my findings confirmed that the capacitance measurements did indeed come back within specification after a half-hour’s treatment in an oven.

      It’s also interesting to know that the aging effect starts all over again after it has been reset by an SMT/hand solder process. So, the capacitance value of the component you’ve fitted to your board will decrease with time. If the component value is at all critical, then the aging effect is extremely significant.

      I found this via a quick google:

      http://www.johansondielectrics.com/technical-notes/general/ceramic-capacitor-aging-made-simple.html

      Brian

      • Mark

        Brian,

        Thanks for the info! I’ll keep this in mind.

        Mark

    • http://lerch.no-ip.com/atm James Lerch

      Hey Dave,

      Could this audio vibration effect of multilayer ceramic capacitors be what you saw back during your O-Scope mystery? You know, when you stood up quickly and the scope showed 140mhz signal?

      I wonder what the resonant frequency of a small smd cap is?

      • http://www.alternatezone.com/eevblog admin

        No, it’s definitely static. I can get the same effect by rubbing my jumper on the chair (which does not produce noise or vibration).

    • Mark

      Dear Dave,

      Your blog is wonderful. I’m learning so much from it. I get very excited about electronics and it’s very hard to find other people who do as well. Keep it up! I eagerly look forward to every update.

      Thank you,
      Mark

    • John McVirgo

      Dave,

      a great blog that is very informative as usual.

      What has been the most difficult project you’ve been involved with that required the greatest level of technical know-how from you?

    • Mark

      Dave,

      I had an idea for an interesting topic you could speak about in one of your segments… “tin whiskers”. I find it interesting that such a small, little known problem can cause so much damage(tin whiskers were responsible for the failure of the Galaxy IV satellite). I hear they can cause lots of problems in high frequency circuits because they act like little antennas. Have you ever run into this problem?

      Thanks,
      Mark

      • http://www.eevblog.com EEVblog

        Haven’t run into the problem personally, but know of some who have. Usually it’s one of the more obscure problems and not something your ordinarily worry about, but when it does happen the proverbial !@#$ can fit the fan!
        Haven’t heard of the antenna aspect. Off the top of my head it would have to be incredibly high frequency to be an issue, given the typical lengths involved.

        • Mark

          Yes, I’ve heard it’s a big problem above 3 GHz… not anything I’ve ever worked on.

      • Sean

        The helical resonators in old GE mobile business radios tended to have a lot of problems with whiskers according to a radio tech friend.

    • http://blog.n-rd.com Adam

      Wow! More like these please. Amazing. For newcomers like me to this, these brain dumps on basic components are very helpful.

    • Sean

      Tech application notes from Intersil on choosing bypass capacitors.

      http://www.intersil.com/data/an/an1325.pdf

      Pages 5 and 6 tie in with your whiteboard graph on using more than one capacitor.

    • David Wise

      Two more things.

      First, the piezo effect works both ways; class 2′s with a lot of AC voltage across them can “sing” at the ripple frequency.

      Second, not only do they decrease with temperature and time, they’re also affected by voltage. I’ve seen Z5U’s drop 80% just by charging them to rated voltage. This isn’t widely known, and manufacturers rarely spec it.

    • http://wardyprojects.blogspot.com Adam Ward

      That “Microphonics” phenomenon is fascinating.
      It got me thinking actually, I have one of these backlit computer keyboards which uses some electroluminescent layer to glow. When I turn on the backlight, I can hear a faint but distracting high-pitch whine from the keyboard.

      Could this be caused by some caps on there vibrating because of some power saving PWM controller driving the backlight?

      I’ll have it in bits tonight and see what’s what :)

    • Jeffrey

      Hi Dave, great videos and well done in explaining the tiny details of a topic you post. I have a question regarding replacing a 4.7uF electrolytic capacitor with a 4.7uF 1206 ceramic capacitor. It will be used for a 555 astable mode circuit ( it will be part of the R x C section) will there be any difference or is it ok to do so ? Thanks.