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    Electrytic Capacitor and Ceramic capacitor

    I uses a 10uF electrolytic capacitor on a bread boar to test the circuit,the diagram shows part of the circuit with act as a timer circuit when the opamp goes high,when voltage from the charging capacitor is fed to a OpAmp voltage follower,and to other part of the circuit.

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    when I use a electrolytic capacitor,the RC time constant is about 8 second and when I fabricate the circuit on a PCB and uses a SMD ceramic capacitor (Non polarized),the time constant drop to about 2 second.
    I desolder the ceramic capcitor and put in a electrolytic capacitor into the PCB,the time constant work as design again which is 8 second,why this happened?

    The discharge resistor is a 910K 1% metal film resistor

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    Re: Electrytic Capacitor and Ceramic capacitor

    What are the voltage ratings of the respective caps? Capacitance can change with temperature and voltage especially when you get near one of the limits.



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    Re: Electrytic Capacitor and Ceramic capacitor

    Hi,

    Time constant depends on capacitor value (uF), not on dielctric.

    Klaus



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    Re: Electrytic Capacitor and Ceramic capacitor

    Sorry I had forgotten to mention,both capacitor capacitance are the same ,value of 10uF with a voltage rating of 10uF,except that the ceramics is a non polarized 50V capacitor and the electrolytic capacitor is a 16V polarized capacitor



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    Re: Electrytic Capacitor and Ceramic capacitor

    I seriously doubt that you are using a ceramic capacitor with that value.
    At least 10uF is not too usual for this dieletric.
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    Re: Electrytic Capacitor and Ceramic capacitor

    Quote Originally Posted by sabahan View Post
    ,value of 10uF with a voltage rating of 10uF,
    Check both with a good multimeter (that can measure C) and report the reading. What is the value of R and the voltage applied?

    10uF ceramtic SMD caps are not common and you need to doubly check the value.



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    Re: Electrytic Capacitor and Ceramic capacitor

    I'm using a Digimess HUC3100 LCR meter to measure both the capacitor and the resistors,all component are ordered from RS components,which I believe it should be in its stated tolerance ,even if the meter is with a accuracy of +_ 10%,the time constant should not be of by 6 second,anout 400% off,and the PCB I'm using I tried both capacitor,the electrolytic Capacitor and the ceramic capacitor,all other component remains the same,only variable is the capacitor itself
    The measured capacitance of both capacitor are as follow
    Electrolytic Capacitor - 10.6uF
    Ceramic Capacitor - 9.8 uF

    with the same meter
    can there be a possibility that a non polarized capacitor cause the error?



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    Re: Electrytic Capacitor and Ceramic capacitor

    This is the capacitor I ordered from RS components
    http://my.rs-online.com/web/p/cerami...itors/7900610/



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    Re: Electrytic Capacitor and Ceramic capacitor

    It is very difficult to explain the difference; only thing I can think of is surface leakage is causing faster discharge.



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    Re: Electrytic Capacitor and Ceramic capacitor

    Hi,

    Do you know about voltage-to-capacitance relationship of X5R capacitors?
    Look at figure 4.
    http://www.analog.com/library/analog...capacitors.pdf

    But i think even this can not completely explain your problem.

    Klaus



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    Re: Electrytic Capacitor and Ceramic capacitor

    Most likely explanation is a broken chip showing excessive leakage current or varying capacitance.



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    Re: Electrytic Capacitor and Ceramic capacitor

    soldering a 10uF SMT cap, even X7R dielectric, can severely reduce its capacitance, as does the DC bias, to get the full 10uF at 10V, you need at least a 50V part, it just gets worse for other dielectrics, e.g. Y5V much worse with temp...



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    Re: Electrytic Capacitor and Ceramic capacitor

    I'm willing to believe that this problem is likely due to load coupling problems caused to different ESR of each kind of capacitor, as well as a non negligible impedance of the operational/ comparator ( open collector ? ). What is the P/N of the OpAmp, and the supply voltage is under 16v ?
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    Re: Electrytic Capacitor and Ceramic capacitor

    Yes,the working voltage is 12VDC,from a SLA battery,here are no alternating wave form from the circuit,all are DC operation,the circuit is to control a set of 12V LED light that I'm planning to install around my house,the light will only turn on for about 10 second when ts dark and sense something moves,the circuit above is the time delay circuit

    I will test the ESR of each capacitor when I back on wednesday,thank you for the assistance



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