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RC time, using a Stop Watch, RC time is longer in time is the capacitor bad?

Discussion in 'General Electronics Chat' started by Billy Mayo, Nov 26, 2013.

  1. Billy Mayo

    Billy Mayo Member

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    Circuits that use RC for charging up a capacitor to cause a Delay in time

    What value capacitors or resistors would it be to have a charging time of 1 second? , 5 seconds?, 10 seconds?

    When using an RC network to do a charging time of 5 seconds but you get 20 seconds , what is wrong with the circuit? since the capacitors are old in the circuit can a capacitor that is in an RC network or a capacitor in an Op-amp intergrator circuit cause the charging time to get longer?

    circuits at my work that are GOOD, work are 5 seconds in charging time for the lights to either turn off or on in 5 seconds after I push a button

    I have to use a Stop watch to makes sure its 5 seconds the lights turn on or off after I push a button

    If the capacitor is old , would it cause the time to get longer?

    Because the bad boards at my work have different Turn on and off times that are not 5 seconds and they values on the RC networks are all the same values on the GOOD boards, What can be wrong with the RC timing?

    BAD boards turn on and off the lights 30 seconds, 50 seconds , etc.
     
  2. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    The RC timing equation will give you the time for a charge to 66% of its final value.
    To reach a 'full' charge, at least 4 or 5 times the RC value is required.

    Look up exponential charge/discharge equations.
    E
     
  3. kubeek

    kubeek Well-Known Member

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    Also electrolytic caps have leakage current, which can be modelled as a high value resistor in parallel with the cap. With such long delays it is quite possible that it will change that much from the calculated value to the actual measured value.
     
  4. dave

    Dave New Member

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  5. Billy Mayo

    Billy Mayo Member

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    Why would leakage current Raise or increase the Time? or charging time? get longer delay time?
     
  6. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    If you think about it, a 'leaky' capacitor is effectively a discharge resistor connected across the capacitor.

    So charging time will be longer, discharge time shorter.
     
  7. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It's best not to try and get long timings from an RC circuit, leakage means they are inaccurate and unreliable.

    Assuming you have to try, use Tantalum capacitors rather than Electrolytic as they are less leaky.

    But better still, use a small PIC to provide your delays :D

    An 8 pin PIC with internal oscillator can provide far more accurate and reliable long delays (years if required), and requires no other components. If you need even better accuracy then add a crystal and a couple of capacitors as well.
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    The value of a large electrolytic capacitor is not accurate since its value could be -50% to +100%.

    Does anybody remember Billy Mayo's original name (a few years ago like now he was asking millions of basic questions about the index pages of books).
    Walters?
     
  9. Mike odom

    Mike odom Active Member

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    So, basically, you're just moving the RC circuit inside the PIC... so these two statements ("It's best not to...timings from an RC...", and "An 8 pin PIC with internal osc...") are mutually exclusive?
     
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  10. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    No, you're moving from a leaky (and unreliable) large electrolytic to an on-chip tiny capacitor. which won't be leaky :p

    I notice you removed the 'long' from the timings quote :D
     
  11. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Walters (Billy Mayo) calls himself Danny Davis today on another website forum.
     
  12. Billy Mayo

    Billy Mayo Member

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    What I do at work is test the Timing of circuits

    GOOD BOARDS:
    1.) Timing of Output voltage to go from +10 volts to - 10 volts in 5 seconds with a switch is pressed
    2.) When a Switch is pressed , measure the timing when a light turns on , should be 2 seconds

    But I'm getting boards that are Longer timing
    When the Capacitors are leaky in an RC network or In an Op amp Intergator circuit or the Op amp has input leakage, can this cause the Timing to get longer?

    3.) There is Other Switches when TURN On takes 10 seconds to activate an Op-amp to switch it state, What is this timing RC networks called? they do this delay timing for 10 seconds so the input of the Op-amp comparators don't get false triggering's from noise or cause they want the Switch Turn ON state to stay on for 10 seconds before the Op-amp switches states or polarity on its output, What is this called when a designer puts RC delays charging like this?

    4.) There is a Momentary Switch that Ground GND the Reset of an IC.
    a.) Press Momentary switch GND Reset Once it will turn off the LIGHT
    b.) Press Momentary switch GND Reset Again it will turn ON the LIGHT

    What Kind of IC uses an Reset Pin that you can Ground GND that will turn On or off a light using the Reset Pin?

    5.) Also what I do at work is I have to measure the TURN ON and OFF points of Lights by using a Pot. I turn the Pot very slowly until the light turns ON or OFF and have to measure the voltage and current of how much voltage and current is I guess going into the Op-amp Comparator circuit.
    What is this called what I am doing? I don't know the electronic name for what I'm doing for this

    The Pot is either hooked up to an Op amp Comparator or to TTL or CMOS Logic GATES
     
  13. Billy Mayo

    Billy Mayo Member

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    6.) Also what I see a lot of times on the bad boards is the company's CMM manual , component maintenance manual says the RANGES of the min and max voltages or the min and max Times in seconds are on a GOOD board.

    I gets Voltages and Timings that are very close to the MAX values, can this be caused by leaky Capacitors go to the Max Voltage value or Very close to it? I guessing

    Example#1
    Test Point#1 , is 9.8 volts , time is 4.7 seconds ( BAD BOARD )
    CMM manual will say voltage min is 7 volts , max is 10 volts
    Timing Min is 4 seconds , Max is 5 seconds
     
  14. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    hi,
    What you need is a 'pass/fail' tester, which has the lower and upper limits of the 'timed' period programmed.

    If you do not know the timing range limits and you use the 'ideal' time for testing, most of your capacitors will be unnecessarily failed.
     
  15. WTP Pepper

    WTP Pepper Active Member

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    Have a look at the Linear Technology TimerBLOX range. They are designed to get around this problem of long time delays.
    A 4060 CMOS divider/oscillator can do similar.
     
  16. Billy Mayo

    Billy Mayo Member

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    I do know, The CMM manual tells the electronic test technician what the timing range limits are

    The Timing's have drift very close to the Upper limit on the BAD BOARDS

    The timing's for the GOOD boards are Right in the middle on the timing range

    When you have leaky capacitors in RC , leaky op amp inputs, Leaky capacitors in op amp intergator circuits, can this cause the timing to drift up to the upper limit?
     
  17. ericgibbs

    ericgibbs Well-Known Member Most Helpful Member

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    What operational problem is seen on the suspect Bad boards??
     
  18. Billy Mayo

    Billy Mayo Member

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    The good boards and bad boards have the same values and parts, the only difference is there is a Timing difference on the bad boards

    The timing problems are that the timing is very close to the upper LIMIT or right on the money on the upper limit

    The Capacitors on both the good and bad boards have the same values , so the only thing I can think of is that the caps are leaky which has caused the timing to drift up? cause long time
     
  19. kubeek

    kubeek Well-Known Member

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    Well, did you try measuring the leakage at the cap, preferably without desoldering the cap?
    Disconnect the charging resistor, connect to a voltage source and mesure current through the cap.
    Four basic sources of the timing error through leakage would be: your boards might not be cleaned properly and the flux residue adds to the leakage, or the capacitors have degraded, or some of the caps are worse but still within their specs, or the input leakage of the timing chip varies.
     
  20. Billy Mayo

    Billy Mayo Member

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    No, I'm not sure how to measure the Leakage of a cap, how do you do this?

    So how would I know if the cap is leaky from measuring the current through the cap?

    Voltage source---> current meter ----> cap?
     
  21. kubeek

    kubeek Well-Known Member

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    Exactly. Depending on the circuit you will directly measure the leakage through everything that is conected after the current meter.
     

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