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Decoupling capacitor

Discussion in 'General Electronics Discussion' started by Darren Booker, Oct 22, 2017.

  1. Darren Booker

    Darren Booker

    11
    0
    Oct 22, 2017
    Hi I need to keep a constant supply to an alarm module when the voltage switches
    I need to fit a decoupling capacitor
    Anybody any idea what I exactly need 20171022_105942.jpg
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    9,400
    1,920
    Nov 17, 2011
    It depends on:
    • The switching time (how long does the capacitor have to hold the voltage): dt
    • The current consumption of the alarm module (more current means a bigger capacitor): I
    • The allowed voltage drop during the switching time (a capacitor cannot hold the voltage completely at the powered level, a certain drop needs to be allowed for):dv

    The capacitance can be calculated from C = I*dt/dv.
    Chose the next bigger standard value for an added safety margin. The capacitor needs to be rated at the operating voltage plus some margin. For 12 V at least a 16 V capacitor is required, a 25 V capacitor is recommended (taking into account that this is for an alarm circuit where a bit of added reliability is worth every penny).
     
  3. Darren Booker

    Darren Booker

    11
    0
    Oct 22, 2017
    The voltage hold would be rather quick lot less than half a second it will be just switching from the on timer relay to the off timer relay
    The module consumes 300 milli amp maximum
    Can you calculate for me
     
  4. twister

    twister

    161
    6
    Feb 12, 2012
    I think the formula is
    time to 60% of charge=CxR
    Where C= capacitance in uf
    R=resistance
    T=time in seconds
    Example 100ufx10M=1000seconds
     
  5. Darren Booker

    Darren Booker

    11
    0
    Oct 22, 2017
    Im not really good with equations
    What capacitor would I need
    With the details I've provided
    If some one could work it out for me would be much appreciated
     
  6. Ratch

    Ratch

    1,044
    308
    Mar 10, 2013
    Have you considered a battery? If you can get the right voltage and capacity, your problem will be solved. Otherwise, the use you propose for a capacitor is not decoupling. So you want to supply 300 ma for 0.5 sec? What is the nominal voltage and how low can it decrease and still be acceptable. When I know that, I can calculate the capacitance needed to sustain that voltage range for that length of time while supplying that amount of current.

    Ratch
     
  7. Darren Booker

    Darren Booker

    11
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    Oct 22, 2017
    Nominal voltage would be 9 volts in a 12 volt system
    It takes spproximately 0.1 seconds for the timer relays to switch over
     
  8. Ratch

    Ratch

    1,044
    308
    Mar 10, 2013
    Let me get this correct. The system will operate at 12 volts until it switches to another voltage source. You do not want to go below 9 volts during the switchover. The time for the switchover will be 0.1 seconds, not 0.5 seconds. The current drain will still be 0.3 amps. Do I understand everything correctly?

    Ratch
     
  9. Darren Booker

    Darren Booker

    11
    0
    Oct 22, 2017
    Yes that's correct
    Used 0.5 seconds as a switchover as a maximum but don't think its quite that much according to spec of module
    Could you work out for 0.1 second switch over and 0.5 second switch over please
     
  10. Ratch

    Ratch

    1,044
    308
    Mar 10, 2013
    OK, when the voltage is down to 9 V, and the current is 0.3 amps, the alarm looks like a 30 ohm resistor to the capacitor. We want the RC time constant to be about 5 times 0.1 sec, so that makes the capacitor value 16,667 mf. You may want to double that value because electrolytics are notorious for drying out and losing their capacitance. If the cap is 16.667 mf and the resistance is 30 ohm, the cap will go from 12 volts to 9.82 volts in 0.1 sec. If you want to sustain the same current for 0.5 sec to a drop down to 9.82 volts, you will need a 83.333 mf cap. Those are fairly big units. Are you sure you don't want to use a battery?

    Ratch
     
  11. Darren Booker

    Darren Booker

    11
    0
    Oct 22, 2017
    A battery is feesable
    This would have to be a rechargeable one I presume and would it charge through the normal 12 volt system
    What battery would I use?
    Also do you have a link to were I can get these capacitors or battery
     
    Last edited: Oct 22, 2017
  12. Ratch

    Ratch

    1,044
    308
    Mar 10, 2013
    That is a question for your local parts supplier or a catalog, but not me, I would think the 12 volt system could keep the right kind of battery fully energized without over energizing it.

    Ratch
     
  13. motardo

    motardo

    1
    0
    Oct 22, 2017
    Hi,

    Assuming a constant current discharge of 300 mA, and C = Q/V = (I * t)/V
    - for 0.1 seconds, C = (.3 A * .1 s) / (12 V - 9 V) = 10 mF
    - for 0.5 seconds, C = (.3 A * .5 s) / (12 V - 9 V) = 50 mF
     
  14. Ratch

    Ratch

    1,044
    308
    Mar 10, 2013
    It doesn't work that way. A capacitance that small would never be able to sustain that much current for that length of time. The cap would de-energize in an instant.

    Ratch
     
  15. Ratch

    Ratch

    1,044
    308
    Mar 10, 2013
    The usage determines what kind of cap it is. In your case, what are you bypassing? Caps that big are sure to be electrolytics.

    Ratch
     
  16. Darren Booker

    Darren Booker

    11
    0
    Oct 22, 2017
    It's just used to keep a supply to the module
    But was under the impression that a decoupling capacitor is what was required
    Also is the capacitor fitted in parralel on live and earth
     
  17. Ratch

    Ratch

    1,044
    308
    Mar 10, 2013
    You need to learn about decoupling and bypassing.

    Ratch
     
  18. Darren Booker

    Darren Booker

    11
    0
    Oct 22, 2017
    Ok thanks
     
  19. duke37

    duke37

    5,254
    726
    Jan 9, 2011
    The capacitors which have been specified are colossal. Do you mean μf rather than mF?
     
  20. Ratch

    Ratch

    1,044
    308
    Mar 10, 2013
    Yes, microfarads. I never heard of anyone saying millifarads, although the prefix is correct.

    Ratch
     
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