Capacitor choice or replacement

Discussion in 'The Projects Forum' started by Netwaves, Mar 19, 2015.

  1. Netwaves

    Thread Starter Member

    Feb 9, 2015
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    I have a piezo buzzer that causes noise on the bus. I'm using a 100uF 16V electrolytic capacitor and that works fine to dampen the noise, but it holds more charge than needed and when the circuit powers down it keeps the piezo going for a couple of seconds until the cap drains.

    My question is, what capacitor can I use that won't store as much power, and will drain more quickly and still work? Should I be looking at lower uF values like 50uF?
     
  2. bertus

    Administrator

    Apr 5, 2008
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  3. Netwaves

    Thread Starter Member

    Feb 9, 2015
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  4. PlasmaT

    Member

    Feb 19, 2015
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    You could also try a resistor in parallel to the capacitor. Let the cap be at 100uF value.
     
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  5. Netwaves

    Thread Starter Member

    Feb 9, 2015
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    It's a 12V piezo buzzer. What value resistor would you recommend?
     
  6. Netwaves

    Thread Starter Member

    Feb 9, 2015
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    I think my piezo is 4KHz. What would be a good capacitor value and/or how could I calculate it?
     
  7. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Post the complete schematic of the set-up.
    This way we can see what needs to be changed.

    Bertus
     
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  8. Netwaves

    Thread Starter Member

    Feb 9, 2015
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    Thanks you! Here is a small/partial schematic I've been working on. The piezo buzzer is remote, approx 100', from the main circuit. The dashed lines are the run out to the sounder/sensor combo. Also, the optoisolator is a PS2505 bi-directional, not a PS2504 as pictured. As you can see, this is a two line system. I can't run more lines. I have this entire circuit on a breadboard and it works fine except for the 100uf electrolytic cap on the piezo side. If I remove the cap then the noise in the system causes lots of havoc.

    remote_sounder.png
     
  9. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What kind of wires do you use for the piezo?
    It can be there is a pickup of signals.
    Can you use a coax or other shielded cable for the piezo?

    Bertus
     
  10. Netwaves

    Thread Starter Member

    Feb 9, 2015
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    No. The wires are old security system wiring. Approx 32 of them. 18 gauge stranded. About 30 years old. I can't replace them. :-( I'm only having the problem on the wiring with the piezo, so it is the piezo causing it. I would like to replace the 100uf electrolytic with a smaller cap or a ceramic if possible.
     
  11. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    What kind of signal is going to the piezo?
    Is it DC or a switched/pulsed signal?

    Bertus
     
  12. Netwaves

    Thread Starter Member

    Feb 9, 2015
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    Bi-directional DC. The piezo is activated by reversing the polarity. The switch in the schematic is a reed relay. This way I can constantly monitor the door switch in both polarities and simply turn on the piezo buzzer by reversing the polarity.
     
  13. Netwaves

    Thread Starter Member

    Feb 9, 2015
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    I wonder if I could use a choke?
     
  14. ScottWang

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    Aug 23, 2012
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  15. Netwaves

    Thread Starter Member

    Feb 9, 2015
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