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Ceramic capacitors

Discussion in 'Electronic Basics' started by Simon Smith, Jun 14, 2005.

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  1. Simon Smith

    Simon Smith Guest

    I have a simple question,

    I have to build this circuit for interacing with a soil moisture circuit.

    The circuit has shows it needs two 4.7uF non-polar ceramic capacitors, used
    to isolate the sensor from the rest of the circuit. The capacitors have to
    be non-polar as there is the voltage across the sensor is AC. However i've
    only been able to find one possible suitable ceramic capacitor from one
    supplier (Farnell, which is out of stock). All the other suppliers i
    normally use (RS, Maplin etc..) dont stock a ceramic capacitor of this
    value.

    So - can anyone recommend an alternative type or value of capacitor that
    could do the same job ?
     
  2. Chris

    Chris Guest

    Hi, Simon. You didn't mention the AC voltage applied, or the frequency
    of interest. However, most hobbyist circuits of this type use audio
    frequency low voltage signals. So, just about any low leakage
    non-polarized cap will do (audio non-polarized electrolytics are out).
    Ceramic might have been chosen by the circuit designer because of cost.
    Availability is a problem for the biggest ceramic caps.

    A good fairly low cost non-polarized cap of this type would be
    polyester. Maplin doesn't have one of that value, but Farnnell does.
    Look at their order number 108799 for a smallish 100VDC/63VAC radial
    lead poly cap that should do the job. Be sure to check applied voltage
    first. If you need a higher voltage, just use the component selector
    to menu Capacitors-->Polyester-->4.7uF, and choose what you need.

    Good luck
    Chris
     
  3. John Fields

    John Fields Guest

     
  4. They are kind of large, but mylar film capacitors in this size are
    available. For instance, Digikey sells a Panasonic E series 4.7 uF
    100 volt cap for about US$3 each. But it is 30.5 mm wide.

    There are lots of non polar electrolytic caps available, if tolerance
    and leakage do not have to be so tight. For instance Digikey sells a
    Panasonic SU series 4.7 uF 25 volt non polar electrolytic (radial) for
    about US$0.24, each. It is only 5 mm diameter, 11 mm tall. With 6.5
    uA worst case leakage current, it is not nearly as good a capacitor as
    the film type, though.
     
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