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Ceramic capacitors

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

Reply to
Chris
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you could parallel smaller value ceramics or, if you\'ve got the room
and the capacitance isn\'t critical, you could use a couple of
series-opposed 10µF electrolytics rated at somwhat higher than the
peak voltage ever to be expected out of the sensor.
Reply to
John Fields

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.

Reply to
John Popelish

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 ?

Reply to
Simon Smith

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