Ceramic capacitors

[Simon Smith]

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 ?

 

[John Fields]

On Tue, 14 Jun 2005 15:39:19 +0100, "Simon Smith"
<notreal@example.net> wrote:

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 ?

---
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 10ľF electrolytics rated at somwhat higher than the
peak voltage ever to be expected out of the sensor.

--
John Fields
Professional Circuit Designer

 

[John Popelish]

Simon Smith wrote:

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 ?

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.

 

[Chris]

Simon Smith wrote:

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 ?

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