polystyrene capacitor

[Pachinko7]

A project I m trying to build calls for a 47p polystyrene capacitor. What are these ? I could not find it in a general catalog (Digi-Key). Can I use other kind of capacitor ? Thank you for your help.

 

[Phasor]

For general purpose use, you should be ok with a ceramic capacitor - depends though, on the application - what is your project, and can you post a schematic?

 

[TheOne]

Polystyrene capacitors are noted for their low distortion, low dissipation factor, and excellent temperature stability. Typically used in equalization and tuner circuits

 

[Pachinko7]

Thank you. So I guess I will have to find the real thing ... :roll:

 

[Nigel Goodwin]

Thank you. So I guess I will have to find the real thing ... :roll:

It depends on what it's used for, you still haven't mentioned that!.

 

[Pachinko7]

Thank you. So I guess I will have to find the real thing ... :roll:

It depends on what it's used for, you still haven't mentioned that!.

Well, I am building the theremin project that appeared on EPE, October 2004, page 706. It should be cool... I am just trying to gather as many components as possible and start, I already prepared the PCB...

 

[TheOne]

Makes sense that you will want a stable capacitor for the oscillators.

 

[Nigel Goodwin]

Thank you. So I guess I will have to find the real thing ... :roll:

It depends on what it's used for, you still haven't mentioned that!.

Well, I am building the theremin project that appeared on EPE, October 2004, page 706. It should be cool... I am just trying to gather as many components as possible and start, I already prepared the PCB...

If you can find a silver-mica capacitor of the right value, they are even better than polystyrene.

 

[Pachinko7]

Thank you. So I guess I will have to find the real thing ... :roll:

It depends on what it's used for, you still haven't mentioned that!.

Well, I am building the theremin project that appeared on EPE, October 2004, page 706. It should be cool... I am just trying to gather as many components as possible and start, I already prepared the PCB...

If you can find a silver-mica capacitor of the right value, they are even better than polystyrene.

I found mica film capacitor of that value on Digi-Key catalogue. I does not mention anything about silver, though. (Part # 338-1017-ND). Is that the right kind of capacitor ?

 

[TheOne]

Try here https://www.web-tronics.com/passive-electronic-components-capacitors.html

Here is a discusssion about polystyrene vs silver mica
https://www.diyaudio.com/forums/showthread/t-20205.html

 

[TheOne]

If your cap will be used in an oscillator, here are some facts to consider.
(This may not be critical to your application, but it is good to know)

Up to about 10MHz it is often practical to use polystyrene caps. They have a slight -ve temperature coefficient making them ideal to use with magnetic cores with a +ve coefficient.

Silver-mica can also be used but their individual drift characteristic may vary from -ve, zero or +ve (even for caps from the same batch!). The only way to grade them will be to monitor the capacitance with a cap meter for change in value over say a 1 hour period in a temperature chamber when the temperature is varied from about 50-100 deg F.

Once graded they make excellent capacitors for use in oscillators because of their high Q like polystyrene.

 

[Pachinko7]

If your cap will be used in an oscillator, here are some facts to consider.
(This may not be critical to your application, but it is good to know)

Up to about 10MHz it is often practical to use polystyrene caps. They have a slight -ve temperature coefficient making them ideal to use with magnetic cores with a +ve coefficient.

Silver-mica can also be used but their individual drift characteristic may vary from -ve, zero or +ve (even for caps from the same batch!). The only way to grade them will be to monitor the capacitance with a cap meter for change in value over say a 1 hour period in a temperature chamber when the temperature is varied from about 50-100 deg F.

Once graded they make excellent capacitors for use in oscillators because of their high Q like polystyrene.

Thank you very much for this information. It makes sense to me that the characteristics of the silver-mica vary widely even for caps of the same batch, since the dielectric mica is a natural mineral, and is not produced under carefully controled conditions, like the man-made polymers (polystyrene, polyester, etc.). It is very likely that small impurities or variations in composition in the mica, even in the same specimen, will change its characteristics.
Why is the temperature coeficient called ve ?

 

[TheOne]

Why is the temperature coeficient called ve ?

It is just my shorthand way of saying positive (+ve) or negative (-ve)