Polyester capacitor question.. How to verify which end is + or - ?

Other Hardware General Electronics
1

I just picked up my Poly 0.1UF capacitor from Radio Shack for my LM386 audio amp project.
Both ends are the same length. There's numbers printed on one side, but it's not marked plus or minus anywhere.

HELP! ?

Thanks in advance. I've gotten nothing but great support here.

2

but it's not marked plus or minus anywhere.

Doesn't have to be, they are not polarised capacitors.

3

polyester is one of the plastic materials used for plastic film capacitors - only electrolytic
capacitors (aluminium, tantalum or niobium) are polarized I believe.

4

Actually for a film capacitor you may get a slightly reduced noise pickup when you connect the outside foil to lower impedance side or ground. If you think that matters for your audio application then there is a tutorial how to figure out which way to install it - Are Your Capacitors Installed Backwards? Build this and find out - YouTube

5

Yes but this is an Arduino forum, there is little likelihood that it is some Hi-Fi foo question.

6

kallek:
Actually for a film capacitor you may get a slightly reduced noise pickup when you connect the outside foil to lower impedance side or ground. If you think that matters for your audio application then there is a tutorial how to figure out which way to install it - https://www.youtube.com/watch?v=BnR_DLd1PDI

Sounds very unlkely to me, a couple of picofarads is not something audio signals notice
unless there's very significant interference (HT leads?) - and you would normally screen
a sensitive audio circuit anyway to reduce the more plausible RF interference.

At RF frequencies a few pF does become significant, but then you normally have to start using
screening cans anyway...

7

kallek:
Actually for a film capacitor you may get a slightly reduced noise pickup when you connect the outside foil to lower impedance side or ground. If you think that matters for your audio application then there is a tutorial how to figure out which way to install it

Disturbing to know that there are "bodgie" labelled capacitors floating about. The band - which has a specific meaning - is only put there for decoration. :astonished:

MarkT:
Sounds very unlkely to me, a couple of picofarads is not something audio signals notice unless there's very significant interference (HT leads?) - and you would normally screen a sensitive audio circuit anyway to reduce the more plausible RF interference.

Ah, but he is talking about thermionic valve ("tube" for the Yanks) equipment. He is a valve enthusiast. We are talking high impedance like CMOS and Arduinos. And his version of "RF" means just up to a couple of Megahertz. :grinning:

Also, his version of digital logic comes straight from Don Lancaster's Cookbook. :grinning:

If I was going to do it, I would lose the CRO and simply use a microprocessor to implement a balance bridge to determine to which side the finger capacitance coupled, and indicate that directly on the LEDs.

Like the Ardutester. Hold the PCB (ground), pick up the capacitor in the other hand, insert it in the two sockets and see the result.

8

Ah, I'd forgotten people still use thermionic devices for audio - how quaint (although this may be the
only sensible way to drive electrostatic speakers I suppose).

This century we use digital signal paths and class D amplifiers, so the only plastic film caps are in the
microphone pre-amp circuit before the sigma-delta converter.