100n Capacitors

[donrecardo]

Hi
I know I must sprinkle liberal quantities of 100n caps around my picaxe chips
but what kind?

I never know what difference it makes if any, as to whether I use polyester, polypropylene, polystyrene , or even ceramic

Does it matter as long as its 100n ?

The reason I ask is like many people today I am taking advantage of the Rapid sale and they have 100n caps at 5.2p for polyester but only 3p for polypropylene . If polyprop will do I cant see the point in buying polyester which give me only half as many for the same price .

OK so I am a cheapskate . but now I am retired all the pennies count .

So why do we use ceramic or polyester or polyprop etc and when , if ever, will it be ok to use what ever is at hand ?

Don

 

[eclectic]

Don.
I'll leave the technical answer to the experts, but

as far as I can see, ALL the project boards are fitted with
the "box" polyester capacitors.
(Blue or white, label 1K63 ).

The 100nF onsale in the RevEd shop is also a poly-box.

e

 

[Dippy]

I too will leave it to the experts and Google cut'n'pasters.

Hey, you forgot polyethylene naphtalate types

Bottom line is horses for courses.

Different dielectrics have different properties.
This affects size (related to voltage rating), tolerance, stability, losses and ESR.
Oh, and frqueceny characterstics.
Oh,oh, and cost of course.

The cheap ones you refer to are perfectly good for general purpose decoupling in non-arduous applications and non-precision RC circuits - which applies to most of the decoupling applications most of us use.

Polystyrene are popular in tuned circuits mainly because they (usually) have lower tolerances and are more suited to that app.

Ceramics (and there are sub-sections of dielectric within 'ceramic') are generally for decoupling where you might have some nasty HF spikes kicking around.
For example in MOSFET driver ICs it is recommended to have a healthy ceramic cap right next to the chip. Low ESR and god HF performance benefits the switching speed of chips like that.
There are many similar apps where ceramic are recommended.

So, consider polyprop/polyester as good general purpose low cost.
Also cheap hiogh K ceramics come into that category.
Low K and fancy dielectric for more arduous apps and timing apps.
And, generally, polystyrene for timing apps.

But there are hundreds more variables to be considered in special apps especially where high frequencies are kicking about.

If you REALLY want to know the nitty gritty then have a nerdaround or just wait for someone to do it and post a link or massive cut'n'paste.
But note, the long answer will be long....

Bottom line; for average general purpose stuff those ones will be just fine.
But, to save postage later, get a selection of high and low K ceramics. You don't need COG unless you're doing something a bit special. Or are you.... (spooky music).

 

[gengis]

Typically you want a low impedence cap like a ceramic disc or monolythic ceramic. The obvious place is between the power and ground of the chip as close to the chip as possible. Avoid capacitors that use wound type of construction - layered with connections on the ends works better for transients.

Learn some theory though . . . good layout and good design practices can eliminate problems that a ton of corrective measures won't touch. All conductors have some resistance and inductance - small as it is, it will cause voltage drops when currents are switched - that causes problems.

Connect the high current runs as close to the power source as possible and provide seperate wire for the low current and transient sensitive parts of the circuit. Elimination of problems is best done at the source; it is usually much more vexing to try and filter something out after it is already affecting the sensitive circuits.

 

[hippy]

I have no idea of type; they are often the only 100nF capacitors available from suppliers who do not stock a wide range of types, typically yellow, sometimes blue or red, plastic rectangular 'bricks', legs on 0.2" pitch, in my experience and looking in my component drawer.

 

[donrecardo]

I have no idea of type; they are often the only 100nF capacitors available from suppliers who do not stock a wide range of types, typically yellow, sometimes blue or red, plastic rectangular 'bricks', legs on 0.2" pitch, in my experience and looking in my component drawer.

I went with the concensus of opinion which was polyester layer, as it happens they also turn out to be red, plastic rectangular 'bricks', legs on 0.2" pitch as Hippy quoted .

I already had ceramic disk and ceramic SMD , but wanted to get some other usefull stuff while the Rapid sale was on

Thanks every one for the advice

Don

 

[fernando_g]

Many moons ago, I had the ooportunity to perform a characterization of capacitor decoupling performance, utilizing a very expensive Hewlett Packard Network analyzer.

To summarize some tedious data:

1) The very best are multilayer SMT ceramic caps. Good all the way to ~20 Mhz at high ripple currents.

2) Followed by thru-hole ceramic caps.
In both these instances the X7R ceramic is more stable than the X5R or Y5U, but either will work in non critical apps. And in general, avoid ceramics for timing and filter apps where the corner frequency is important.

3) For PolyFilm caps, to reiterate what other poster has already noted: Avoid wound types, use stacked metal film construction instead. For decoupling purposes, the film type is usually not a major contributing factor. However for timing or filtering, it is.

For non-critical applications, your purchase was a good one.

 

[donrecardo]

Thanks for the reasurance Fernando

Don