Capacitor Value Again : This time I've gone crazy....

[lord loh.]

Preface : I have asked this question on the forum twice and have read Tony van Roon's tutorial about reading capacitor values. I have been authoratatively told that if a capacitor has values like 1.5, 4.7,2.2 etc. it should be assumed as μF.

Now : I have got a 555 IC in astable mode and the on time on triggering is given by

1.1xRXC

When I try a combination of a 220μF and 150kΩ I get the perfect timing of about 36s. SO my circuit is perfect.

I then replace the 220μF capacitor with the one marked 4.7 and expext a .7s on pulse(There is an LED at pin 3). However what I get is not distinguishable by observation. A .7s pulse is largeenough to be observed by eyes :!:

Then I use a 1μF and 220kΩ comination and get noticable ¼ second pulse.

So obviously I am confused about reading capacitor values. :cry: :cry: :cry: :cry: :cry: :cry:

The various capacitors I have are marked 2.2, 4.7, 3.3 and 1.5. These do not behave as if their values were in μF.

Please Help. :cry: :cry: :cry: :cry: :cry: :cry: :cry:

 

[Nigel Goodwin]

Just giving us random numbers isn't of ANY use, post pictures of the capacitors so we can see what they look like.

You should also be aware of the extremely wide tolerance range of electrolytics, they can be no where near there marked values. Using large electrolytics in 555 timer circuits is also a big problem, as the leakage of the capacitor completely messes the timing up.

 

[lord loh.]

I tried to get pictures, but the capacitors were un resolvable or out of focus... Just couldn't get the details...

I am not using electrolytic capacitors at all... All the capacitors I mentioned are dish/disk type. Presumabley ceramic.

 

[Nigel Goodwin]

I tried to get pictures, but the capacitors were un resolvable or out of focus... Just couldn't get the details...

I am not using electrolytic capacitors at all... All the capacitors I mentioned are dish/disk type. Presumabley ceramic.

In that case they are most probably NOT 4.7uF, if they are small disk types they most probably will be pF - a million times smaller.

Don't you have a macro mode on your camera?, or do you have a flat bed scanner?.

 

[akg]

.. All the capacitors I mentioned are dish/disk type. Presumabley ceramic.

if they are of disk type they are not in the uF range .. most probably they are on nf/pf range. what excatly the text in the cap ?

 

[lord loh.]

The exact text on the capacitor is "1.5", "2.2", "4.7", "3.3" some have 104(which means 100000pF I get that.)

All 1.5 marked caps have a plack dot on the top of it.

2.2 a line under it and AEC

.01 a line under it and AEC

4.7 NO line and AEC

3.3 a linr under it and AEC

33 and a line under it.

5.8 a line under it and AEC

82 and a line under it and AEC

27 a line under it and a black mark on the capacitors

How do I interprete these?

 

[windozeuser]

Please correct me if I'm wrong.. But Standard convention uses whole numbers for pF, and fractional decimals for uF.

2.2 = pF

0.22 = uF

Anything 1 uF or over will usually have the value printed on the capacitor itself; along with the "uF"

 

[mstechca]

Now : I have got a 555 IC in astable mode and the on time on triggering is given by

1.1xRXC

When I try a combination of a 220μF and 150kΩ I get the perfect timing of about 36s. SO my circuit is perfect.

I then replace the 220μF capacitor with the one marked 4.7 and expext a .7s on pulse(There is an LED at pin 3). However what I get is not distinguishable by observation. A .7s pulse is largeenough to be observed by eyes :!:

Then I use a 1μF and 220kΩ comination and get noticable ¼ second pulse.

Capacitor size can give you a hint, but look at the letters after the number, because they can help determine the value.

I'm going to make the assumption that the values you have used so far are in uF, because when you used a 4.7(uF), you couldn't distinguish the speed. If you have followed the equation, you should be able to tell that the 4.7 is either in uF or a lower unit (pF, nF, etc).

 

[lord loh.]

Okay, if windozeuser is correct, i guess it might me 4.7pF and not 4.7μF

I was under the impression that if there is a point, the value is in μF. I did not know that it was μF only if the value printed on the capacitor was less than 1 ex :.01 etc...

 

[Nigel Goodwin]

Okay, if windozeuser is correct, i guess it might me 4.7pF and not 4.7μF

I was under the impression that if there is a point, the value is in μF. I did not know that it was μF only if the value printed on the capacitor was less than 1 ex :.01 etc...

There's no 'rule' about it, you just need to apply common sense, a 4.7uF is electrically 1,000,000 times larger than a 4.7pF - a 4.7uF is almost certainly going to be an electrolytic, once you get over 1uF most capacitors are (or they are rather large). You should be able to tell just from a glance at a capacitor if it's pF or uF, it's normally pretty obvious.

 

[lord loh.]

I knew that the μF is a vary large unit and capacitors are large. But all the repeated misconceptions made me believe that there possibly might exist capacitors of μF range in ceramic packages(he pF type package).

Looks like I found the missing piece of the jigsaw puzzle. :idea:

 

[mstechca]

I knew that the μF is a vary large unit and capacitors are large. But all the repeated misconceptions made me believe that there possibly might exist capacitors of μF range in ceramic packages(he pF type package).

I have not seen capacitors in the uF range in ceramic packages.

 

[DigiTan]

The closest I've seen were some quarter-sized ceramics from an old rotary phone. Are there are a good web guides on how on the capacitor conventions (maybe including tolerance and other parameters)?

 

[Dean Huster]

Whether or not you can say that a capacitor can be in the "microfarad" range depends upon your country's convention for cap values. In the U.S., we idiots refuse to use nanofarads and millifarads, stubbornly sticking with picofarads and microfarads only. So, our values for ceramic caps can range from 0.001µF (1000pF or 1nF) to around 0.68µF (680,000pF or 680nF). So, a lot of the answer lies in your perspective.

As Nigel says, it should be obvious to the most casual observer whether a cap is microfarads or picofarads. But folks new to electronics are hardly "casual observers" since they're still on the flat slope of the learning curve.

Aluminum electrolytic caps are usually characterized by an aluminum can with a crimped circumference around one end, whether it has axial or radial leads. They tend to have values of 1µF or greater, although I have seen a few in rare instances as small as 0.22µF (220nF). The sky is the limit for an aluminum electrolytic's value, although they tend to ease into the millifarad catagory as their value equals or exceeds 1000µF, at least in technologically progressive countries. As timing components for 555 timers, they aren't the best for precision work as they tend to have tolerances of +80%/-20% and the larger ones have lots of leakage resistance which really screws up the timing equations.

Tantalum electrolytics tend to be "conformal coated" or "dipped", looking more like blobs. They tend to have values ranging from just under 1µF to as much as 470µF, although it isn't common to find them much over 47µF these days. They originally had their values marked on them as a color code and tantalums were some of the prettiest components in electronics. But these days, they're marked just as any other cap is. Although they have better tolerances than their aluminum brothers and a lot lower leakage (higher leakage resistance), they don't have the best temperature coefficient in the world.

Polyester (Mylar) caps tend to have values ranging from around 0.001µF (1nF) to 0.47µF (470nF). They are non-polarized and great for most audio work but usually horrible for RF work because of their wound construction. For your 555 timer work, they are usually the best choice for they have a low temperature coefficient and have good tolerances, usually available in 10% and 5% tolerances but sometimes found with 2% and 1% tolerances.

Ceramic capacitors range in value from around 1pF to as high as 0.47µF (470nF) although values much over 0.1µF aren't very common. They shine as RF bypass capacitors, but suck as timing components because they have some of the most awful temperature coefficients around. Of course, that could put them into a use in a temperature-to-frequency converter! Ceramic capacitors in low pF values are available with a nearly zero temperature coefficient for frequency-sensitive/temperature-sensitive work. They're called NP0 (that's NP-zero, not NP-oh) ceramics and will usually have the edge opposite the leads marked with a black sploch.

Dean

 

[lord loh.]

As Nigel says, it should be obvious to the most casual observer whether a cap is microfarads or picofarads. But folks new to electronics are hardly "casual observers" since they're still on the flat slope of the learning curve.

I have been with electronics for about 1 had a half year now. I am not exactly new... :lol:

But reading capacitors was one thing I never learnt.

The trouble was that several people told me several different things. And there was not Least Common Explaination... So everything confused me. I was aware that 1µF was mostly electolytic but all doubious explainations (from books and teachers) made me believe that I might be wrong.

 

[Dean Huster]

Time isn't always a factor in learning practical electronics. I've seen electrical engineers who've been practicing in the field for years who can't read color codes or component values, simpley because they do all their design on paper and never actually handle components. 'Tis a shame.

1µF is one of those values where you can find caps in both electrolytic and non-electrolytic/non-polarized versions. You can get polyester caps in values as high as 10µF. They're huge components, but sometimes necessary components that have no other alternative. Some filters and speaker crossover networks (a specialized filter, of course) cannot use electrolytic caps, yet because of the low frequencies involved, require large, non-polarized caps.

Dean

 

[mstechca]

The best way to confirm a capacitor's identity is to go to the store, buy it, and then if it isn't labelled properly, label it.

Most electrolytics are labelled the best. they even say xxx uF right on the cylinder.

as for the small ceramics, they tend to be more fussy. You have to unscramble the code on them.

Tony Antoon has a great webpage on capacitors.

go to: **broken link removed**

 

[lord loh.]

I've seen electrical engineers who've been practicing in the field for years who can't read color codes or component values, simpley because they do all their design on paper and never actually handle components. 'Tis a shame.

I agree. In India, 'dignity of labour' does not exist. White collar jobs of sitting at a office table with just paper is what people want.

I have been told by several people to stop wasting my time making PCBs and outsource my project by just doing the schematic design on paper. I do not agree to do such things. An error while making a PCB teaches me a lot more than paper work and books.

**broken link removed** - I have read this.

 

[RCinFLA]

There are many labeling conventions for caps. Some large suppliers create their own convention.

In cellphones, almost all electrolytics, tantalum capacitors have been eliminated and replaced with low cost ceramic. Highest value, very high K dielectric, ceramic cap is presently 22 uF but their capacitance is highly a function of applied D.C. bias. At about 90% rated voltage their capacitance can drop to half their marked value.