Capacitors with colored dots, values?

[cab42]

Recently I bought a grab bag of assortered components in the last remaining electronics  store in Copenhagen. A lot of junk but also usable stuff.

There was +100 ceramic capacitors marked with colored dots. Most of them with five dots, but some of with a colored top. I did some searches, but did not find anything conclusive on how to interpret the dots. My impression is that I should use the three top dots read from ledt to right, and those with acolored top should be read top, left, right

Some of them are here





Click to get larger image

From left to right:
Yellow, violet black: 47p
Blue, White Black: 69p
Red, red, red: 2200p 2,2n
Blue top, blue, White: 6,6p??
Violet top, White Black: 79p
The last one has a two colored top, violet and orange and the a Black dot on the right: 73p

Are the values correct?

[Rob Strand]

Never seen such caps in my life.

Some of the values are non-standard (not E12 or E24 series) so they are probably not correct.

I can't read the dots on your pics very well.   Not enough samples for me to guess the pattern.

Here's some older coding schemes, it might help
https://i.stack.imgur.com/wnENM.jpg
http://www.electronicstudio.net/ebay88spec89/capa-colorcode.jpg
http://seattleclouds.com/myapplications/greenman02/NewAp/capcity.png

(In some of these schemes, you might only has two dots for the value digits instead of three.)

Do you have a cap meter?
---------
Found these,
http://www.pcbheaven.com/wikipages/Reading_Part_Values/
http://audiokarma.org/forums/index.php?threads/mica-capacitor-domino-id-question.665790/

[thermionix]

Many of the old rectangular box silver mica caps have colored dots.  I just use a cap meter, a cheap BK Precision that works great.

[cab42]

Thanks Rob

I found diagrams similar to those in the pcbhaven link, but also some that read the values from bottom right.

My multicenter has a capacitance setting, but not for such small values.

Link to full size image in Gallery :

http://www.aronnelson.com/gallery/main.php/v/Cab42s-layouts/album137/72BC31F2-9772-44FD-B56B-580903B68D29.jpeg.html?g2_imageViewsIndex=1

I found this diskussion in another forum, but not much help.

www.dutchforce.com/~eforum/index.php?s=3247216f9725ef0a8499a5e1c0a4af85&showtopic=17840

[Rob Strand]

I found this diskussion in another forum, but not much help.

Well at least you aren't the only one to struggle with it.

Thanks for pic.  I still can't figure it out.  One problem is I'm not sure if the meaning of a dot at the top is the same as the top being dipped. 

I start off on the left with 47pF, then 680pF, then 2200pF=2.2nF (maybe), then I get to the dipped tops and I think hmmm, maybe I'm on the wrong track, or maybe they are coded differently.

One trick in measuring small values is to put many parts of the same values in parallel, measure them, then divide the measurement by the number of parts in parallel.     If you still can see 1nF with 5 parts in parallel then at least you know they are less than 200pF - it does give you some information.

I'd start with the third cap, then the second then try the end two.  The fourth one is weird.   

Another way is to build an RC oscillator with say an NE555 and measure the frequency.  The easiest way is to put in an unknown cap, measure the frequency, then by trial and error put in known caps to get a frequency close to the unknown value.   

It is also possible find the values using calculations but you have to work out the stray capacitance  in the circuit first.   eg.   put in a known 100pF cap , measure the frequency, calculate the expected frequency for a 100pF cap.  You will find the measurement is lower then you estimate what extra capacitance the circuit had, which might 30pF or so. 

It is often better to leave a cap in the circuit (maybe an extra 33pf or 100pF) so the oscillator frequency has a maximum value.  Using this scheme I have been able to detect 0.1pF or so.

[amptramp]

It pays to have some measurement instruments for capacitors because some colour codes are confusing, ambiguous or misleading.  I recently sold my classic EICO 950B bridge and picked up an AideTek DM4070 direct-reading LCR meter which will see use along with my Leader LCR-740, Heathkit IB-2A, a Heathkit IB-5281 and a purpose-built combination meter that included a variac, R-C bridge, multimeter and resistor sub box.  Yeah, I have a test equipment hoarding problem as my 12 oscilloscopes will confirm.  There have been enough different colour dot codes that I would be inclined to confirm the values.

[Rob Strand]

Yeah, I have a test equipment hoarding problem as my 12 oscilloscopes will confirm

Make a 3x4 grid like those multi-screen arrays.  It would be really annoying to adjust the position.

[cab42]

It seems, I have to measure the  things.

Thanks for the measuring advicess. A lot to get started with.

Yeah, I have a test equipment hoarding problem as my 12 oscilloscopes will confirm

Make a 3x4 grid like those multi-screen arrays.  It would be really annoying to adjust the position.

Well, my wife thinks I have hoarding problem as well, and I only have one. I found these two on the trash a few years ago, a tektronics 340 digital scope and a Leader function generator



I found a video on how to measure capacitance with those as well.

[Joe]

I wish I could find stuff like that in the trash = )

[cab42]

I wish I could find stuff like that in the trash = )

If you keep on looking, once in a while you will find something Interesting. But I must admit, this was one of those once-in-a-lifetime scores.

Carsten