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Topic: Other capacitors for crystal (Read 2 times) previous topic - next topic

JanD

Hello,

I want to build an sanguino, but I just realized I (but I'm not shure) don't have any 22pF capacitors. I have some with the codes:
330 (33pF?)
180 (18pF?)
22 (200pF?)

As you may see I'm not 100% sure if i have the right codes. But if I have the right codes these are the capacitors I have witch are nearest 22pF.

Jan

cr0sh

For most crystals out there, anything between 18pf and 33pf will get them to resonate (generally, the closer to the crystal's design spec, if you know it, the better, though).
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

JanD

i don't have a datasheet, but the crystals product name is "Crystal 16.000 MHz HC49/S 30pF"

what sounds like I need a 30pF capacitor, so the 33pF (330) would be the best choice? Or do I get anything wrong?

Jan

florinc

#3
Dec 29, 2010, 08:09 pm Last Edit: Dec 29, 2010, 08:10 pm by florinc Reason: 1
You may get it to resonate even without capacitors (I tried it many times and it worked). I am not saying this is a good practice; it may give you some play time until you get the right capacitors.

liudr

JanD, just feeling my math today.
Take one 33 and 3 200, put them all in series, the result is 22.1pF.  ;)
Just in case you want to be kind of exact.

-||---||---||---||--
33 200 200 200

JanD

But would my 33pF (330) capacitors work????

liudr: But thanks, what could be a other way, but 8 capacitors insteed of 2 are a little to much.

Jan

cr0sh

Quote
But would my 33pF (330) capacitors work????


Are you sure its 33pF? I would measure it first, but if it is - try it out; the worst thing that will happen is it won't work.

Like I said before - anything between 18pF and 33pF should work - try them all until you find one that is closest to the "spec" you have that works, then leave it at that. For hobby-level use, you won't care. If this were a project where things needed to be as accurate as possible, you would probably want to make it "exact" to the value needed, but for general purpose usage, it likely won't matter.
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

JanD

The project is pretty time sensitive and I need pretty accurate readings from millis(), but if it my crystal is named "Crystal 16.000 MHz HC49/S 30pF" and I use a 33pF (measured to 0.324nF) capacitor I think I don't lay to wrong. Or do I think wrong?

Jan

westfw

I would use your 18pF caps, as that's closer to the 22pF "standard."  There are other factors and math in between the 30pF rating of the crystal and the actual value of the caps to use; there are various app notes out there (IIRC, it's like the series capacitance of your caps, plus internal capacitance of the chip, plus stray board capacitance, should equal the crystal rating.)

0.324nF is closer to 300 pF, BTW.
a cap labeled "22" is almost certainly NOT 200pF; I've never seen the "last digit is power of 10" thing used with fewer than 3 digits/colors.

cr0sh

Quote
Or do I think wrong?


How about simply trying it out? Write some test-case software to determine what, if any, error exists compared to a standard Arduino. If it is too much, then work on getting the proper parts. If it seems OK, then all's good.

Arguing and worrying about an unknown like this (as compared to something that might make things let out the magic smoke) does your project no good; you'll advance much quicker by just trying to make it work, and doing some testing to verify results.

I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

JanD

Quote

0.324nF is closer to 300 pF, BTW.
a cap labeled "22" is almost certainly NOT 200pF; I've never seen the "last digit is power of 10" thing used with fewer than 3 digits/colors.


I just measured the capacitors labeled "22" and the value was 0.023nF = 23pF (I looked up this now). So no problem, I have two capacitors with 22pF.

Thanks for all help everyone!

Jan

westfw

Sigh.  In the old days, you could sorta tell the multiplier of a cap just by looking at it.  Under 1nF you had disks and mica caps of various sizes.  Between 1nF and 1uF, you started to see film caps.  Above 1uF you had electrolytics.  Above 500uF, they got too large to mount on boards.  Above 10kuF they started to be major structural components.  And you didn't ever see a 1F or high cap, just to keep the physics teachers truthful...   Nowdays there is so much magic in electrode and dielectric materials that you pretty much need to read the markings.   Which they now leave off.  :-(

Udo Klein

#12
Dec 30, 2010, 04:17 pm Last Edit: Dec 30, 2010, 04:18 pm by udoklein Reason: 1
@jan: if a crystal requires a load of 30pF this does NOT imply that you need 30pF caps.

You may want to have a look at this thread: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1270573737. Especially you may want to follow the links found in this thread.

BTW: I figured that the board layout may add so much parasitic capacitance that you do not require any caps at all. At least most of the time the caps can be significantly smaller than the theoretical values.

Udo
Check out my experiments http://blog.blinkenlight.net

cr0sh

Quote
Sigh.  In the old days, you could sorta tell the multiplier of a cap just by looking at it.  Under 1nF you had disks and mica caps of various sizes.  Between 1nF and 1uF, you started to see film caps.  Above 1uF you had electrolytics.  Above 500uF, they got too large to mount on boards.  Above 10kuF they started to be major structural components.  And you didn't ever see a 1F or high cap, just to keep the physics teachers truthful...   Nowdays there is so much magic in electrode and dielectric materials that you pretty much need to read the markings.   Which they now leave off


Ohsomuch THIS!

;D

/thinking about investing in a cheap LRC meter...
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

JanD

Udo: I looked into the thread, i bookmarked a lot of the pages linked to from there for future projects.

Again, Thanks all,

Jan

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