Other capacitors for crystal

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

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 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

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.

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

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

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

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.

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

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.

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.

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

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. :frowning:

@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

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...

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

Again, Thanks all,

Jan

I've recently seen super capacitors in the size of a "sub"capacitor, from Cooper Bussmann, the powerstor, up to 100F. I guess I really don't know what they do inside of those magic cylinders as a physics teacher. I'll have to find out. They can discharge very slowly to provide almost battery-like actions or very fast to fuel a flash light of something. Just amazing. Anyone used them in the past? How do you like them?

I have a few of them on a few scrap PCB I got from a surplus store. They are very short like mushrooms. ;D

The supercaps essentially use a carbon electrode with massive surface areas compared to their weight/volume. Earlier models use activated charcoal, and have a relatively high internal resistance. Later versions use carbon aerogels with significantly better conductivity. I imagine nanotubes are being hotly researched in the same space, but I don't think I've heard anyone claiming to be using them yet.

We have them in a few products for memory backup. Your cellphone probably has some variety that saves cpu start long enough to change batteries...

There power density compared to batteries is pretty (very) low, but they lack other problems that have been inherent in batteries.

Very interesting. My dad used to work on these nanotubes. They have quite a bit of surface area over volumn.