Holy crap at the capacitance of some SMD ceramic caps

[Evil Lurker]

The other day I decided to yank off some some SMD monolithic ceramic capacitors just to see what all values I could come up with for a lead acid battery charger/desulfator circuit I'm working on. Figured I might try my hand at fabricating my own PCB using SMD components and the caps were to be used for bypassing and what not.

What I recovered blew my mind... I ain't ever seen anything like it. These high performance SMD ceramic caps nowadays in like the 0805 and 1206 packages come in an insane amount of capacitance for their size... like 10uf+. A few of them actually tested 20uf+!

Hell I didn't have a clue they went that high... I actually triple checked my meter against some electrolytics to make sure it was reading correctly.

I wonder how the compare to electrolytics?

[PeteS in CA]

Re: Holy crap at the capacitance of some SMD ceramic caps

EL, try an experiment. Hold your capacitance tester leads on the cap terminals while warming the cap with your finger. I'm pretty sure you'll see the capacitance reading change significantly. Or you could spray a cotton swab with freeze spray and touch the swab tip to the cap and see the same kind of change. High value monolythic ceramic capacitors are usually Z5U or Y5V temperature coefficient, which is horrible - losing 70%-80% of the nominal value at the cold and hot ends of the rated temperature range. Also, those types have horrible DC voltage coefficients, losing, IIRCC, >50% of nomminal value with an applied DC voltage near the rated voltage. But at room temp, 0 VDC, the capacitance numbers are getting up there, especially with caps rated for 25V or less.

[b700029]

Re: Holy crap at the capacitance of some SMD ceramic caps

The large capacitance comes from having many many layers of dielectric and plate, and the dielectric is very thin.

http://parts.digikey.com/1/parts/122...bj107ml-t.html

They will produce an impressive amount of photons when overvolted.

[PeteS in CA]

Re: Holy crap at the capacitance of some SMD ceramic caps

Some I work with produce photons even when the voltage rating is not exceeded. I've been teaching a younger tech I work with that large form factor SMT monolythic ceramic capacitors are evil. Experience has him believing me.