Quick question about capacitors

pfroment

New Member
I have some 330 uf and 220 uf caps i want to use as decoupling caps. One of the legs on the cap is longer than the other.
Does this mean that one leg is positive and the other negative? Or does it not matter?
 

pxgator

Senior Member
The longer leg is usually positive. If there is any markings on the caps
the negative side should have a stripe. I'm assuming that these are radial
lead style capacitors..??
 

rq3

Senior Member
I have some 330 uf and 220 uf caps i want to use as decoupling caps. One of the legs on the cap is longer than the other.
Does this mean that one leg is positive and the other negative? Or does it not matter?
Indeed, it does matter. Devices with the capacity you have are almost completely certainly electrolytic (notice the wiggle words, I haven't seen everything...yet). If there is a stripe on the side of a can type cap, that usually is aligned with the negative terminal. The positive terminal will also (usually) be longer. The negative terminal is also (usually) connected electrically to the outside of the capacitor "can", if it is a can type.

For surface mount devices, all bets are off, although MOST tantalum surface mount devices have a stripe at the POSITIVE end, just to confuse the issue.

I use a large quantity of surface mount LEDs, in three different colors, but the same package (0603). Red and yellow are marked ->+ (bass ackwards), while the green is +>- (normal). When I pointed this out to the manufacturer(s), they said pay attention to the data sheet.

Pay attention to the data sheet. Pay close attention to the data sheet.
 

Goeytex

Senior Member
330uf and and 220uf would almost surely be aluminum electrolytic and should be marked with either a black stripe or minus signs on the negative terminal, Getting these backwards can cause a loud bang with some sparks included.

But these are rather large for most decoupling applications. What are you wanting to decouple? Bigger is not necessarily better when it comes to capacitors.
 

erco

Senior Member
For decoupling, a 10 uF electrolytic is usually plenty, plus a 104 disk cap (0.1 uf).
 

inglewoodpete

Senior Member
.....Does this mean that one leg is positive and the other negative? Or does it not matter?
Yes, it does matter. An electrolytic capacitor connected the wrong way or if it is connected to a voltage higher than its rating will tell you this fact. Often quite dramatically. Do you remember back to scouts, when an unopened can of baked beans is placed in a camp fire? Very similar results but an exploded electrolytic capacitor smells a lot worse and makes a mess or your project!
 

westaust55

Moderator
For decoupling purposes the normal case is to use a ceramic type non polarized capacity of a round 100 nF.
Decoupling capacitors are used to filter higher f squelch components (aka noise) from the power supply lines at the IC terminals. The electrical noise arises due to the high frequ NYC switching of signal states within the Ics.

Electrolytics and Tantalum capacitor are more typically used for bulk energy storage. These provide an energy reserve to help stop the supply voltage dipping under momentary higher currents.
 

westaust55

Moderator
330uf and and 220uf would almost surely be aluminum electrolytic and should be marked with either a black stripe or minus signs on the negative terminal.
Taltalum capacitor are also available up to these uF values but are rather pricey so not used unless space is at a real premium.
 

Goeytex

Senior Member
Taltalum capacitor are also available up to these uF values but are rather pricey so not used unless space is at a real premium.
Of course they are as well as huge oil filled HV caps the size of a car. I doubt the OP has either of these types.
 
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