cap between vcc and gnd

[Spectacular Butter]

I have noticed some circuits where they place a 0.01uF capacitor between VCC and GND. What does that do?

 

[evandude]

smooths the power rails. the capacitor will absorb small transients/ripple present in the power supply output. the more capacitance you put there, the smoother the output. they're called decoupling capacitors... and they become VERY necessary in certain situations, like when using microcontrollers, where strange things happen when they see ripples.

 

[Oznog]

I have had circuits fail when this was excluded.
The type of cap is very important. Ceramic is best because the ESR (internal resistance) is very low.

 

[evandude]

if you want to be extra careful you can use 2 or more different caps in parallel. i often use one large value (10uF, 100uF, 1000uF, depends on the application) electrolytic for large transients/ripple, and a much smaller (.01uf or .1uF) ceramic for smaller ripple.

 

[Russlk]

In high speed CMOS circuits, the ICs pull transient current from the power rail. The inductance of the power rail can produce transient voltages that screw up adjacent circuits, so a small bypass at each IC is a good idea.

 

[samcheetah]

i agree with evandude. i also usually connect two capacitors. one is a large value electrolytic cap. but these can have rather large ESR so the transient response suffers. so one usually adds a smaller ceramic type to provide low output impedance to step changes in load. the bigger cap is for the stability and the smaller cap is to improve the transient response. in some cases just using a bigger electrolytic cap would be enough. furthermore, the smaller ceramic also helps suppress high frequency differential mode noise which will sail right through the regulator.

i hope that helps

 

[zachtheterrible]

yup. 100uf is good for stuff to do w/ audio. for radio circuits around 100 MHZ i always use a .001uf cap. im not sure if this makes any difference, but in a receiver that i made, the schematic called for 2 .001uf caps.

i'm sure that you've realized this butter, but WHY it works is because capacitors do not pass DC, only AC, so if the voltage supply ripples (AC), that ripple passes through the capacitor directly to ground.