Cap

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RustyH

Senior Member
Im all out of 100 uF Caps which I need for a voltage regulator circuit. Im regulating first down to 12v (used to power some bits and bobs) and then down to 5v for the PICAXE side of things.

I just had a quick look at regulator circuits on the internet, and the cap values vary greatly. I was wondering if it actually matter what value you put in there??

I only have 2.2, 33 and 220 uF left untill i get somemore
 
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jtcurneal

Senior Member
If the bits and bobs include parts that create surges of any kind when they start,
the higher capacitance ( 220 uF ) will provide more reserve and your voltage may not dip as much.

Joel
 
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RustyH

Senior Member
Oh ok, so the caps arnt there cause of a dirty input then, its more for anything the circuit drives

I will only be driving mainly LEDs, so I guess they wont cause any major great shakes?
 
T

TAMeyer

Member
This explanation helped me understand one of the roles that caps play.

Like a Water Tower

One way to visualize the action of a capacitor is to imagine it as a water tower hooked to a pipe. A water tower "stores" water pressure -- when the water system pumps produce more water than a town needs, the excess is stored in the water tower. Then, at times of high demand, the excess water flows out of the tower to keep the pressure up. A capacitor stores electrons in the same way and can then release them later.

Found at http://electronics.howstuffworks.com/capacitor1.htm
 
D

Dippy

Moderator
They are also there to prevent/reduce oscillation - read the regulator data sheet. In many cases the above values should cover most small, undemanding apps.
 
W

westaust55

Moderator
RustyH said:
Oh ok, so the caps arnt there cause of a dirty input then, its more for anything the circuit drives
QUOTE]

Smaller value ceramic capacitors such as 100 nF are used for decoupling purposes to help filter noise caused by the switching of transistors within the IC and other parts of electronic circuits.
Click to expand...
 
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westaust55

Moderator
RustyH said:
Oh ok, so the caps arnt there cause of a dirty input then, its more for anything the circuit drives
Click to expand...
Smaller value ceramic capacitors such as 100 nF are used for decoupling purposes to help filter noise caused by the switching of transistors within the IC and other parts of electronic circuits.
 
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B

BeanieBots

Moderator
I'd like to re-emphasise what Dippy has already mentioned but maybe didn't hammer home as this often comes up.
Many regulators REQUIRE a certain value of capacitance to prevent them from oscillating. However, some will oscillate if there is too much capacitance close to their output. Read the datasheet for the specific regulator you are using.

Switching LEDs on and off is not often a problem, however, many LED circuits such as seven segment displays use multiplex system which often involves driving the "on" segment at very high current for a short time. This type of circuit requires two types of capacitor. A large (~100uF) for reservoir action like that described by TAMeyer and a smaller one (~100nF) close to the driving chip a bit like your loft water tank.

Do a search on "decoupling" for even more details on this complex and often overlooked subject.
 
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