Author Topic: Which capacitors to use?  (Read 488 times)

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bhsmith

• Jr. Member
• Posts: 29
Which capacitors to use?
« on: November 02, 2010, 06:37:57 PM »
I'm going to be building something like this:

Except the screen will probably be different and the power source may be different.  What I'm trying to figure out is, how do you figure out which capacitors to use. Not necessarily the type but the capacitance(1uf,.1uf,1pf....).  Thanks!

waltr

• Supreme Robot
• Posts: 1,944
Re: Which capacitors to use?
« Reply #1 on: November 02, 2010, 07:18:05 PM »
The data sheets for the parts used usually have recommended cap values. This is a good starting point.

For example: PICs recommend a 0.1uF cap across Vdd and Vss.
The PIC DS also has recommendations for the caps on the XTAL.
The Voltage regulator DS usually specs the minimum to maximum input and output cap value.

bhsmith

• Jr. Member
• Posts: 29
Re: Which capacitors to use?
« Reply #2 on: November 02, 2010, 10:21:00 PM »
I know some hardware have the recommended caps to use but not all the time.  Is there a mathematical formula to figure out what cap to use?  But I will look at the datasheets to see what I'll need.

waltr

• Supreme Robot
• Posts: 1,944
Re: Which capacitors to use?
« Reply #3 on: November 03, 2010, 07:58:26 AM »
Much of cap selection is by 'rule of thumb' and learned by experience. This is mainly for the by-pass caps that are placed across the plus and minus pins of ICs.

Caps are used for difference reasons in circuits so one needs to understand why a cap is in that part of the circuit. There are equations when a cap is combined with other passive components such as resistors and inductors. Look up RC, LC and RCL in Wiki for equations.

Circuit simulation tools (programs) are also used to determine cap values. LTSpice (free from Linear Tech) works well and is nice for visualizing the AC response of a signal with an RC, LC or RLC components.

Good question but there isn't a quick easy answer.