[sdiy] Capacitors on modular power buss?

[Simon Richardson]

My ha'pennys worth:

It is possible to calculate values depending on demand for current, 
ripple tolerance, etc. In practice I, and I suspect many others, use 
rule of thumb values - what boat builders call 'scantlings'.

At the power input to a circuit board, at some distance from the power 
supply, put in a reservoir capacitor. Its job is to [1] provide a local 
reservoir of power in case the circuit suddenly needs it (transient 
demand) and [2] to isolate the circuit from incoming ripple/noise. The 
value depends on how big the circuit is (or more precisely, how much 
current it needs):

small	1u..2u2
medium	10u..22u   (a vcf for example)
big		100u..220u
huge    1000u      (whole synth on a board, ASM, for example)

It will be electrolytic with a generous voltage rating.

This is not the same as the smoothing capacitor in a power supply which 
is going to be larger. According to my textbook:

demand        smoothing cap

125mA         470u
250mA         1000u
500mA         2200u
1A            4700u
etc

Electrolytics don't function at high frequency, so it is common 
practice to put a ceramic in parallel with them, often 100n. Despite a 
bad press, ceramics have one outstanding virtue - they work across a 
very wide range of frequencies.

You will also find decoupling capacitors dotted around the board, 
typically near the supply to any chips. I almost always use 100n 
ceramics. They provide an even more local reservoir of power and stop 
one part of the circuit interfering with another.

If there is anything that is likely to suddenly draw a lot of current: 
a relay, a timer chip like the NE555, a big LED even, then it may be 
worth providing that with its own reservoir too.

Not very scientific but it works.

--------------------------------------
Simon Richardson