[sdiy] Capacitors on modular power buss?

[harrybissell]

Capacitors on modular power buss?

(or... too many cooks spoil the broth :^)

You can add capacitors to the power distribution bus.   Large capacitors may actually cause
you trouble unless your power supply is up to the task.

When a power supply first starts, all the capacitance in the system is discharged... ie all the
capacitors 'look' like a short circuit.  The initial current drain can be so great it can stop the
supply from turning on...

(as I know... my latest supply has a special 'soft-start' circuit of a relay, resistors, and a timer to
charge the system capacitance slowly and avoid this problem :^)

Any number of small caps can be used... usually caps like .1uF are enough to stop transients from
propogating down the bus.   It also helps to space the caps apart... maybe a set of caps near every
power connector.

Think... the power bus has parasitic inductance, and resistance.   You wish the bus was zero ohms
and that there is no inductance.   Since it really has resistance annd inductance... spreading the
caps across it allows them to function as local power reservoirs (decoupling) and the entire bus as
a lowpass filter.

A couple of larger electrolytics probably will not hurt, just don't over-do it.   You can have too much of a good thing.

H^) harry

Julian wrote:

> Hi there,
>
> I asked this before, but it was in the midst of the list stutter malaki, so that could have been the reason why i still dont know...
>
> After Harry's detailed explaniation of earthing, i was wondering as to capacitance of the supply rails.  In particular ive been
> lookign at the MOTM boards, and see large caps there...
>
> So, im wondering as to the type and size of caps that i should be using on my +/- 12v system, and, i have to ask to, not being that
> smart, exactly where i should install these caps?
>
> Thanks,
>
> Julian
>
> (in quest of nice smooth running modular)