Tal Bejerano - AMC wrote:
> Hi 2 All
>
> Look at the attached files, it's a schemes from 2 sites , look at the
> capacitors mounted on the negative rail.
> which one is correct or I miss something here? in which order should be the
> capacitors on the negative rail?

The capacitors are oriented correctly in both drawings. The positive end
of a polarized capacitor always goes toward the circuit point where the
voltage is more positive.

In your first drawing, ground is shown in the middle. V+ (drawn above
ground) is more positive than ground. V- is more negative than ground.
So the cap going from V+ to ground has the + end at V+, and the cap
going from ground to V- has the + end at ground. This voltage divider
will only work well when the current demand is low (the rule of thumb
about circuit impedance says that the input impedance of the load should
be at least 10K - 10 * the 1K resistors in the divider - which would
limit the current to about 1mA), though it will also behave if the
current drains from the V+ and V- terminals are equal or nearly so
(again giving you a divider with equal resistances on each leg).

In your second drawing, the grounds are shown near the regulator
components. The in and out terminals of a 7815 (fixed voltage positive
regulator) are more positive than ground, so those caps have the + ends
on the in and out terminals. The in and out terminals of a 7915 (fixed
voltage negative regulator) are less positive than ground, so the + ends
of the caps go to ground. This circuit can supply more current (1A each
from V+ and V-, though I might use larger diodes than 1N4001s if I were
going to approach that limit), and will remain well-regulated even if
the positive and negative current draws are unequal.

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