There has been a fair amount written recently about the use of
capacitors to "drop" the mains voltage down to values suitable
for transistor circuitry. I was recently asked to explain some
aspects of such circuitry and looking through my references, was
taken by the fact that one of the circuits described looked
remarkably familiar. Above are shown two circuits. Only the bare
essentials are shown, things like surge limiting resistors are
omitted......... Their similarities are self evident. Now, if we
wish to analyze Fig. 1. in terms of current available, it is
natural to think in terms of impedance. The reactance (
impedance ) of a capacitor is given by the formula ( 1 ) Xc =
1/2p fC where f is the frequency in Hz and C is the capacitance
in Farads. We can now apply Ohms Law to this and we find that,
in a circuit where there is capacitance only, ( 2 ) I = 2 p fCE
where I is the AC current and E is the RMS value of the AC
supply. As D1 is a half wave regulator, ( 3 ) Idc = p fCE. Let
us see what a bit of simp