[R-390] Cap Confessions

[Charles Steinmetz]

Tisha wrote:

>The risk you run is that it is a choke fed supply so on power-up those
>capacitors appear like a dead short on the B+.

The reason the SP-600 draws high(er) current at turn-on is because it 
is NOT a choke-input supply.  It is a capacitor-input filter (C161A 
is attached directly to the rectifiers).

Power supply filter capacitors always appear as a low impedance until 
they are charged, typically within a few cycles of the AC mains 
supply after it is applied -- it doesn't matter whether the filter is 
capacitor input or choke input.  In a choke-input supply, the choke 
limits the inrush current somewhat, both because of its inductance 
and also because of its resistance -- 170 ohms, in the case of L51 in 
the SP-600 (note that if the SP-600 had been designed with a 
choke-input filter, L51 would almost certainly have been larger than 
8.5H, as well).

Sometimes a choke-input supply SOUNDS like it is drawing a greater 
surge current, because there are two magnetic things to go "thunk" 
instead of just one (the power transformer and the choke).  But the 
actual surge current is less than it would have been if the first 
capacitor was moved upstream of the first choke.

Best regards,

Charles