Trouble analysing this power supply ckt..

Thread Starter


Joined Feb 27, 2016
Power Supply ckt.PNGThe capacitor is a 33 micro farad/400v
Zener diodes 2 & 3 are used in forward biased mode according to me.
If anyone can relate this to something helpful...please revert back...


Joined Nov 30, 2010
It's true. The zeners are forward biased.
This is a current limited power supply with the 33 uf capacitor being the dominant limit at about 100 ohms of reactance.
Another hundred ohms at R14 and That allows about 1/8 amp total. about a milliamp through the forward zeners and that makes the transistor conduct about 6ma.
It doesn't make a lot of sense without knowing what it is supposed to serve for its load at PL2.


Joined Jun 22, 2012
It's a transformerless psu with a small transformer put on the mov, the zeners are drawn backwards and will drop 1.4v, it maybe a constant current circuit, or you have drawn it wrong.


Joined Sep 9, 2010
It's not constant current, but the 24V zener and the transistor will perform some filtering of the voltage peaks. The 24V is a sort of peak clipper to absorb any voltage >24V. The transistor seems to be set up as a varying dummy load. These two loads combined with the high input reactance give some filtering - peak dampening - without a capacitor. It's a silly design, though. R4 is redundant to R2, the zeners are backwards and don't need to be zeners.


Joined Aug 1, 2013
The red node is clipped pulsating DC, steady at 24 V for most of the AC half-cycle, then dipping down to 0 V (WRT D1 anode) for each AC zero crossing. At anything above 1.4 V the transistor b1 kicks in as a constant current load of 7 mA, assuming Vbe = Vf.