# DC Voltage Source - circuit problem

I'm currently trying to build a DC voltage source from an AC source to a school project. The circuit worked fine when it had no voltage regulator, as it was just a transformer + full-wave rectifier + capacitor filter; the output was a DC voltage varying from 23V to 21V.

The desired output is 15V. So, the goal is to use a regulator (which is actually a 10V-Zener in this case) and a controller built using a comparator amplifier to keep the output voltage as closest to 15V as possible; there's also a NPN BJT in amplifier's output that's there to supply the needed current to the load - it's written 'Carga' on the load, which means Load in portuguese.

So, the final circuit is the following:

simulate this circuit – Schematic created using CircuitLab

The problem is that when I run the simulation and check the load's voltage, it's 15V for an interval of time and then it suddenly goes to 23V, which is the not-regulated-circuit output voltage; then it goes back and do it all over again. I guess the transistor is actually going into the saturation region of operation, and I really don't know why. The result of the simulation is in the image below; the red lines are the output voltage and the blue lines are the amplifier output voltage.

If anyone needs I can upload the .asc file (LTSpice schematic), but I guess it's just a stupid mistake I'm doing.. anyone got any idea?

I really appreciate any help. Thank You.

edit: for anyone who needs a better schema of the regulator, which is my problem: https://i.sstatic.net/iDpwf.png. sorry for the bad drawing.

• I think it would help if you redraw the circuit from left to right, input to output; and from top to bottom, higher to lower voltages. Try to get rid of wires crossing. That exercise will give yourself (and us) a much better insight of how the circuit actually works. It also wouldn't hurt to put some labels at the most important nodes. electronics.stackexchange.com/questions/28251/… Commented Aug 13, 2014 at 20:39
• Apart from that, I frown upon how D2/Q1 are connected to each other. Commented Aug 13, 2014 at 20:42
• I do know the circuit is not well drawn, that's why I tried to explain it at the most. But thanks for the tips. Also, the left part is just a transformer, what's on the bottom is the rectifier, filter and load; the regulator sits on the top. For a better schema of the regulator (where the problem resides): i.imgur.com/LiA4x1w.png Commented Aug 13, 2014 at 20:46