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.

Simulation results

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.

  • \$\begingroup\$ 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/… \$\endgroup\$
    – jippie
    Commented Aug 13, 2014 at 20:39
  • 1
    \$\begingroup\$ Apart from that, I frown upon how D2/Q1 are connected to each other. \$\endgroup\$
    – jippie
    Commented Aug 13, 2014 at 20:42
  • \$\begingroup\$ 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 \$\endgroup\$ Commented Aug 13, 2014 at 20:46

1 Answer 1


jippie's right. D2 is your problem. On the reverse cycle, you've got no regulation on the voltage/current from D2.L3's top is at "virtual ground". L3's bottom is at positive 23 volts or so. This forward biases D2. The output is the cathode side of D2 I assume. That means you've only got regulation on 1/2 of your cycle (through D1). Your regulator sees the high voltage and tries to shut it down, but the power isn't coming from the regulator side. It's coming from D2 so all it's effort to shut the voltage down is in vain.

In short, you want the cathode of D2 connected to the cathode of D1. This will force both sides of your transformer to go through the voltage regulator circuit you've created.

  • \$\begingroup\$ Uh, when I added the regulator I just didn't connect the wires correctly. Can't believe I spent a couple of hours on such a stupid mistake. Thank you! \$\endgroup\$ Commented Aug 13, 2014 at 21:18

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