Power Supply

Thread Starter


Joined Oct 29, 2020
Hello colleagues,
I have a few questions about the attached photo, which represents а power supply and it is:
1. Is it possible to improve this circuit?
2. I cant understand how current regulation works here. The output of the stabilizing voltage must fall on the transistors Q7-Q9?
3. Why are so many p-n-p transistors used?

I don't think in the current implementation of this circuit, it will work?
Тhanks in advance !



Joined Jun 4, 2014
As shown this circuit will not work.
All the PNP transistors are used as if they were NPN but the part numbers are indeed for PNP transistors.


Joined Jun 5, 2013
You can get a circuit that works in a 3 pin package. LM723 became obsolete when these came out, I think when I when I was a young man, about 35 years ago.

Look up LM317.



Joined Mar 14, 2008
Obviously the circuit was never built by the author of the schematic.

There are many circuits that use the old 723 that work, but this is not one of them.


Joined Sep 24, 2015
I'm enjoying looking at this schematic. It shows R10, 11 & 12 as being 0.22Ω in parallel, forming a 73mΩ (0.073Ω) resistor network. The +V is otherwise directly connected to the positive of BR1. I see no regulation in that at all. +V feeds back to U1 pin 4 via R25 (100KΩ) which "should" act as a voltage sense line. RV2 sets up a reference adjustment, but it's dependent also on the output from BR1. -V appears to be tied directly to ground (though that might not be a ground, could be a common ground, which BR1 (negative side) is grounded. Across +V and -V there is no regulation going on.

Also, though the circuit identifies PNP transistors, the tell tale sign that they are NPN is the emitter arrow. The way I learned to recognize NPN is by the Not Pointed iN arrow. PNP is Pointed iN Pointer. I'm still working on MOSFETs though. I think N type has the middle arrow pointed iN where as P type has it Pointed out. I'm sure others have their favorite mnemonic method of remembering which is which. Still others probably have just learned to recognize the difference without all the foolishness I use.


Joined Mar 14, 2008
I'm sure others have their favorite mnemonic method of remembering which is which.
I just remember that the normal current flow for a BJT is in the direction of the emitter arrow (and that NPNs thus operate from positive voltage and PNPs operate from a negative voltage), and the direction for the MOSFET arrow indicates the forward direction of the substrate diode (which is reversed biased in normal operation).


Joined Jan 23, 2018
In addition to ome of the tranistors being wrong there is the fundamental flaw that the pass transistors are not connected to the output, as has been noted already. It is unfortunate that so much garbage is published, mostly because it is confusing to a lot of people.
There is still a place for the 723 voltage regulator control, but it is not everywhere.