Fork me on GitHub

Project Notes

#384 LM723/HighVoltageRegulator

Playing with the basic high voltage output circuit for the LM723 voltage regulator.



The LM723 voltage regulator is a classic IC designed by Bob Widlar. Although not recommended or particularly useful for modern designs, it is widely discussed and documented, so makes an excellent learning tool.

This little project is a straight-forward build of the reference circuit for high voltage regulation.

Some great resources for the LM723 are:

See also LEAP#383 LM723/LowVoltageRegulator for more background and resources for the LM723.

Basic High Voltage Regulator


Output Voltage Set

A resistor divider (R1, R2) is used to set the target voltage as a multiple of the reference voltage. This is what is meant by “high voltage regulator” - the output voltage will be greater than the reference voltage.

I’m using R1=1.2kΩ, and R2=4.7kΩ, so assuming Vref=7.15V, the expected output voltage is 8.976V

The reference voltage is fed to the error amplifier via R3. For minimum temperature drift, R3 = R1 * R2/(R1+R2) = 960Ω, say use 1kΩ

Actual performance: I’m reading a regulated output of 9.06V, slightly over the predicted value. I measure the actual R1 and R2 values and they are accurate within 10Ω. But I measure the Vref output of the LM723 at 7.26V. At this value, the expected output is closer at 9.1V.

Current Limit

Adding current limiting control is optional. It is achieved with a series sense resistor Rsc on the output, used to bridged the current sense and current limit pins, which are effectively the base and emitter pins of a current limiting transistor in the LM723.

When the current through Rsc produces a voltage drop of over about 0.5V, the current limiter starts throttling the output control transistor.

Assuming a limiting drop of 0.5V and a target current limit of 50mA, Rsc = Vbe/Ilimit = 10Ω

I haven’t tested the effectiveness of the current limit yet.


I have an LED and 1kΩ current limiting resistor as a dummy load.

Supply Rail

The 723 requires a relatively high input voltage of between 9.4 and 40 volts (it was designed before “low dropout” was a thing). The main constraint on the minimum voltage is the need to generate the 7.15V reference voltage.

I started running the circuit at 12V. As I reduce the supply voltage, it starts to lose regulation at about 10.6V.





Credits and References

Project Source on GitHub Project Gallery Return to the LEAP Catalog

This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.