0
\$\begingroup\$

I am building a power supply with LM723 and my intention is to have 0-30V and a range of 0-5A variable. I am regulating voltage normally, however for current limiting, if I want to regulate small current amounts like 10mA, 100mA I need a shunt resistor with high value, which is not viable for 5A, since with LM723 I need a 0.67V voltage drop to limit current.

Currently I am simulating my design with three parallel 0.33 Ohm shunt resistors, which will give me approx. 600mV voltage drop. That works for 5A, but nothing lower. When I calculate the resistor needed for a low current scenario, the value is too high to withstand the 5A.

I tried using a non-inverting amplifier to sense the difference and increase the voltage output, but the difference between R1 and R2 on the amp-op won't work well.

Any suggestions on how to do this? Is amplifying the voltage drop to input it on the LM723 even a good idea?

Thanks.

enter image description here

\$\endgroup\$
1
\$\begingroup\$

The current limit function of the 723 is fairly crude to begin with. The "reference" is fixed; it's simply the forward drop of the BE junction of an internal transistor. Therefore, if you want an adjustable limit, either the series resistance itself must be variable1, or you need to put a variable gain stage between the sense resistor and the 723.

At this point, you're really starting to go beyond the kinds of applications the 723 was intended for, and there are far better solutions on the market today.


1 That's how I did it when I built myself a little bench power supply back in high school (mid-1970s).

\$\endgroup\$
2
  • \$\begingroup\$ Thanks Dave for the answer. I couldn’t agree more, do you have any hardware to recommend so I can replace the LM723? I searched the internet but wasn’t very successful \$\endgroup\$ Jul 11 '20 at 17:46
  • 1
    \$\begingroup\$ I'm afraid not. I gave up building my own bench power supplies many decades ago. I do still design custom power supplies for client projects, but they are very much fixed function, specific to the application. \$\endgroup\$
    – Dave Tweed
    Jul 11 '20 at 19:50
0
\$\begingroup\$

Turns out another LM723 did the trick! I just had to add another LM723 as a comparator, that way I have a variable current control. Any tips or suggestions? Any flaws? Thanks Dave for the help.

enter image description here

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.