I have a 28V steady DC input, and I want to use it to charge 2x12V lead-acid batteries in series.

I want to use the LM723 IC to make an adjustable (26V to 28V) regulated output, as it has current limiting capabilities. So I used the Positive Switching Regulator figure from the datasheet with the recommended values for R1 and R2, but the circuit is not working neither in simulation nor reality.

Circuit from the datasheet Circuit with ISIS Proteus for simulation (**updated**)

Am I missing something? I checked multiple times.

Is my choice for the transistors right? I can't find 2N5005 and 2N5153 locally.

Can anyone help me please? Thanks.

  • \$\begingroup\$ What is Vref set to? Is the Vout high or low? \$\endgroup\$
    – Aaron
    May 1, 2020 at 16:05
  • \$\begingroup\$ @Aaron Vref is set to 7.15V, and Vout I believe is high \$\endgroup\$ May 1, 2020 at 16:07
  • \$\begingroup\$ @jsotola You are right. I used UA723 datasheet, and the LM723 datasheet is different. I updated the schematics but still the same result. Post updated \$\endgroup\$ May 1, 2020 at 16:21
  • \$\begingroup\$ the cap on the output may be required \$\endgroup\$
    – jsotola
    May 1, 2020 at 16:24
  • \$\begingroup\$ To charge a 24 volt battery (or two 12 volt in series), you need to supply about 29 volts to the batteries, so your input voltage must be over 30 volts. \$\endgroup\$ May 1, 2020 at 16:24

1 Answer 1


For fully charging a 24V lead-acid you need almost 29V, and this topology cannot supply it.

On the other hand, remember this IC has two main Vref connections, one for Vo<7V and one for Vo>7V. So I think you naeed to change the connections, and I agree with earlier comments the Potentiometer connections could be interchanged in order to vary what N.I.input compares. For reference, I attach a 1973 diagram, intended for 5V output

My last remmark, is you're using a legacy IC meant for linear application as a switcher.


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