LM723: Adapting 0-30vdc regulated power supply for lower input voltages (like 12 or 16vac)

[afarooq]

• Hi! My LM723 based 0-30 VDC regulator kit (see circuit diagram below) requires 30 VAC input transformer but I only have nominal 12 volt transformer instead of the required 30 VAC input. Using this 12 volt 3 amp transformer operating at line voltage of 230 volts, I get 0.25-22.4 VDC output (instead of 0-30 VDC) at no load, which is OK, however potentiometer only functions over 1/3 of its travel range (it has no effect in remaining 2/3 travel). I want to use full travel range of potentiometer to maximize voltage adjustment sensitivity.
• What should be value of resistors or potentiometer so I can use full range of potentiometer?

 

[Reloadron]

Try around 2K and keep in mind you will never get more out than you put in. You mention 1/3 of travel so 1/3 of 5K. That gives you 1.5K so try a 2 K pot.

Ron

 

[afarooq]

Thanks Ron. Considering lower than design input voltage (12 vdc vs. 30 vdc), wouldn't that reduce Vref that LM723 sees (compared to it's required 7.15v), and wouldn't I need to change some resistor values (which ones) to compensate for it?

Also the one-third potentiometer travel is not at the start or end of the potentiometer range but rather somewhere in the middle of its range. Should I add any resistor in series with the potentiometer wiper if I change its value from 5k to 2k?

 

[Reloadron]

If I remember correctly the voltage at pin 4 is a sample of Eout (in your drawing). The voltage at pin 4 is compared to an internal reference. So with that in mind we have 100K, 10K and VR making up the divider. The internal reference should be around 7.15 volts. The LM723 data sheet should show the math for selecting the resistors and for the variable output we use a variable resistor. These things are far from my forte so hopefully another member can help.

Ron

 

[deKees]

You should keep the potmeter. 5K is Ok.
But 100K from Pin 4 of LM723 to Output is too high. 50 K (2 x 100K in parallel) is much better.
Will give you a range of almost 0 to 25 Volt using 100% travel of the potmeter.

LM723 is specified for minimum supply of 9.7 volt, so using at 20 Volt is Ok.