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1.25 V to 30 V Variable Power Supply Circuit using IC LM350

Last Updated on April 8, 2020 by Admin Leave a Comment

The variable voltage current power supply might look very small and compact physically, but it has the supply capacity of up to 3 A at 1.25-25 V output voltage.

However, when the power dissipation goes up to 30 W, the on-chip overload protection of the integrated voltage regulator IC1.

The basic schematic of this LM350 variable voltage power supply is shown below:

LM350 power supply circuit

How the Circuit Works

The junction of the R1-P1 potential divider has the ADJ(ust) pin of the regulator connected to it. Output voltage Uo can be calculated using the formula

Uo = [ 1.25 (1 + P1/R1) ] V

3 amp 30V power supply

As shown in the above detailed circuit diagram the unit of P1 and R1 is ohms. P1 lies between 1-2.5 kΩ, measured between the wiper and the junction with R1.

C1 here is a normal filter capacitor while C2 and C3 help improve regulation. D1 and D2 are protection diodes needed to ensure a higher positive potential at the output of IC1 than the input at switch-off.

R1 is so chosen that its value maintains a minimum load current of 3.5 A through IC1. IC1 must be mounted on a 1 K/W rated heat sink using sufficient heat-conducting paste.

At low output voltage requirements, a mains transformer with a lower secondary voltage should be used ( 9 V secondary voltage for Uo = 5 V ).

With a 24 V secondary and 1.25 V required output, the maximum output current should be 1 A from LM 350 variable power supply circuit.

Otherwise, the dissipation in the LM 350 will go over the maximum permissible limit.

This will cause the internal protection to turn the regulator off. On the other hand, the maximum load current comes to 2.5 A when the secondary is 9 V and the output is 1.25 V.

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About Admin

Hey friends, Thanks a bunch for stopping by this site! I am an engineer with a Bachelor of Engineering in Electronics and Telecommunication. One of my passions is gathering information from all sorts of electronics books and tutorials. I then take that information and compile it into a language that is super easy to understand. My goal is to make those complex electronics circuit concepts and technical terms much more accessible for all the new and budding electronics engineers out there. I can also design customized circuit diagrams as required by the users.
If you have any questions related to this field, please do not hesitate to drop a comment! I am always here and ready to help you out with any queries you might have. I cannot wait to hear from you!

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