LM338 power supply problem

[Anubhav Sharma]

Hello Everyone,
This is for the first time am posting here, ans hope ill get a solution from experts like you guys.

First of all i must tell you that am hobbyist and dont know much.

I have built a LM338 power supply using 12-0-12 1Amp transformer and diode (IN5399) bridge rectifier.
Also 2200uf 50v cap + 0.1uf ceramic cap, LM338 with nice big heat sink, 220ohms 1watt resistor (between adj pin and out pin), 10K pot (also tried 5k 1watt pot), 1K load resistor with LED and 0.1 ceramic cap.

Everything is working fine except 1 thing. I am getting 30v max as output BUT cannot go below 7v.
I turned pot to its very starting position but not showing below 7v.
7 volts is the minimum output voltage showing.

As per data sheet LM338 should produce 1.2v as minimum volts.

Please see the attached diagram of same circuit.

Please guide me where am wrong.
Help me please.

Thanks in advance.

 

[BobK]

Sounds like your 10K pot does not go down to zero. Try shorting it out and see if you get 1.2V.

Bob

 

[Anubhav Sharma]

Sounds like your 10K pot does not go down to zero. Try shorting it out and see if you get 1.2V.

Bob

Hi, thanks for the reply. I checked POT with multi meter its showing 0 ohms when extreme left and 5K when on full (5K POT)
I have already joined wiper pin and right pin of POT when supply is running. Can this be a problem?

 

[Amar Dhore]

Anubhav,

Check the datasheet for LM338 Dropout Voltage. I guess for 1 Amp, the headroom is few Volts. your Vin -Vout should not be more than allowable limit otherwise your output might saturate.

 

[dorke]

Check the voltage between pins 1 and 2 of the LM338 should be 1.25V in all positions of the pot.
Check the voltage on the pot in all positions.
What do you get at max cw position and max ccw position?