LM338T Voltage regulator - I can't arrange the output voltage less than 13V DC

Question

I have a circuit which purpose should regulate to voltage down to at least 3V. But when I try to adjust the 5kΩ potentiometer I only get minimum 14V voltage drop. I have a transformer which goes to bridge rectifier and my circuit input voltage is 19V DC after the rectifier. So I am very curious that why I can not arrange the voltage drop to less than 14V. You can find the specific information about my circuit and also schematic and pcb drawings down below.

Transformer: 220V AC/ 15V AC 2A LM338T Input Voltage: 19V DC

My doubts:

1. In LM338T datasheet it has a circuit diagram which LM338T input voltage is minimum 28V DC. So is that why I can not get the voltage I want?
2. LM338T is 5A regulator. I have max 2A input.

I tried to change the resistor values but nothing changes. Let me know what you think.

[UPDATE] I used 120Ω 1/4 watt resistance but resistance burned. That's why I am using 5 resistance as a paralel. I don't have any 120Ω resistance left so when i get some i will update this topic.

Answer 1

My doubts:

1. In LM338T datasheet it has a circuit diagram which LM338T input voltage is minimum 28V DC. So is that why I can not get the voltage I want?

2. LM338T is 5A regulator. I have max 2A input.

1. The diagram you are referring to is a specific example. For that example the minimum input voltage is 28V because that is an example of a 25V regulator (as stated in the figure caption). Section 6.2 of the datasheet says you need a Vin that is at least 3V higher than Vout and the max difference is 40V. In your case you have a 19V input so you cannot output more than 16V.

2. no problem

Answer 2

After your update, in which you tell us that you burned out a 120Ω resistor between OUT and ADJ, this is very suggestive that your regulator is damaged.

Normally there is exactly 1.24V across that resistor, for a power dissipation of:

$$ P=\frac{V^2}{R}=\frac{(1.24V)^2}{120\Omega} = 13mW $$

That is tiny, not nearly enough to kill it. It implies that the regulator is dead, or you have something connected very differently from how it is shown in the schematic.

Perhaps you have D3 or D4 reversed? Either would result in an uncontrolled increase of regulator output.

Answer 3

I solved the problem by replacing LM338T regulator with LM317. When I look closer to the LM338 IC, I saw that manufacturer of the IC was ST. ST doesn't have any product as LM338. I got a conclusion that never buy electronic devices from somewhere you don't know.

[CHANGES]

1. I replaced the R1 resistor value to 120Ω and the potansiometer value to 2kΩ.
2. Replaced the LM317 instead of the LM338 IC.

[CONCLUSION]

Now I can get any voltage between 1.25V and the input voltage of the IC.