IC LM338 Application Circuits - Explained in Simple Words

Apr 1, 2012

The IC LM338 by TEXAS INSTRUMENTS, is a versatile IC which can be wired in numerous different ways for obtaining high quality power supply circuit configuartions.
The following circuit examples simply depict few of the very interesting useful power supply circuits using this IC.

Let's study each circuit diagram in details:

Simple Adjustable Voltage Power Supply Circuit

The first circuit shows the typical wiring format done around the IC. The circuit provides an adjustable output right from 1.25V to the maximum applied input voltage which shouldn't be more than 35 vots.

R2 is used for varying the output voltage continuously.

Simple 5 Amp Regulated Power Supply Circuit

This circuit produces an output that may be equal to the input supply voltage but the current is well regulated and can never exceed the 5 Amp mark. R1 is precisely selected so as to maintain a safe 5 amp maximum current limit that can be withdrawn from the circuit.

15 Amp, Variable Voltage Regulator Circuit

The IC LM 338 alone is specified for handling a maximum of 5 Amp of current, however if the IC is needed to handle higher currents, in the region of 15 amps, it can well be modified to produce that much of current with the appropriate modifications as shown below. The circuit utilizes three IC LM338 for the intended implementations with output voltage that's adjustable as explained for the first circuit. R8 is used for the voltage adjustment operations.

Digitally Adjustable Power Supply Circuit:

In the above designs, the power supply utilized a pot for implementing the voltage adjustment procedure, the below given design incorporates discrete transistors which can be digitally triggered separately for obtaining the relevant voltage levels at the outputs. The collector resistance values are chosen in an incremental order so that a correspondingly varying voltages can be selected and becomes available through the external triggers.

Light Controller Circuit

Other than power supplies, LM338 can also be used as a light controller. The circuit shows a very simple design where a phototransistor replaces the resistor which normally acts as the component for adjusting the output voltage. The light which needs to be controlled is powered by the output of the IC and its light is allowed to fall on this phototransistor. As the light increases the value of the photo-transistor decreases which in turn pulls the ADJ pin of the IC more toward the ground, forcing the output voltage to decrease which also decreases the light illumination, maintaining a constant glow on the lamp.

Current Controlled Power Supply Citcuit:

The next circuit shows a super simple wiring with the IC LM338 whose ADJ pin is connected to the output after a current sensing preset. The value of the preset determines the maximum amount of current that becomes permissible through the IC at the output.

12V Current Controlled Battery Charger Circuit

The circuit below can be used to charge a 12 volt lead acid battery safely. The resistor Rs may be selected appropriately for determining the desired level of current for the connected battery. R2 can be adjusted for obtaining other voltages for charging other categories of batteris.

Slow Turn ON Output Power Supply

Some sensitive electronic circuits require a slow start rather than the usual instant start. The inclusion of C1 makes it sure that the output from the circuit rises gradually to the set maximum level ensuring the intended safety to the connected circuit.

Heater Controller Circuit

IC LM338 can also be configured for controlling temperature of a certain parameter like a heater. Another important IC LM334 is used as the sensor which is connected across the ADJ and ground of the IC LM338. If the heat from the source tends to increase above the predetermined threshold, the sensor lowers its resistance  correspondingly, forcing the output voltage of LM338 to fall, subsequently decreasing the voltage to the heater element.

10 Amp Regulated Power Supply Circuit

The following circuit shows another circuit whose current is restricted to 10 amps, that means the output can be made suitable for high current rated loads, the voltage is adjustable as usual via the pot R2.

Adjusting Many LM338 modules via a Single Control

The given circuit shows a simple configuration which can be used for controlling the outputs of many LM338 power supply modules simultaneously though a single pot.


  1. good day sir,

    regarding the 12V Current Controlled Battery Charger Circuit, can is use rectified DC from a motorcycle as V_in?

    if possible, what voltage of the capacitor would you recommend? is it ok to use 1/4w for all resisitors?

    thank you.


    1. Good day Jhunix, yes you can use the motorcycle alternator input after rectification, but it should not exceed 35V.

      Use 10uF/50V at the input and 100uF/25V at the output of the IC.

    2. Can i use LM317 or any equivalent IC replacing LM338? because I can't find LM338 in any electronic/electrical store here in or place.

    3. 317 will charge only upto 10AH batteries not above that.

  2. sir some times 7805 get short circuits & it gives 12v output how i save my mobile from 12v output
    give your reply

    1. prashant, 78XX ICs are internally short circuit protected, they will never get shorted unless tampered intentionally. Even if this happen, the cell phone is protected against upto 12V supplies and will simply show"not charging"

  3. good day sir. i wat to build a powersupply with lm338 but i want to be able to adjust the curent to not just the voltage i saw a schematic above with a simple curent limiting circuit.but if i make that circuit how can i adjust the voltage not just the curent?

    1. good day,

      please refer to the first diagram in the following link:


      Replace the transistor with a Darlington pair (two BC547) and replace the base resistor with a 10k pot. The slider arm will go to ground while the two ends across base and 0.6 ohm resistor.

      this pot can be used for setting the current limits.

  4. hello sir
    how to find R1 values(ohms,watts) for current controlled power supply . i am using 12v 80ah battery and need output current : 3.5 amp to drive led array of 30 * 1 watt leds

    1. hello aashish, please go through the following article, it has all the required info in it:


    2. thank you
      i hav read that article and got the formula R1 = Vref/current,
      1.25/3.5A=0.357 ohms ,4.375watts is it correct

  5. good morning swagatam
    regarding 12V Current Controlled Battery Charger Circuit
    I have 12v 35Ah battery, 0-12v 5A transformer. Charging current required must be around 3.5A.
    Is ther anything i should modify about
    Rs .1 ohm
    R1 250 ohm
    R2 2k7 ohm

    1. Hi Phoenix, If you have calculated them correctly nothing more needs to be done in the circuit...that's all is needed.

    2. is Rs .1 ohm appropriate for a 35 Ah battery. I didnt calculate this value. I took it from the image above.

    3. 0.6/3.5 = 0.17 ohms is the approximate value

    4. i want to charge 12v 4.5ah battery form 17v 11 watt solar panel is the value Rs=0.1 is correct for the arrangement ?

    5. I am not quite sure about it, I think the following circuit would be more suitable for you:


    6. sir but my input is 17vdc from a solar panel...the link you have provided is for 220ac supply...

  6. Hello sir, I'd like to know how to charge a 12V7Ah lead gel-acid battery, I know that the "slow charge" must me 700mA, so I have to supply the battery with 700mA, and I don't know how to calculate the external components. Thank you very much for the help beforehand

    1. hello Jeronimo, you can build the first circuit from the above article exactly as shown, just replace the pot with a 10k pot.... for setting up the circuit adjust the pot to get around 14V at the output without any battery connected. once this setting is done connect the battery and switch ON power...let it charge for about 8 hours and then you could switch OFF the supply manually, you would find the battery fully charged by this time.

    2. LM317 may be used instead of LM338 to make the circuit cost effective...

  7. For the 5amp power regulator you have a resistor set for 2W and .24ohms
    If we use I^2*R=(25amp)*(.24ohm)=6W, So wouldn't the resistor need to be a minimum of 6W?

    1. by using the more general VxI formula and by considering the fixed reference voltage across the resistor = 1.25V, we get P = 1.25 x 5 = 6.25 watts

      yes, you are right, the datasheet's calculation is incorrect, the resistor should be rated at above 6 watts and not 2 watt

  8. Hello sir, i want to build a 5A 12v battery charger, what value should i use for Rs, R1,R2 and what is the resistor watt preferable? Thank you

    1. Hello Hardy, you can keep Rs and R1 as shown in the diagram, R2 can be replaced with a 10k pot and this pot could be adjusted to set 14.3V at the output, for the recommended 12V/5A battery charging

  9. Sir I want to make a mobile charger out of it can u pls help me by the spec if I want the output as 5v 1.5amp and it should be stable . the input is taken from a bike battery which is about 12v

    1. Ashraf, you can try the following linked circuit:


      adjust P1 to get precise 5V at the output and use it for your purpose, also calculate R3 appropriately for limiting current to the desired limit.


    1. Hi Gajendra ji, you can try the last circuit from this article...


  11. Hey sir.. Smit here
    Sir i m trying to make battery charger and for that I have used lm317 but using this circuit I can charge mobiles upto certain mAh. If I connect mobile which is having higher mAh than mobile is not charging.so give me solution how can I make it useful for all mobiles.
    In my circuit I have used lm317 and and one 10k potentiometer is connected with its adjustable leg and than other all of potentiometer is connected to 180 ohm resistor. Out terminal of lm 317 is connected with that resistor and one more resistor of value 4.7 ohm. And than I have connected one zener diode of 6.2v, 0.5 W,, after that I connect one diode 1 N4007. And this diode is connected to female USB connector help me pls..

    1. Hi Smit,

      a good LM317 would be capable of generating in excess of 1amp which should be sufficient for most cellphones.

      I think the 4.7 ohm could be causing some significant drop in current.... try reducing its value and see the response...and also make sure the LM317 is mounted over a large heatsink and adequately cooled.

  12. I'm looking at your "Simple 5 Amp Regulated Power Supply Circuit" using the TI338. I have an application where I need to limit the current to a 24 vdc solenoid to around 1 amp. will this circuit work and what would the resistor value need to be. Thanks scott s.

    1. yes it will work, the resistor value would be

      1.25/1 = 1.25 ohms 1/4 watt

      for more info you can refer to the following article


  13. Hi Swagatam,
    Say we have a pc smps that supplies 15A at 12v rail,would lm338 current regulator work if a 10W LED requires 12v and I want to drive it at 700mA.Can current be fixed to 700mA using LM338 current regulator?
    If it can be used then R value would be R=(1.25/0.7A)=0.87ohm.Right?
    Whats the power dissipated by resistor and power rating of resistor should be used?

    Following the above can we cascade the above leds in parallel,say 7 (i.e 700mA x7=4.9A),
    so the ckt should pass on 4.9A to the led array.
    Hence R required would be R=1.25/4.9A=0.25 ohm.Am I right?What should be the power rating required by the resistor in that case?

    And if I want to work on above 5A,can LM 196 be used in similar fashion?
    Sorry for asking too many question,Just want to learn and experiment :)

    1. Hi Abhishek,

      you can try the following design for achieving any desired current control


      yes your calculations are correct.

      the power rating of the resistor would be

      1.25 x 4.9 = 6.1 watts.

      LM196 can be replaced directly with LM338 for getting 10 amp control



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