In this post we will try to analyze a few interesting IC LM338 based power supply circuits and related application circuits which can used by all hobbyists and professional for their day to day electronic circuits and experiments
Introduction
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 configurations.
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 R1 is precisely selected so as to maintain a safe 5 amp maximum current limit that can be withdrawn from the circuit.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 Circuit:
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.
In the above section I have explained a few of the important application circuits using the IC LM338, which were basically collected from the datasheet of the IC, if you have more clues regarding such LM338 based circuits, please let us know through the comments below.
Comments
Swagatam
i want to design a four wheel led lamp with switching LEDs i had used total 8 LEDs 2S2P*2 in which it should be operate at 12V DC but the thing is i want to operate it with switching function so it can changes the colour temperature i used the 4 LEDs are of 3000K and 4 LEDs are of 6500K
so can you guide me on it ?
Thanks in advance
Waiting for your reply
Thanks Brijesh,
Do you want the bulbs to be switching alternately and continuously?
Please let me know this then I will provide you with the required circuit design.
Dear Sir
I want to make a circuit with the function of controlling the output voltage (0-30V) and output current (1mA-2.5A). I use 2 IC LM338. The first IC is for voltage regulation and the second IC is for current regulation. I use BC547 to protect the IC LM338. I have the schematic diagram and would like to ask you to check if it really works.
Thank you very much!
here is my schematic link
https://www.homemade-circuits.com/wp-content/uploads/2024/09/LM338-with-voltage-and-current-control.jpg
Hi Long, your circuit looks OK to me, however, if you are using an additional LM338 for the current regulation then the BC547 stage is not required.
The LM338 already has an internal over current protection, above 5 amps.
Hi Engr
In the heater control circuit, i can’t find Lm334, any replacement ic i can use to work with the Lm338 regulator?
Thanks alot for your effort.
Hi Abba,
I cannot seem to find any direct replacement for the LM334. You can perhaps use an LM35 IC with a transistor to replicate the same function.
Why are there 0.1 ohm resistors at the inputs?,these are not load resistors as you would expect on the output side,are they just for impedance matching and what wattage would they be?
thanks
The 0.1 ohm ensures both the LM338 conduct uniformly, and none of them are able to conduct current more than the other. Wattage = R x I^2 = 0.1 x 5×5 = 2.5.
Thanks for your prompt reply.I am making a temp solar power charger for 12v batteries, before I buy an mppt controller for this 320w panel I am testing this ti 10 amp circuit and most circuits use a forward diode on the input
To prevent any back current,would this not be better?
You can use a diode at the input side to prevent an accidental polarity reversal of the solar panel.
thx swagatam
can you please direct me to a simple temperature regulator? preferably 12v or so and drives a relay
thanks
Here’s a circuit which you can try: It has a relay which operates in response to temperature variations.
https://www.homemade-circuits.com/simple-thermostat-circuit-using/
Hello Swagatam
for LM338 in constant current mode 10Amp for 12V bat ; if the input voltage is less than required minimum voltage ; then what will happen? Will it continue with less current or stop fully?
thanks
Hello JK, 12V battery will require minimum 14 V output from LM338, and the input must be at least 3V higher, that is 17V minimum. However, LM338 cannot be used to provide 10 amp current.
Hello Swagat
I want to use LM338. Constant current 10A for 12V battery 100AH C10.
What is the minimum input voltage required to supply 10Amp current?
thank you
Hello JK, you cannot get 10 amps from LM338. You can get 5 amps max, but even at 5 amps the IC can get extremely hot, and might need a large heatsink.
you could piggyback 2 lm338 ‘s to allow 10 amps surely?
Hi, I am a retired Electronic Engineer but like all engineers we find it hard to give up tinkering, i do enjoy
looking at your circuits, and i am building your simple ajustable power supply circuit to power my new project
Hi, that’s great, wish you all the best with this simple power supply project!
Halo sir
May i ask why is the 10AMP regulated power supply limited maximum output of 20v can it go higher?
Hello Joshua, you can increase it by replacing the pot with a 4k7 pot
What is the value of R2 if the output Voltage is 17 ,15 ,13 ,9 ,22 ?
you can try this software
https://www.homemade-circuits.com/lm317-lm338-lm396-calculator-software/
And also whats the use of the 0.1 ohm resistors ? Is it a must to use them if so what how much wattage should i use ?and can i use any opam instead of the lm107 if not available ?
It can be 0.2 x max voltage. If the max voltage is 30V then the result will be 6 watts and so forth.
It helps to keep the dissipation uniform across the two ICs, a simpler idea can be to remove the op amp the 0.1 resistors entirely and put the upper IC in parallel with the lower one, and mount both over a single common heatsink
Halo sir
Thank you for the quick response , let me try to apply the circuit .
Please can you send me your email so as i can send you images of my finished product and let me know your thoughts .
Thanks Joshua, you can send it through your google drive or microsoft drive links, I’ll be happy to post them in the above article.
https://drive.google.com/drive/folders/1ld_qS1tknmIZyV5fzX9Dszrm1FupRiAw
Halo sir please help!
https://drive.google.com/file/d/1L10NtgJ9LOPnGvGniEpiIQdeHHpxaNCO/view?usp=drivesdkI
Joshua, I can help only if you provide the complete schematic of your design…and also tell about the issue that you are facing
How is the project finishing ,cable running and case design … what should I improve ??
All look good to me!
Halo sir
The design has multiple sub circuits but the main circuit is lm338 circuit which has 2 variable resistors to adjust the voltage level, there is the Dual power supply which gives 5v and -5v to power the voltmeter which uses CL7107 ic there is also the sub circuit which provided 5v 1A to with short circuit protection ( it lights red led and buzzer beeps when short is present)which can be used to test digital circuit
Thanks Joshua, for the clarification!
Sorry for the inconvenience pls try again the link below
https://drive.google.com/drive/folders/1ld_qS1tknmIZyV5fzX9Dszrm1FupRiAw
That looks cool, but which circuit did you exactly build? because the 10 amp design explained in the above article does not seem to have so many parts.
please change to sharing mode so that i can access the link
If I want to make 2.5 Amp Regulated Power Supply Circuit, then I have to change the 0.24 ohm resistor to 0.5 ohm. But I want the input voltage is 26V, is it possible?
Yes you can use 0.24V for getting 2.5 amp limit, it will work for the entire voltage range
I have tried to give 26v for input voltage. but, voltage reference doesn’t 1.24V, it becomes around 15V and the output current is more than 3 amp (I’m using 0.5 ohm to get 2.5 amp). I’m sure that my circuit is right, so what are the problems?
Please check the voltage across the resistor, is it 0.6V? if not increase its value a bit more…
No, it is not 0.6V. It is more than 15V. I dont know why. If the input voltage is 4V, the voltage across the resistor is already 0.6V, when I give bigger input voltage then the voltage across the resistor is bigger too. It seems impossible to reach 26V input voltage and has 2.6 Amp on the output.
Or you can suggest me, how to build a constant current source with 26V input voltage and 2.6 Amp output?
That is strange, because anything above 0.6V across base/emitter of the BC547 will cause the BC547 to conduct and ground the ADJ pin of the IC which will in turn shut off the output.
Alternatively you can try the following options for getting a constant current output
https://www.homemade-circuits.com/universal-high-watt-led-current-limiter/
Do You mean 0.24 ohm?
OK. What is the maximum input voltage? Is It 40V?
Yes, 0.24 ohm, I mistakenly wrote 0.24 V. 35 V is the max input
I have 6 32 AH 12 VDC batteries in my 200 watt Solar array.
I have set the charge voltage to 13.7 VDC
But the amps is more than a LM338 can handle. it also passes through a 3 wire Volt/Apere meter 0-100VDC and 0-50 Amp which is via a shunt in the Negitive line.
Can I put a transitor or MOsfet accross the LM338 of around 25 Amperes to increase the current.
If so can you recomend types and circuit mods
yes that’s definitely possible, you could probably try the concepts explained in the following articles:
https://www.homemade-circuits.com/lm317-with-outboard-current-boost/
https://www.homemade-circuits.com/how-to-make-solar-inverter-circuit/
Tolu, please search for “741 null offset” and you will be able to find all the required details
Thanks sir, what value of preset is appropriate to use in the 741ic with added null adjustment
you can use opamp in that place. If you decide to use IC 741 make sure to add an offset null adjustment to it.