The IC LM338 as we know is a highly versatile device when it comes to controlling voltages and current levels.
Circuit Configuration
In the present design, the device is configured in the automatic current control mode.
White LEDs specifically need a well dimensioned input, technically the current to these LEDs must be strictly controlled.
By connecting its ADJ pin with the output makes sure that the current at the output is constantly monitored by the ADJ pin and is never allowed to go beyond the predetermined level set by the 0.6 Ohm resistor.
The device can support at least 3 amps of current through it, therefore easily becomes compatible for driving 1 to 5 numbers of 1 watt LEDs, each having their own current limiting resistors.
The current limiting resistors can be experimented with, probably lesser values may be tried for increasing the brightness levels of the LEDs, however anything less than 50 Ohms should not be tried, because it might cause a permanent damage to the LEDs.
Simulation and Working
The input to the LM 338 IC can be from an regulated DC power supply, capable of supplying 12 volts at 3 amps or more.
The IC LM 338 should be mounted over an heatsink for better performance.
The input diode should be rated at 3 amps, so a 1N5408 becomes OK for the application.
If the circuit is intended for operating outdoors, the input may be taken from a 12 v battery, like from an automobile battery.
Thus the proposed 4 watt LED driver circuit becomes especially suitable for illuminating vehicle interiors, for example as a roof light etc.
Calculating the Limiting Resistor
The formula for selecting the resistor value if a single 4 watt LED is conected is R = 1.25/LED current.
Here, LED current = 4/3.3 = 1.21A
Therefore R = 1.25/1.21 = about 1 ohm (0.6 is not correct)
R watts = 1.25 x 1.21 = 1.5 or 2 watt
for 4 nos 1 watt LEDs, total current is 300mA
Therefore R = 1.25/0.3 = 4 ohms
Circuit Diagram
You would also want to make a transformerless SMPS version of a 1 watt to 12 watt LED driver circuit
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sriramkp says
I will be using a 12v battery. how to get a constant 12v from that battery to connect the 4x1w led? Because 13.5v will be fully charged battery?
Swag says
If you put 4 LEDs in series then 13.5V will be perfectly alright, however for current control you may have to add the following concept between the 13.5V source and the LED string
https://homemade-circuits.com/2013/06/universal-high-watt-led-current-limiter.html
sriram says
Hi, I like to connect 4 nos 1 watt led (3.3v, 300mA) with 12v lead acid battery. So it required a resistor of 4 ohms (R=1.25/0.3= 4 ohms, R watts= 1.25*0.3= 0.37 watts).So what should be the wattage for resistor for better safety? 1/4 watt or 1/2 watt?
Swag says
Hi, it should be 1/2 watt for better safety.
syed shajan says
hello sir, I need your help in designing a LED driver circuit for running two 10W LED connected in series with a forward voltage of 10-11 V and current 850-950 mA. The input for the circuit will be a car battery which has ranging voltage from 8 to 16V.
The output expected is constant 24V and 850-950 mA, please help me sir.
Thank you very much.
Swagatam says
hello syed, you can simply connect the above explained LM338 current controller in between the source voltage and the LED, and achieve the required results…however at 8V the LeDs will become little Dim…
but make sure to the LEDs in parallel….
Swagatam says
and set the LM338 resistor to limit current at max 1.8 amps
Unknown says
hello sir, I need your help in designing a LED driver from car battery.
the input voltage will be ranging between 8 to 16V and the required output voltage is constant 24V and 950 mA current( if possible then need to have a very low value capcitor)
Thank you sir
Swagatam says
hello, you can probably try the following concept:
https://homemade-circuits.com/2014/11/12v-car-laptop-charger-circuit-using.html
satheesh k Nalluswamy says
Swagatam sir Thank you so much!, I will try it.
satheesh k Nalluswamy says
Swagatam sir Your blog is useful and very pleasant
Can you help me please provide transformer less 5w led AC circuit
Swagatam says
thanks satheesh, you can try the following design:
https://homemade-circuits.com/2014/02/simple-1-watt-to-12-watt-smps-led.html
paul timmy says
please sir,what is amount of current that ic338 can give out as output if I use a 12v 300ma input by setting the ic to 5v?can I get the same 300ma current at the output?
Swagatam says
you will get full 300mA, it won't be affected.
Rami Jaimin says
hey what is about current….is it constant with voltage…..???
Swagatam says
current is constant, not the voltage, voltage needs to be constant from the source.
satish kumar says
please provide the circuit for 3w automatic emergency light and how many LED's need to put in this circuit
Swagatam says
you can try this circuit:
https://homemade-circuits.com/2013/02/make-this-automatic-10-watt-to-1000.html
Anonymous says
what changes I've to make for 1w six led in 6 volt battery, please reply.
Swagatam says
Use two strings of two leds each and connect them in parallel at the output of the above shown circuit.
The resistor can be calculated with the help of the given formula.
Rajib.digi says
If i want to use it in 230v AC Also then what would be the changes and Tell me the Size of Proper heat sink for 4 led in area and thickness also.
Thanks,Rajib Banerjee
Swagatam says
use a 12V 2amp smps at the input of lm338….PCB should be aluminum back type…additionally attached a finned type rectangular heatsink at the back of the PCB…..size can be experimented.
mandar says
what is the voltage tolerance for a 12 volt ZENER Diode i.e. 12v =/- ? And can i connect one in series before the IN5408 for a constant 12 volt supply as i want to connect it in my car and as we know that while the engine is running the battery is charged with 14.2 volts?
Swagatam says
zeners should be connected across the supply terminals and not in series, but anyway, a zener will not work for anything that delivers more than its rated capacity…for example a 1 watt zener would tolerate upto 80mA……anything above this would blow it off
The above circuit needs some corrections, I'll do it soon and make it a constant current, constant voltage type of driver.
lihtnesr says
Hello Sir,
Im an avid DIYer. Recently, i was looking to make a DRL(daytime running light) for my car using 1 watt smd LEDs.
But i couldn't find an appropriate circuit for my need. I want to drive Eight 1watt LEDs from my car battery.
I would very much appreciate if you could design a simple and rugged circuit to drive 8 x 1watt leds from a 12-14v input.
Im also planning to add a heat-sink to dissipate any heat generated by the leds.
Thanks & Regards,
Senthil
Swagatam says
Hello Senthil,
I'll do it soon, and post it in this blog…keep connected.
Naveen Abhyankar says
hi swagatam. i am naveen abhyankar here i have some want issue i am making 15 watt led by using 12 volt dc supply.i am making them in series of 3(1watt*3*5=15watt)amp 4.5.but only 3 led of 1 watt is lightining. tell me the right wat to make 15 watt led.
Swagatam says
Hi Navneet,
did you use resistors in series with each string?
you will have to incorporate 6 ohm, 2 watt resistors in series with each led string.
gurmeet singh says
i want to operate led on car what should be the changed to be made in the circuit.
Swagatam says
I'll have to update the above article, I'll do it soon, because it requires a few corrections, I'll reply to your query then…
Chetan M.P says
Its working and the LED's are very bright now, thank you.
Swagatam says
you are welcome!
Chetan M.P says
Thanks, I will try.
Chetan M.P says
You mean I have to use 2 Ohms 1 watt resistor instead of 100 Ohms 1/2 watt resistor which are connected to the LED's.
Chetan M.P says
so i have to use 2 ohm 1 watt resistor for each led.
Swagatam says
Yes that's correct…
Chetan M.P says
what changes I've to make for 6 volt battery, please reply.
Swagatam says
reduce the series resistors to 2 Ohms/ 1watt
SVG says
U applied 12V to each led, i hav 1W leds, but i never applied 12V, i fact i use 3 leds in series if i am using 12V….
so my question is what the ideal rated voltage for these 1W leds ???
r u using any different ones?
Swagatam says
you can connect three in series with a 7 Ohm resistor, no issues.
In the above circuit each led has its own high value resistor so there's nothing to worry, moreover the output is current controlled.
Anonymous says
can i use lm317 instead of lm338 if yes what diode should i use
Swagatam says
317 will work but may dissipate a lot of heat, you will need to mount it over a large heatsink.
Use 30 Ohms in series with each LED instead of 100 Ohms which is wrongly shown in the diagram.