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2 Best Current Limiter Circuits Explained

2 Best Current Limiter Circuits Explained

The post explains 2 simple universal current controller circuits which can be used for safely operating any desired high watt LED.

The universal high watt LED current limiter circuit explained here can be integrated with any crude DC supply source for getting an outstanding over current protection for the connected high watt LEDs.

Why Current Limiting is Crucial for LEDs

We know that LEDs are highly efficient devices which are able to produce dazzling illuminations at relatively lower consumption, however these devices are highly vulnerable especially to heat and current which are complementary parameters and affect an LED performance.

Especially with high watt LEds which tend to generate considerable heat, the above parameters become crucial issues.

If an LED is driven with higher current it will tend to get hot beyond tolerance and get destroyed, while conversely if the heat dissipation is not controlled the LED will start drawing more current until it gets destroyed.

In this blog we have studied a few versatile work horse ICs such as LM317, LM338, LM196 etc which are attributed with many outstanding power regulating capabilities.

LM317 is designed for handling currents up to 1.5 amps, LM338 will allow a maximum of 5 amps while LM196 is assigned for generating as high as 10 amps.

Here we utilize these devices for current limiting application for LEds in the most simplest possible ways:

The first circuit given below is simplicity in itself, using just one calculated resistor the IC can be configured as an accurate current controller or limiter.

current limiter using LM338 circuit

Calculating the Current Limiter Resistor

The figure shows a variable resistor for setting the current control, however R1 can be replaced with a fixed resistor by calculating it using the following formula:

R1 = Vref/current

or R1 = 1.25/current.

Current may be different for different LEDs and can be calculated by dividing the optimal forward voltage with its wattage, for example for a 1watt LED, the current would be 1/3.3 = 0.3amps or 300 ma, current for other LEDs may be calculated in similar fashion.

The above figure would support a maximum of 1.5 amps, for larger current ranges, the IC may be simply replaced with an LM338 or LM196 as per the LED specs.

Making a current controlled LED tubelight.

The above circuit can be very efficiently used for making precision current controlled LED tube light circuits.

Application Circuits

A classic example is illustrated below, which can be easily modified as per the requirements and LED specs.

30 watt Constant Current LED Driver Circuit

30 watt led current limit design

The series resistor connected with the three LEDs is calculated by using the following formula:

R = (supply voltage – Total LED forward voltage) / LED current

R = (12 - 3.3+3.3+3.3)/3amps

R= (12 - 9.9)/3

R = 0.7 ohms

R watts = V x A = (12-9.9) x 3 = 2.1 x 3 = 6.3 watts

Restricting LED Current using Transistors

In case you do not have an access to the IC LM338 or if the device unavailable in your area, then you could simply configure a few transistors or BJTs and form an effective current limiter circuit for your LED.

The schematic for the current control circuit using transistors can be seen below:

transistor based LED current limiter circuit

How to Calculate the resistors

In order to determine R1 you may use the following formula:

R1 = (Us - 0.7)Hfe/Load Current,

where Us = supply voltage, Hfe = T1 forward current gain, Load current = LED current = 100/35 = 2.5 amps

R1 = (35 - 0.7)30/2.5= 410 Ohms,

Wattage for the above resistor would be P = V2 / R = 35 x 35 / 410  = 2.98 or 3 watts

R2 may be calculated as shown below:

R2 = 0.7/LED current
R2 = 0.7/2.5 = 0.3 ohms,
wattage may be calculated as = 0.7 x 2.5 = 2 watts

Using a Mosfet

The above BJT based current limit circuit can be improved by replacing T1 with a mosfet as shown below:

The calculations will remain the same as discussed above for the BJT version

mosfet based constant current limit circuit

About the Author

I am an electronic engineer (dipIETE ), hobbyist, inventor, schematic/PCB designer, manufacturer. I am also the founder of the website: https://www.homemade-circuits.com/, where I love sharing my innovative circuit ideas and tutorials. If you have any circuit related query, you may interact through comments, I'll be most happy to help!

163 thoughts on “2 Best Current Limiter Circuits Explained”

    • Keep the battery connected with the solar panel through a high value resistor, this will keep the floating charge always on.

        • Initially try a 1 K resistor and check whether it prevents the battery voltage from dropping from its fully charge level. If it does, then try some other higher values like 4k7, 10K etc and see which highest value succeeds in preventing the battery voltage drop from the full charge value. Select this highest value.

  1. Hi Sir,
    I’m thinking of making a Lucas dynamo regulator myself and I need a current limiting circuit
    The dynamo itself is rated at 100W, so I would like to limit the current draw at about 8 Amps
    According to the diagram in your article, is it right that I have to go with the first schematic and get LM196 with 0.16 Ohms resistor?
    Also, if i’m running LM196 at current limiting mode, is the main heat dissipation from the chip itself or from the resistor?

    • Hi Alec, You can use LM196, the dissipation will be from the IC, and not from the resistor.

      I would rather recommend the last circuit.

      • If I use the last circuit, will there be any difference between high side and low side current limiting?
        As I could see the last circuit limits current on the low side, but in my application the current must be limited at the high side ie. the dynamo output.

        • Sorry did not understand what you meant by high side low side current? You can set it at any desired threshold current. The only criterion is to select the mosfet appropriately for handling the specified amount of current

  2. I’m thinking a current regulator could also be the shunt resistor voltage drop in the BJT circuit somehow affecting the duty cycle of a simple bistable multibrator. Not as simple as yours though!

    Also can I use a small car bulb as this resistor (the shunt or emitter follower resistor)?

    • A filament bulb can be used in place of R2, but the the response of the bulb may not be as linear as a normal resistor

      • A 12v festoon filament bulb of 15w, (car interior bulb) should have 1 ohm cold and 10 ohms hot (it draws 1 amp normally).

        I wonder therefore if one uses this as the shunt resistor (on the NPN emitter), and just use one BJT transistor (my Power transistor) I could then have a 10k pot(entiometer) feeding the base between the 24 power supply rails? (the 3 connectons on the pot)?



        • A shunt is supposed to short the power supply rails on overload. In your case the bulb is in series with the load so it’s basically acting like a limiting resistor. It’s a very crude way to control current and probably won’t work for sensitive devices like LEDs

          • I’ve seen it referred to a shunt resistor or a sense resistor in other current limiting circuits.
            Do you think dispensing with or leaving out T2 would work therefore as a current limiter if one uses a pot as a voltage divider on the base of the power transistor instead of T2 etc?
            My understanding is that as the voltage across the emitter resistor, shunt or whatever rises the forward bias between the base and emitter will drop so I could have current adjustment using the pot?

            • A Shunt is normally connected in parallel to the load, not in series. R2 is a sensing resistor that’s right, since it activates T2 when over current is sensed.
              T2 does two things, it saves power and also enables a complete shut down of power to the load on over-current. On the contrary if only a limiter resistor is used it will keep dissipating power at different levels regardless of the over current, and never allow a complete shut down of the supply on over current conditions.

              Your understanding is correct but won’t be as efficient as including T2 would be.

    • All the designs shown above will work with 24V without any issues. So you can use any of them for your specific need by calculating the parameters accordingly.

      • Thanks very prompt.
        I may not have a IRF 540 so the link was for the power nower transistor I will try and use.
        If I use a potentiometer of 10k instead of the zener will there be problems please?

        • My pleasure! If you do not want to use a mosfet then you must try the second last design. Zener is required only for a mosfet not for BJTs.

          • Hello again I really want to adjust the current limit a little, so thought to feed T1 with a voltage divider (a pot) of 10k?

            • You can do that, but the zener for mosfet is placed for a different reason, it’s for protecting the mosfet from high voltage. For your case you could add a pot in series with R1. Make sure to calculate them correctly.

  3. Thank you for your response!
    Obviously, the heating up tendency will be controlled by the heatsink but I think that the absence of the current controller may lead to loose more current in the form of heat which economically is not efficient enough. Is that true?
    Thanls again!

    • Yes that’s true only for those devices whose temperature rise affects its current consumption, otherwise current control is not required.

  4. Good afternone Sir!
    I read here somewhere the following sentence :-“If your load is rated at 12V, then the current would have not effect on its functioning because if the load and source voltages are correctly matched, current becomes immaterial. Even if your battery is rated at 1000 Ah still a 12V load would work without any issues.”
    My question:-Just now ,I am makeing ca. 36 watt LED light,12v .If the above sentence is right,it means no need of current controling curcuit.Am I right?
    God bless you again and again!

    • Hi Shigida,

      that’s absolutely correct, but only as long as your LED does not heat up. If it has the heating up tendency in that case a current controller will be required, otherwise the LED will start drawing more current as it heats up! and eventually get burnt

  5. hello swag
    i used the above circuit with 30 watt led .all going well but i couldn,t find 0.7ohm 7watt resister..
    so i used 2 resisters of ( 0.33ohm 5watt + 0.33ohm 5watt )in series . these resisters become hot after some time , what should i do?

  6. thanks for this one
    but i really don,t get it. if i am using single ( 5watt-3.3v ) led instead of 3.leds in series then what will be the R1 & R2. & also will the circuit be the same for 1-led ?

    • for a 3.3V led you won’t need any of the above circuits, you can simply use a 7805 IC and power the LED through the 5V from this IC. Make sure to add a 1.2 ohm/5 watt resistor in series with the LED, and mount the IC and the LED on suitable heatsinks.

          • tomorrow i used LM338 with specific resisters to regulate the voltage at 3.37V . but Led burned due to excess of current so i red about LM338 and i found that it allows 5A current,,
            all i need is 3A & 3.37V.
            CAN you give me a circuit of LM333 for the Led mention in previous comment ( cree XML-t6 ( 10watt 3.3V 3A)).
            Thanks for all your help .
            i m looking forward for your reply 🙂

            • At 3.3V even if you apply 100 ampere current to your 3.3V LED it will not burn. Your LED burnt due to heat, and thermal runaway. When an LED heats up it starts consuming more current, this causes more heating and the LED consumes even more current….. this goes on until it is destroyed. So it is not the current rather heat which destroys an LEd in the absence of cooling. But controlling through external cooling can be difficult, therefore it is better to have a current controlled input.You can employ both voltage and current control by applying the design shown in the following article:


  7. Hi Swag,

    Writing it again. Not sure if previous question was posted properly because I cannot find it after post.

    Hope you are doing well.

    I need your help to limit high amp input. let me explain the case. One of my friend bought 12 volt 4.2 amp DC fan and bought 125 AH battery to get a longer backup. when he connected the fan to the battery he felt that fan wires are getting hot. I am sure something should be connected to limit the current. Can you please help me generating a schematic of a circuit that can limit the current for 4.2 amp fan?

    • Hi Saqib, all comments are held for moderation without any notice, so they will always reach me, no issues. I have deleted your previous comment to avoid duplication.

      If your fan is rated at 12V, then the current would have not effect on its functioning because if the load and source voltages are correctly matched, current becomes immaterial. Even if your battery is rated at 1000 Ah still a 12V load would work without any issues.

      I am sure the fan is not rated for 12V operation or it might have had some other internal problems.

  8. Hi Sir.! First of all, congratulations for the help that you give us. All here are electronic’s lovers!!!
    Secondly!… I want to make a fog lights with 3 power leds of 10w.
    I’d like to use the circuit above but the steps for calculate the component values (mainly resistance), are not clear for me.
    If we have:
    Power Source: 12v..(from car battery)
    Power leds 10Wx3( 10V, 0.8A each one)
    Can you give some sugestion about the diagram and the configuration and wiring?
    Thanks by advance.
    Greetings from México!!!

    • Thank you Fremin, actually if you use the first circuit with a proper limiting resistor, then the series resistor with the LED can be avoided.

      for 3nos of 10 watt in parallel the total wattage would be = 30 watts, dividing this with 10V would give 3 amps, therefore the current limiting resistor for the LM338 IC would be 1.25/3 = 0.41 ohms, and wattage = 1.25 x 3 = 3.75 watts

      However you may have to add a voltage control circuit also to ensure a fixed 10V to the LEDs.

      you may require the the first circuit from the following article:

  9. Hi Swagatam i’m really learning a lot from your website.
    i made a small observation
    in the second diagram in the smp you wrote 12amp and 5amp, i think the 12 should be voltage value and not current, am i write?

    • Thanks Abba, glad you are learning from here…yes it seems like a printing mistake, but I am sure readers will understand and ignore it…

  10. Can i skip the current limiting section from the above ckt to drive n nos. of high power leds of 5W,12v.Will be driving it by pc smps 12v rail.

  11. Hi Swagatam,

    Firstly, wow, our names are quite similar.
    Secondly, I plan to connect about 15 pcs of 10w leds (9-12V,1050mA)using a computer PSU (12V,20A), but I hear there's a risk of burning the leds out due to variable current and heat. Is there any way to implement your circuit to this application. Sorry, I'm a complete newcomer to this, so can't figure it out myself.

    Great site. Thanks in advance

    • Hi Sattam, our names do sound similar 🙂

      a current controller is strictly required for all high watt LEDs along with a heatsink.

      for your application you can add one current control stage for each LED.

      Instead of the above explained LM338 design you can rather go for the following design and apply with each of the LEDs….and then you can rest assured your LeDs will be perfectly safe


    • Thanks Swagatam,

      Even though I kinda got the basic concept of the circuit from the link you provided, I have no clue how to practically apply it, what components do I need to procure?

      On a side note, I had some 1n4007 diodes, anyways they can be used?

    • Sattam, you will have to calculate the parts with the help of the formula as suggested in the article.

      for 15 LEDs you may have to make 15 such controllers and then combine their supply rails together with the input supply from the 20A PSU

      the transistors could be TIP31for T1 and 2N2222 for T2

  12. Hi SWAGATAM MAJUMDAR,first I would like to say, "this is a great site and thanks for sharing all the info with us". I would like to use the second circuit with the fixed resistors to drive my RGB led in a Mood Lamp Arduino Circuit using PWM. My question is, how do you hook this up to the Arduino ? I woul be making three of them, one for each color and using the LM317 with a 12V supply.Please guide me accordingly.
    LED Emitter:(Red, Green, Blue)
    Wavelength: RED:620-625NM, GREEN:520-530NM, BLUE:460-470NM
    DC Forward Voltage (VF): RED:6-8V, GREEN:9-12V,BLUE:9-12V
    DC Forward Currect (IF): 350mA Each Color

    • Thanks Garfield,

      Th first design would be more suitable for your requirement…

      you can simply feed the 12V at the input of the first circuit, and use the output to power the Arduino lamp.

      You can use 3 separate current limiter modules for the 3 lamps….

      I hope the Arduino has its own 5V controller circuit

    • Hi Garfield, for 10 watt LEDs the Arduino signals will need to be fed to transistor drivers, and the LEDs will need to be connected across the collectors (for BJT), or drains (for mosfets) and the positive line…this positive line is supposed to come from the above current limiter stage….I hope you got the plan.

  13. I want to make a 7w bulb using Edison 0.5w,5630 led. Forward current 150mA,forward voltage 3.2v. This is my first project please explain in detail. I also want all types of electrical protection in this circuit.

  14. How do I connect 2 x 10watt LEDs with a LM338?
    LED specs: DC Forward Voltage :9V-12 V, Forward Currect: 1050mA.
    What will be the values of the two resistors???
    Supply voltage is 12V 3 amps

    • If the input is a 12V supply then you can connect the LEDs in parallel across the output of the first circuit, with a 24V input you will need to connect the LEDs in series and connect their ends with the output of the first circuit.

      The resistor value will need to be calculated as instructed

    • it's mentioned in the article, ref = 1.25:

      therefore R1 = 1.25/2amps = 0.62 ohms

      wattage = 2 x 1.25 = 2.5 watts

      the above is for a 12V input

    • Doesn't this constant current source has a drop voltage of 3V, so the supply voltage needs be 3V higher than the LED voltage??? So, to power a 12V LED, should I use a minimum of 15V power supply? Or if the current is a constant 1A, the LEDs will glow brightly in 9V also?

    • according to me LM338 series ICs are specified to drop not more than 0.5V across their input/output terminals, can you tell me where did you find the 3V result?

    • Hello, I made the first circuit shown here. I connected 2 x 5watt LED in parallel to a LM317. The LED run at 12V.
      So, from calculation I(LED) =0.4A each.
      The resistance value= 1.25/0.8= 1.56 ohms. I connected 3 x 0.5ohm resistors @ 5W. The LM317 heats up a bit, but it is okay with a heatsink, but the LEDs heat up a lot. I measured the output of the LM317 with a multimeter, it shows 12V and 2.3A?? Also the LM317 became very hot when I was measuring the current only. Why is that happening? Is the LM317 burnt out???
      P.S.: My power is 12.3V & 3A.

    • the 3V drop is due to the generation of heat from the IC and not because of the IC internal circuit, but anyway that means a 3V higher input will be required.

      how did you measure the output current? It must be done by connecting the ammeter in series with the LEDs, if you connect the meter probes directly to the output terminals then the IC will get shorted and heat up.

      The LeDs will need the specified amount of heatsink for cooling, which could be sufficiently large.

      LM317 has internal thermal and short circuit protection so most probably it might be still OK, but if it's a duplicate IC then the results could get seriously affected

    • I measured current using a multimeter, by connecting the same way as when measuring voltage only changing the red wire to the ampere measuring hole. I took the readings from the point where I connect the LED. Can you explain where the 2.3A came from? If it is a current limiter circuit, it should always provide 0.8A whatever the load.
      Anyway, is my calculations correct???

    • Everything okay now, the output is coming 1.2Amps. Previously, perhaps the LM317 had shut down due to overheating. Thank you for your support.

    • yes even with an output short circuit the current should be limited, not sure how it was showing 2.3 amps, anyway it's good to know that it's working now.

      with over heating of the IC the output must shut down to zero.

    • Got a heating issue with the LEDs. I will run the LEDs at 9V so that the heat will be less as the LEDs will be in a closed chamber.
      Will the same circuit run from a 9V power source?
      The LM317 (with heatsink) also heats up a lot after few minutes, is it normal or there is any over voltage or over current?

    • If your current reading is showing the specified 1 amp then the heating of the devices is not an issue, so better confirm the reading by putting the ammeter in series with the LED once again.

      please remember that the LeDs will need a finned type robust heatsink otherwise these may get damaged permanently

      LM317 will surely get hot a lot, so you may want to attach the IC with the LED hetasink for double protection………use a mica isolator for the IC

  15. Dear Mr. Swagatam,
    I would like to make a LED driver circuit which consist 4 nos of 10W LED. And to make it compact it is needed to be transformerless. Voltage and current rating of 10W LED is 9-11V and 1050mA respectively. Please suggest me to build it.

  16. Am I correct in understanding the voltage drop across a led is just a characteristic of that led in relation to the current. So as long as I have the correct current it should be okay because my supply can easily supply enough voltage (Vf). After lots of reading, other web sites indicate that you do not get the voltage out that you put into lm338 as indicated in this post. They state you get somewhere around 4 to 5 volts less making them pretty inefficient and if I understand this correctly the bigger the gap between the input and output voltage the more energy is turned to heat. So finding the sweet spot on the input potentiometer is important to keep things cool and efficient.

    • I have used the IC myself plenty of times and have never faced this issue, if this would be the case the manufacturer would have clearly mentioned it in the datasheet. The datasheet boasts the device to be extremely versatile and efficient as far as voltage and current regulation is concerned, so I don't think the allegations are true

      The electronic parts market is filled with duplicate stuffs, and LM338 id no exception, people who complain could have possibly procured a duplicate or a faulty device from the market

    • By the way input should be adequately rated with the amp capacity, meaning it should be rated much higher than the required specs of the load.

    • …yes the device could dissipate appreciable amounts heat at optimum loads, but that's a different issue and could be compensated by applying a heatsink to the device and using sufficiently rated input source.

  17. Ok got it work with a different IC. The output voltage is quite a bit lower than the input voltage though. I found winding the input voltage from 20 volts up to 28 volts the output voltage driving the led remained the same at 19.7 volts. The current increased till it hit 24v and started decreasing above 24v. So it seems 24v input would give me 19.7v 3600mA output which was the sweet spot where I could get a max of 70 watt to drive the led. I am not sure why I was limited to 19.7 volts as I would like to drive the led a bit harder. Any ideas? I thought maybe ohms law fixed current and resistance locked the voltage at that level but then I remembered leds don't abide by ohms law.?????

    • Hi Shane, if the voltage is not going above 19V without the LED connected, there could be something wrong with the IC again or the wiring, if it's happening with the LED connected, remove it and check the same without any load, it should increase to 24V, and that could be considered as normal.

      Under any circumstances, the voltage should be on par with the input and current not exceeding the calculated value, without load and with load respectively

      The resistor is not supposed to lock the voltage in the recommended configuration, it's specifically designed for controlling current, not voltage.

      Everything will abide by Ohm's law, only the way to understand it could change as per the specifications of the particular component

  18. Hi, Swagatam Majumdar,

    I did as you suggested and built the cct but was unable to get the results indicated above. I am hoping you may point out my error.
    My supply voltage is 24 volts, IC is LM338T. I=3.78A R=0.36ohms.
    Left to right pin out of IC (writing to front) Adj, Vout, Vin.
    Resistor connected between Vout and Adj.
    As a variation I also tried a 0.8ohm, 0.33ohm and no resistor and get exactly the same values on Vout across the load (100 watt 24v led) as I do with the 0.36 ohm resistor.
    The values I get is 19.7 volts and 3.7 amps.
    I tried turning my pot up to 28v on the input and that just reduced my current to the load but kept the voltage on the load at 19.7 volts.
    Do you have any pointers?
    Thanks for your help.


  19. sir i used transformer 9-0-9 which draw total 19.1v AC after that is used LM317T for DC supply. At output i get 24.3v DC constant. I used this supply for 5 1watt ultra bright LED in series along with 6ohm 1 watt resistor. but still i dont get sufficient illumination. As i used total 5 LED in series and 4 in parallel so its 20 LED i used. my question is how we can improve illumination & is it required current limiter circuit in above article in this application.

    • each LED series will require a 25 ohm resistor, not a 6ohm….may be all your LED sgot damaged and could have become weak.

      Use this formula for calculating the resistor

      R = Us minus total LED drop divided by led amp

      R = 24 – 0.3×5 divided by 0.3 = 25 ohms.

    • i think above calculation are like this R= (24- (3.3*5))/3= 2.5ohms &
      Watt= [ 24- (3.3*5)] * 3=22.5 watt as per your calculation in above article.
      in last comment calculation value= R= ( 24-0.3*5)/0.3= 75ohm
      so how much value of Resistor value with watt for each string for maximum illumination.

    • sorry I wrote 0.3 instead of 3.3…the above calculation should be as like this:

      R = (24 – 3.3*5)/0.3 = 25 ohms…wattage = (24 – 3.3*5) * 0.3 = 2.25 watts

    • when the LEDs are in series the amp will not increase, it will be equal to the single LED amp spec, so here I have taken it as 300mA or 0.3 amps for each of the strings

  20. Hi sir can you tell me what is current generator circuit & current limiter circuit? actually how functioning those for LED driving. I have 24 volt 5 amp supply & 1 watt LED in series 7 no so is it necessary to use current generator or limiter after supply?

    • Hi Ashok,

      your power supply is a current generator.

      an example current limiter design is shown in the above article, please read for a detailed view.

      current limiter becomes essential when high watt LEDs are incorporated, so it's a must for your application too.

    • ok sir. I want to used 1 watt LED in 7 no series and 14 no line in parallel total 98 no LED's. Supply is 24 volt 5amp. so how can i connect them?. as you comment above (vinu subhash 20 may) as 6 ohm resistor. Is it use or not? Current limiter circuit is used or not for this my LED bank? if yes how & what no of current limiter circuit is used?

    • connecting 7 in series for a 24V supply would allow you to avoid and eliminate the resistors for the individual strings, however make sure the voltage from the source never exceeds 24V.

      For limiting current you may use the first design given in the article above…..use LM338 for the IC and 0.3 ohms, 1/2 watt for the resistor

    • hi sir i got it. supply is constant because i use SMPS supply. one another question is 0.3ohm is minimum value? because below 1 ohm thr is no resistor available in local market

  21. Great now that all makes sense. But in practice I still am unable to replicate the current using NI Multisim. I have used 32 volt supply 0.36 ohms connected between Vout and adj.
    I figure this would give me approx. 3500 mA. But don't know how to insert LM338 into the software as its not there by default and I can't find any instructions that I understand to insert it. It was suggest by NI to use the LM117 but the readings I get are far from what I expected. It may be due to the current limitation of 2A. I still was getting low load voltage and low current with the correct components for a 50 watt load which should be fine the LM117. Oh well.
    Thanks for all your help. Really appreciated.

    • I don't think a simulation would be necessary, simulators are not always correct, it's better to build the circuit and verify the results using a digital multimeter, that would enable you to understand the design practically, tweak it personally and also confirm the final results.

      A simulator will only confuse you more.

  22. Good work here but I think I misunderstand a little and hope you may be able to clear it up.
    I worked out for first cct I needed 10 ohm resistor for R1 which seems very inefficient. Could you point me in the right direction please.

    I have a 100 watt high power led I plan to supply 32 volts and 3100 mA. What resistor would you use. Could you provide ohms and wattage of the resistor please so I can reverse engineer your maths for different leds I have. I think I am not understanding Vref. I thought it would change with your voltage but you seem to keep it at 1.25v. Maybe you could clear this up for me. Much appreciated. Regards,

    • How did you get 10 ohms:) as per my calculations it should be 0.4 ohms and the wattage should be 1.25 x 3 = 3.75 or 4 watts

      The resistor is calculated in the following way:

      R = 1.25/3 = 0.41 ohms

    • Yes, I was not understanding the maths. I read the data sheet and kept working on your examples and realised the 1.25 was a constant. Thank you for your reply.
      So would I be correct to assume the internals of the IC cause voltage difference of 1.25 volt between Vadj and Vout. Which would then give me 1.25 volts across the 0.41 ohm resistor and supply the load with the input voltage.
      Thanks again. Great work!

    • correct! That's how it's supposed to be…..1.25V across the calculated resistor for achieving the desired current level, and that's why we multiplied 1.25 with the amps for getting the wattage of the resistor.


  23. mosfets like LM7805,7808, 7812 , 78XX series voltage regulators will give constant voltage with max of 1A output. So if I need a constant voltage like 5v, 8v, 12v with more than 1A means wat mosfets I can use??

    • you can use a transistor parallel with the IC as shown in the following post with calculated resistors as per the shown table.


  24. Thankyou… So no need of current limiter circuit. The circuit which I designed is enough for constant voltage and current. Am i correct??

  25. Hai, I designed a circuit like this imgur.com/Xc2J8pl
    LED used is 1watt led which need 3.3v, 350mA. I made two strings. Each string consists of two LEDs. Connected two strings in parallel. So the LED bank need 6.6v, 1.4A. Am using 13.5v smps adaptor. Connecting the adaptor to LM7808 to obtain constant 8v, 1.5A. Then connecting the output from the LM7808 to LED string through resistor. so the output from each resistor will be 6.6v, 350mA. I think this will be a constant output. So do I need to add a constant current limiter circuit??

    • Hi, each string will consume 350mA, not 700mA, the total will be 700mA.

      the resistors will take care of the current. for more safety use a common heatsink for LEDs and the 7808 IC this will prevent the LEDs from getting too hot.

  26. Hai, thanks for the calculation. Then Am having 13.5v ,1 Amp smps adapter and 12v , 5 Amps battery. The full charge of the 12 v battery will be 13.5 v. I like to connect these both adapter and battery with a relay to the LED panel which consists of three 1watt LED in series. I need a constant 9.9v to the LED panel. what can I do for that?? Is that enough to add a resistor in the series of LEDs?? And If I add a resistor means , can I get a constant input to the LED panel from the smps adapter??

    • yes, if your 13.5V is almost constant and the ambient heat does not rise by too much then a series resistor and optimal heatsinking will be enough for the LEDs.

    • Thankyou. suppose if my LED panel needs 13.5v and I am having 13.5v dc smps adapter, can I connect the adapter directly to the LED panel or do I need any resistor??

    • ok. So for the IC LM317, LM338, LM196, the input Amphere can be anything.
      LM317 capable of giving max output of 1.5A. LM338 capable of giving max output of giving 5A. and LM196 capable of giving output of 10A. Am I correct??

    • input current does not matter, neither will the output consumption, it's the input voltage that must not exceed 35V, rest everything is internally protected for these ICs.

      The output capacities that you have mentioned are all correct.

  27. Hi Sir,
    How U??? I have connected 30x1w leds….3 leds in a string totally 10 strings…..my input voltage is 12v 5amps…..how should i wire up these led arrays to the input source…..aim of the circuit: led should glow at full brightness, less heat generation.

    • Hi Vinu,
      connect 6 ohm 1 watt resistors with each string otherwise your LEDs could burn at 12V.

      use the first circuit in the above article in between the 12V and the LED, meaning the 12V must pass through the LM338 circuit before reaching the LEDs.

      select R1 = 0.5 ohms 2 watt

    • Hai, Can u tell me that how u calculated the R1 should be 0.5 ohms, 2 watt in which u replied for Vinu subash?? Bcoz I am also doing LED panels for my home. So need a clarification on calculation.
      And am having another two doubts…
      Q1:- 1. 10 LEDs of 1 watt each connected in series which require 33v, 350 mA,
      2. 10 LEDs of 1 watt each connected in parallel which require 3.3v, 3.5 A.
      which of the above two will be more energy efficient??
      Q2: 1. A 11 watts CFL bulb require 240v AC,
      2. A 40 watts LED panel require 26 v DC.
      which of the above two will be energy efficient??
      If possible means pls explain..

  28. what should be the watts for R1 resistor?? suppose in the above circuit, the input is 12v 1A and the output is 12v 700mA by fixing the R1 as 1.78 ohms. And how to calculate it?

  29. Hai,
    In the above current limiting circuit, the output voltage will be equal to input voltage, Am I right??
    I.e. If the input is 12v, 1A means, by a suitable resistor of R1, I can get 12v,700mA.
    And can give the input as 5V, 1A to get 5V, 700mA as output in the above circuit??

  30. Hi Sir,
    I need a simple boost converter….my spec is 12V 35A car battery to 35v 4A & another circuit is same p0wer source 12V 35A to 70V 7A……

  31. Hi Swagatam,
    LM317 circuit with power supply of 12v 2amp (or 1.5amp?)- by the explanation I can use 5 nos of 1 watt LED and the resistor required will be: using R1 = Vref/current; or R1 = 1.25/current. we get for 12v 2 amp supply R1 = 1.25/2 = 625 ohms and 2.5 watts (3 Watt resistor)
    and for 12v 1.5 amp supply R1 = 1.25/1.5 = 0.833 ohms and 2 watts
    for 12v 1 amp supply R1 = 1.25/1.5 = 1.25 ohms and 1.25 watt

    Are these calculations correct? I want to use five 1 watt Led's for a start. what is the maximum number of LED's I can use in this circuit all (1 watt). can I make a LED tubelight with this (using 40 of them)


    • Hi Gopal,
      yes the calculations are correct.

      with a 12V/1.5amps output, it can accommodate not moire than 9nosof 1 watt leds, made by connecting 3 strings of 3 leds each in parallel.

      for 40 leds you can replace LM317 with LM338

  32. Hi, Thanks for all the help. I understand will do the same as you told. Do i need any kind on rectifier or capacitor in the circuit?

  33. hello. i want to use LM317T IC instead mentioned one. i need 10volt 700mA on output. i have 12v 5A adapter as input. what exactly i need to add or remove? or can you kindly show me another circuit please? i want to run some high watt led. actually 6 of those led at once. LED s are rated 10v and 650mA. please help sir! And thanks for all help.

    • LM317 will not work, since parallel connection of the LEDs will require 650 x 6 = 3.9 amps.

      You will need an LM338 with the first circuit configuration.
      For R1 you can use 0.4 ohms, 1 watt resistor
      the leds can be connected in parallel at the output, and the input of the IC can be connected with the 12V /5 amp supply.

    • Ok. ill then use 338. No extra Resistor needed as 2nd schematic (R2) ? like those resistors of led array? also may i know how to determine R1 ? say, if i need to drive those led at 10v and 500mA, what i need to do?

    • A series resistor could be included as given in the second diagram, however even if it's not included the LEDs would be safe due to R1 which will never allow the current to go beyond the unsafe level.

      The formula for determining R1 is given in the article.

  34. Dear Sir ,
    Will you plz give me materials or link related with power/current limiting ckt . that can be used up to 40 watts which can be used instead of mcb …..as in our market we didn't get the mcb that can meet our requirement (that trip in 40 watts)….I would be grateful if you help me in this project.

  35. Hi Swagatam,

    Its nice to read your knowledgable blog, I'm a Street Vendor in Delhi have 7 Outlets as of now and in search of a lighting system for the vending counters.

    I Don't have electricity available on the streets hence have to rely completely on Battteries
    I'm Currentlky using 80 Watts 1 watt LED Strips that operate with 12 V Battery, But want to increase the lighting to around 400 Watts with minimum powerc onsumption of battery Power.

    Reason being the light shave to stay on with full power for around 7 Hours.

    I'm Not able to find possible solution to my Problem, would be great if you could help.

    • Hi anjain


      An LED system is itself the most economical lighting option available to date, it cannot be further modified in any manner for getting more than what its been specified at.

      But you can ensure that your lights are optimally tuned by employing all the related parameters correctly…. to be precise as discussed in the above article.

      If you follow the conditions as explained in the above article you can be sure of having the most efficient system in hand.

      I you already have a current controlled circuit in your LED system then the above circuit won't be required.

  36. Dear sir,
    If i am replacing LM117 with LM338 for the application of a battery charger which is supposed to be charged at a rate of 5A maximum, what will have to be the specifications of resistor R?''( Value and wattage ). The transformer current is rated above 20 A at secondary.
    And, Will the IC LM338 be able to accept that much current to its input prior to limit it's value???????

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