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LM317 with Outboard Current Boost Circuit

LM317 with Outboard Current Boost Circuit

The popular LM317 voltage regulator IC is designed to deliver not more than 1.5 amps, however by adding an outboard current boost transistor to the circuit it becomes possible to upgrade the regulator circuit to handle much higher currents, and upto any desired levels.

You might have already come across the 78XX fixed voltage regulator circuit which are upgraded to handle higher currents by adding an outboard power transistor to it, the IC LM317 is no exception and the same can be applied for this versatile variable voltage regulator circuit in order to upgrade its specs for handling massive amounts of current.

The Standard LM317 Circuit

The following image shows standard IC LM317 variable voltage regulator circuit, using a bare minimum of components in the form of a single fixed resistor, and a 10K pot.

This set up is supposed to offer a variable range of zero to 24V with an input supply of 30V. However if we consider the current range, it's not more than 1.5 amps regardless of the input supply current, since the chip is internally equipped to allow only up to 1.5 amps and inhibit anything that may be demanding above this limit.

LM317 regulator circuit

The above shown design which is limited with a 1.5 amp max current can be upgraded with an outboard PNP transistor in order to boost the current on par with the input supply current, meaning once this upgrade is implemented the above circuit will retain its variable voltage regulation feature yet will be able to offer the full supply input current to the load, bypassing the IC's internal current limiting feature.

Calculating the Output Voltage

For calculating the output voltage of a LM317 power supply circuit the following formula could be used

VO = VREF (1 + R2 / R1) + (IADJ × R2)

where is = VREF   = 1.25

Current ADJ can be actually ignored since it is usually around 50 µA and therefore too negligible.

Adding an Outboard Mosfet Booster

This current boost upgrade can be implemented by adding an outboard PNP transistor which may be in the form of a power BJT or a P-channel mosfet, as shown below, here we use a mosfet keeping things compact and allow a huge current upgrade in the specs.

adding an outboard PNP transistor

In the above design, Rx becomes responsible for providing the gate trigger for the mosfet so that it's able to conduct in tandem with the LM317 IC and reinforce the device with the extra amount of current as specified by the input supply.

Initially when power input is fed to the circuit, the connected load which could be rated at much higher than 1.5 amps tries to acquire this current through the LM317 IC, and in the process a proportionate amount of negative voltage is developed across RX, causing the mosfet to respond and switch ON.

As soon as the mosfet is triggered the entire input supply tends to flow across the load with the surplus current, but since the voltage also begins to increase beyond the LM317 pot setting, causes the LM317 to get reverse biased.

This action for the moment switches OFF the LM317 which in turn shuts off the voltage across Rx and the gate supply for the mosfet.

Therefore the mosfet too tends to switch OFF for the instant until the cycle perpetuates yet again allowing the process to sustain infinitely with the intended voltage regulation and high current specs.

Calculating Mosfet Gate Resistor

Rx may be calculated as given under:

Rx = 10/1A,

where 10 is the optimal mosfet triggering voltage, and 1 amp is the optimal current through the IC before Rx develops this voltage.

Therefore Rx could be a 10 ohm resistor, with a wattage rating of 10 x 1 = 10 watt

If a power BJT is used, the figure 10 can be replaced with 0.7V

Although the above current boost application using the mosfet looks interesting, it has a serious drawback, as the feature completely strips off the IC from its current limiting feature, which can cause the mosfet to blow-of or get burnt in case the output is short circuited.

To counter this over-current or short-circuit vulnerability, another resistor in the form of Ry may be introduced with the source terminal of the mosfet as indicated in the following diagram.

The resistor Ry is supposed to develop a counter voltage across itself whenever the output current is exceeded above a given maximum limit such that the counter voltage at the source of the mosfet inhibits the gate triggering voltage of the mosfet forcing a complete shut off for the mosfet, and thus preventing the mosfet from getting burnt.

LM317 outboard boost mosfet application circuit

This modification looks pretty simple, however calculating Ry could be little confusing and I do not wish to investigate it deeper since I have a more decent and a reliable idea which can be also expected to execute a complete current control for the discussed LM317 outboard boost transistor application circuit.

Using a BJT for Current Control

The design for making the above design equipped with a boost current and also a short circuit and overload protection can be seen below:

LM317 outboard boost transistor with short circuit protection

An couple of resistors, and a BC547 BJT is all that may be required for inserting the desired short circuit protection to the modified current boost circuit for the LM317 IC.

Now calculating Ry becomes  extremely easy, and may be evaluated with the following formula:

Ry = 0.7/current limit.

Here, 0.7 is the triggering voltage of the BC547 and the "current limit" is the maximum valid current that may specified for a safe operation of the mosfet, let's say this limit is specified to be 10amps, then Ry can be calculated as:

Ry = 0.7/10 = 0.07 ohms.

watts = 0.7 x 10 = 7 watts.

So now whenever the current tends to cross the above limit, the BC547 conducts, grounding the ADJ pin of the IC and shutting off the Vout for the LM317

Using BJTs for the Current Boost

If you are not too keen on using  mosfet, in that case you could probably apply BJTs for the required current boosting as shown in the following diagram:

LM317, LM338 current boost using outboard transistors

Courtesy: http://www.ti.com/lit/ds/slvs044x/slvs044x.pdf


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!

56 thoughts on “LM317 with Outboard Current Boost Circuit”

  1. Hi swagatam first congratulation for your hard work .
    i’m a hobbyist and i want to ask stupid question 🙂 with your design can i upgrade currant i mean from transformer 4 ampere can i have approximately 20 ampere ?
    If not you have project or solution for simple project to have big currant low voltage ? 20amp transformer is so big not practical for the project i want to make thank good jobs br!

    • Thank you Issam, voltage and current can be changed using a transformer but the wattage will always remain the same. For example if you have a 4 amp 25 V supply, it can be changed into 5 V 20 amp. If you multiply the two V x I you get 100 watts on both sides, so it works in this way.

  2. Hello sir, i two questions to ask pls:
    1. can i use TIP127? If yes how can i connect the pins to the circiut?
    2. While the transistor increases the current, is the output current of the circuit equal the transistor’s current or it will be plus the ICs current? Thanks

    • 1) You can use TIP127, pin connections will be similar to what is shown in the diagram.take the help of the datasheet of the transistor
      2) The output current will be equal to the sum of both IC and the transistor

  3. Hi Swagatam,
    I would like to confirm with you the calculation of Rx & Ry.
    I require 3A max ( even though the TIP34C are rated at 10A) from the power supply. I use a BJT (TIP34C) instead of a Mosfet in my circuit.
    Therefore to calculate Rx & Ry I used the following calculations:
    Rx = TIC34C Trig. V/LM317V Optimal Current
    Rx = 0.7V/1A=0.7R
    W = 0.7V x 1A=0.7W
    Ry = Trig. V BC547+LED Forw. V/Current Limit TIP34C
    Ry = 0.6V + 3.6V/3A = 1.40R
    W = 4.2Vx3A = 12.6W
    Are my calculations correct to achieve 3A output.
    Your help will be much appreciated
    Kind regards

    • Hi Jan, referring to the last diagram all your calculations look OK to me, the LED is supposed to be in series with the base resistor of BC547 which is shown as 47 ohm

    • Hi Swagatam,

      In the following post you said the LED must be in series with the emitter of the BC547. Now you say it must be in series with the base resistor? How do I calculate Ry then?
      Swagatam says
      March 19, 2017 at 7:20 am
      connecting it in series with the emitter will be a better idea and a resistor can be avoided.

      but in that case make sure the Ry formula is modified in the following way:

      Ry = 0.6 + LED fwd Voltage / current limit value

  4. Hello, Mr. Swagatam.
    Can I change the LM317 30v IC to the 60v LM317HVT to have a higher Vout and maintain the IRF9045?
    Would something like Vout = 0 – 60v and Iout = 10 amp = +/- 600W!?

    Thank you
    Amadeu Mendes

  5. Hi swagatam, i wanna try this circuit, and i want to ask if is too complicated if i want to make it with adjustable current about 3amp.
    i alway like your design

  6. Hi! Thank you for this helpful article!
    I wonder how much power is dissipated by the MOSFET. I think it is U * I where U is the input voltage minus the output voltage and I the current going through the transistor. Do you confirm ?

  7. Thanks for providing such a clear and simple explanation of how the outboard transistor works with the lm317. I couldn't get my head round how the external transistor was able to provide the correct voltage alongside the lm317 now that I know, it seems so simple, thanks.

  8. Hi Swag,
    I used this circuit, because i tried to reach 5Amps max. Ry=0.14 Ohm.
    But whenever i short circuit the output, the current goes 8-10Amps and the cut-off is not working.
    I did a modification, connecting the Pot (10K) between collector and emitter on BC547 but still the same result. As i observed, whenever i do short circuit i don't have 0.7V on the base of BC547, but 0.55V. And on the collector voltage varies 1.6-2.5 depending of the position of the potentiometer.

    • Hi Nikola, you can try the second circuit and dimension Ry with some trial and error until the current is restricted at 6amps max.

      Yes the the third design might require some improvement, because the BC547 set-up might not be able stop the mosfet from conducting completely.

      0.55V is sometimes enough to enable a BC547 to conduct…

  9. No the shown configuration in the last diagram is correct…the actual ground is supposed to be connected to Ry and the emitter of BC547 because the return path of the current must pass through Ry before reaching the transformer ground.

  10. sir i am feeding the input pin of lm317 with 48V dc.. i need an output voltage of fixed 12V and 10A.. what modification i have to do ?
    Moreover i dont have 0.07 ohm resistors with me.. the only resistors i am having are 3 no.s of 0.1 ohm 10 W resistors.. so please suggest a suitable circuit

    • RT, You can try the last circuit and calculate Ry as per the formula.

      you can use many 0.1 in parallel which may yield 0.07 value approximately.

      No other modification would be required according to me.

  11. Hi Swa,
    how are you ?
    If i want to put an LED as short circuit indicator in series with the BC 547 collector, do we need to put a resistor in series to the LED? If so, how big is the value of the resistor?

  12. The last circuit design link seems to be broken, so I cannot see the circuit please upload a new one..And I think heatsink would be required to LM317 as well.

  13. Hi Swagatham
    An LED or a Piezo buzer with internel ocilator can be added to indicate the short circuit condition. Where can I connect it in the last diagram.
    Can I use an N-channel mosfet instead of P-channel after some modifications in the circuit….? If yes, how.

    • Hi Anil, using an NPN transistor will make the design very complex, so PNP is the only easier option…connecting a buzzer will be difficult because there's no appropriate space for it in the above design, however you can use a red LED in series with the BC547 collector for a short circuit indication.

    • resistor for the led on the collector of bc547 can alter the design? what is the voltage on the collector of 547 transistor and how can I calculate the resistor for the led?

    • connecting it in series with the emitter will be a better idea and a resistor can be avoided.

      but in that case make sure the Ry formula is modified in the following way:

      Ry = 0.6 + LED fwd Voltage / current limit value

  14. Hi Swagatam

    Many thanks for this circuit. This circuit will be very useful for every electronic technicians, especially for beginers.

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