So far in this website we have studied LM317 based linear power supply circuits, here we will learn how an LM317 can be executed as a variable switch mode power or SMPS with zero loss.
LM317 as Linear Regulator
We all know that an LM317 IC is internally designed to work as a linear voltage regulator IC, which has a serious drawback of power dissipation through heating. Moreover such topology also requires the input to be minimum 3V higher than the desired output, adding further restrictions to the given regulator configuration.
Here we discuss how the same IC could be simply implemented as a 0-40V variable power supply using SMPS topology and therefore eliminating the losses mentioned in the above paragraph.
Modifying LM317 Circuit into a PWM Switching Regulator Circuit
The LM317 variable SMPS circuit explained here effortlessly converts an ordinary LM317 IC into an inductor based switching regulator power supply counterpart, as exhibited in the following diagram:
Circuit Diagram
Referring to the above shown diagram we can see that the LM317 is configured in its usual variable regulator mode but with some additional parts in the form of R6, C3, and D1.
We can also see an inductor attached with D1 and an associated power BJT Q1.
How it Works
Here the LM317 IC performs two tasks together. It varies the output voltage through the indicated pot R4, and in turn causes a PWM to initiate for the base of Q1.
Basically, the introduction of R6/C3 transforms the LM317 regulator circuit into a high frequency oscillator circuit, forcing the output of the LM317 to switch ON/OFF rapidly with a varying PWM, which is dependent on the setting of R4.
The BJT Q1 along with the inductor L1 and D1 forms a standard buck converter circuit which is controlled by the above explained PWM generated by the LM317 circuit.
This implies that while the pot R4 is varied, the voltage pulse width developed across R1 also varies proportionately causing Q1 to switch L1 in accordance with the varying PWMs.
Higher pulse widths enable the inductor to produce higher voltages and vice versa.
Capacitor C4 makes sure that the fluctuating output from L1 at the output is adequately smoothed and eliminated, this consequently enhances the ripple current into a stable DC.
In the proposed LM317 switch mode power supply circuit since the IC LM317 is not directly involved with the handling of the load current, it's restricted from dissipating current, and thus ensures an efficient regulation of the high input voltage into the desired low output voltage levels.
The design also allows the user to upgrade the circuit into a high current SMPS circuit simply by changing the Q1, L1, D1 rating as per the required output current specifications.
L1 can be built by winding bifilar enameled copper wire over any suitable ferrite core.
Although this LM317 SMPS circuit promises a near zero loss output, Q1 must be mounted on a heatsink and some degree of dissipation may be expected from it.
franz says
hello swagatam
what is the voltage of the tantalum capacitors of 50uf x? and of 100ufx?
Swag says
hello franz, it can be anything above 60V
lm339n says
can i replace pnp transistor with npn transistor? and instead of connecting base of npn to input of lm317t i rather connect it to vout
Swag says
try the last concept from the following article
https://homemade-circuits.com/how-to-make-current-controlled-12-volt/
jindro says
Sir good morning! I can’t find tip127 sir could I use 2 tip 32 as darlington configuration? any suggestion for any parts changes Sir? tnx!
Swag says
Jindro, try a single TIP32 first, if it doesn’t work efficiently then you can upgrade it into a Darlington with a 2N2907 transistor
H. Foster says
Hello Swagatam,
I first want to say I enjoy reading your articles and how toys. I’ve watched you on YouTube and read your Instructables for a long time now and they are vey informative.
I have a question about power and voltage in this circuit, how to apply the changes and if this would work for a diy bench top variable psu?
If I were to change q1 to a higher current transistor, do I only need to satisfy l1 and d1 voltage and current requirement for the circuit, or is a consideration needed for the base current of q1 in relation to lm317’s output driving q1’s base?
I am still a novice electronics hobbyist, and though I have firm understanding of application and mechanics, the equations are still elusive.
Thank you for all of your wonderful advice and instruction.
H. Foster
Swag says
Thank you H. Foster, I am glad you liked my site, and I appreciate your involvement very much!
For acquiring more output current the base resistor of Q1 can be ignored to an extent, unless the output current requirement is significantly high, in that case the 22 ohms will need to be increased a bit and experimented. Or a Darlington BJT can be used for Q1 which will allow the 22 ohm value to be consistently without any change for any output current.
However, Q1, L1 and D1 will need to upgraded accordingly. For L1, the average wire surface area will need to be increased by winding bunch of thinner wires together, whose mass may be equivalent to the calculated single thick wire’s mass approximately.
Q1 rating will also need to be enhanced as per the required output amp specs….
Similarly D1’s current handling capacity will need to be upgraded as per Q1’s specs.
If you have any further questions, please feel free to discuss with me here…
H. Foster says
I was thinking on using 2n5683 or MJ11015 to get up to 30 amps. I plan on using it in this circuit with a computer psu to make a diy variable bench power supply output. I believe the MJ11015 is a Darlington, not sure about the other. Am I exceeding their specs for this application or would line be better than the other. They are both inexpensive, which is what I’m going for. ?
And I’m correct in thinking I need a pnp, not an non, yes?
Thanks again for the assistance.
Swag says
yes, it should be a PNP, the MJ BJT is Darlington but an NPN, so it won’t work.
2N5684 seems like a better fit, you can find the datasheet here
https://www.onsemi.com/pub/Collateral/2N5684-D.PDF
although it is not a Darlington, can be modified with another attached PNP such as TIP32 for converting it into a Darlington pair.
H. Foster says
Thanks again for the discussion Swagatam. I was looking at the mj11015g specifically because it is a pnp in a Darlington setup. Here’s the datasheet link at alldatasheets – www.alldatasheet.com/datasheet-pdf/pdf/426442/ONSEMI/MJ11015G.html
This way I don’t have to about current and voltage changes on the base and its resistor and its all in one package.
So am I correct in thinking the only additional considerations needed are for d1 and l1 current and voltage requirements? If I pick those to conform to the 30 amp max of q1 I should be set to build?
You have been most helpful and I truly appreciate it.
Thank you.
Swag says
oh yes MJ11015g is a PNP, I am sorry, I mistakenly saw the search description for MJ11016 and assumed it wrongly.
In that case it can be definitely used for the discussed purpose, and the only factors that will need to be considered are L1, and D1, according to my knowledge.
you are absolutely correct with your conclusions regarding picking 30 amp specs for D1 and L1, and you can proceed with the assembly.
If you have more doubts, let me know, I’ll be most happy to help!
jindro says
Sir can I use tip127 for q1? I want to build this for 5 li ion batt parallel charger or cellphone charger. Your other circuit is better but parts here in my town is so hard to find, I need simple and less parts like this tnx
Swag says
Hi Jindro, yes you can try TIP127
febry says
I have a projeck SMPS 48v 20 ampere ,, can you send me the schematic and component list via email ?? your help will be very helpful, .thanks sir,,
febryandespa@gmail.com
Swag says
you can try to modify the following concept and get the required results
https://homemade-circuits.com/smps-2-x-50v-350w-circuit-for-audio/
mayokun says
hello sir,please i have a project i would love you to help me with.will love to e-mail the details to you.i wouldnt mind if you give me your email.thanks alot
Swag says
Hello Mayokon, you can feel free to share it here through comments, I’ll try to help!
Anonymous says
Hi Sir. how can i get 4vol 5amp throught this smps curcuit
Swagatam says
replace Q1 with around 8amp rated device and adjust the pot for getting 4V/5amp
Rabie Rheribi says
HI sir what about curent limitation of lm317 !how much power can give this socket ? what i have to do if i want to use li ion battery 3.7 volt whit this !thank you à lot !you are the best
Swagatam says
Hi Rabie, current is supplied by Q1 not by the LM317, LM317 is used just for initialization of the buck converter and for the PWM control.
the output current is dependent on the inductor specs and the input supply current range.
Sherwin Baculi says
Please tell me the wattage of all resistors. Ty
Swagatam says
when not specified assume it to be always 1/4 watt rated.
bashir abdulkadir says
Sir i have make an an inverter instant to rate at 220v but it rating at 320. what shall i do to decrease it to 220 ?? And later that i just observe that may be because i am using 6-0-6 transformer 12v battery but last time u say if going to use 6-0-6 transfer the battery also must be 6v so please how shall i reduce the battery from 12v to 6v ??
Swagatam says
Bashir, use a 9V battery with your 12-0-12/320V transformer, that will provide you with approximately 240V at the output
Ainsworth Lynch says
Wouldnt that be low voltage at the output if he is to use a 9v battery with a 12-0-12v transformer.
He has a 12v battery wouldnt it be best for him to use a 10-0-10v transformer?
Swagatam says
he wants to reduce 320V to 220V that's why I recommended the above method.
chucks ichoku says
hello sir good morning
please I want to know, wher can someone apply the above circuit???
Swagatam says
hello chucks, you can use it as a power supply circuit for getting an efficient power output and also as a solar charger. It can be also modified to work as an MPPT
Joseph Gynferd says
Sir can I replace bjt/pnp transistor with MOSFET P-channel? sure i will try this after work ^_^
by the way sir is this circuit applicable also on LM7812 or 7805 regulator??
Thanks again Sir. Have a nice day.
Swagatam says
Joseph, if you are using a mosfet then the 22 ohm resistor will need to be increased proportionately such that it develops the required 9V for the mosfet gate to trigger it satisfactorily.
No, it is applicable only with LM317 or LM338.