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.
Interesting Feedback from one of the Avid Readers:
Mr. Swagatam:
I am a retired EE, but continue to have an interest in various areas. Happened to come across your website when I was researching power supplies using LM317.
Saw the interesting switch mode power supply schematic using the LM317.
As it turns out, the exact circuit shows up in the 1978 National Semiconductor Voltage Regulator Handbook, with additional verbiage to explain its behavior.
However, I found it even more helpful to simulate the circuit using LTSpiceVII (which is free to download and use) to get a better idea of how the circuit operates with component value changes.
Anyway, I decided to scan the two pages from the 1978 Handbook and email to you in the event you care to post them with the schematic for others who might be interested in a little more detail.
Regards,
Denton Conrad
Raleigh, NC
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Hi Swagatam
Thank you immensely for all you help and guidance. I shall try to get all the components that are required to build the LM 317 SMPS circuit once the shops are opened again and shall let you know how it went .
Till the next time all the best to you,bless you
Kind regards
Vee
My pleasure Vee, I wish you too all the best!
Hi Swagatam
In the manual it does not show or mention what the 36 volts max is but in the pictures on the net it shows “ Capacitance test sockets” across the two sockets where it says max 36 volts what are these sockets for, does it mean we can test capacitors with the two leads in these sockets and is this the volts of the capacitor value
Thanks
Vee
Hi Vee, I saw the picture of the LCR meter, and also saw the 36 V marking on it.
It seems the meter is suggesting not to check capacitors which may have a stored voltage of over 36V….so before testing any capacitor make sure to discharge the capacitor first, and then connect the probes to it.
Hi Swagatam
Today I received the LCR meter its a PROSTER Model BM 4070 from Amazon and I tested out some inductor coils I had but they are only 145 & 138 uH wound on a common circular core.
In the SMPS circuit in this article we need a 600uH inductor, if I series the coils which would be 483uH could I use this what would be the result, even the equivalent diode you told me in your earlier message i cant get due to all these lockdowns I only have the IN 5408.
I want so much to build this SMPS circuit but the parts are the problem.
I also want to send you a picture of the meter and clear a doubt a label on the meter that says 36 volts Max,what does it mean, can I send the picture by email to you, but i dont have your email , awaiting your reply
Thanks
Vee
Hi Vee, the inductor must be almost exact as given in the diagram. If it is 630uH then it can be 600 uH but not too far away from the actual value.
Now since you have an lCR meter with you, you can try building your own inductor by winding a 1mm thick super enameled wire over any small ferrite rod or ferrite core.
It will be difficult for me to understand the specifications of the meter just by seeing the picture….if you have the manual or the booklet of the meter you can write the specs here, I will try to understand them and explain if possible.
36V may be the maximum volts the meter can tolerate while measuring anything with the power supply ON. However the inductance must be always measured without any power through the coil
Hi Swagatam
One more question
I purchased a variable SMPS power adapter 9v -24v /3A with an LED display from Amazon .
This adapter does not have any heat sinks, so does it mean they are using a circuit like the link you sent me earlier with opto coupler and it does not generate much heat the device gets a little warm
Just curious to know how this works. They do not have any with 9 to 24v //5A
Thanks
Vee
Hi Vee, yes if it is not generating much heat then it should be a SMPS design, because all linear regulators using transistors and ICs will generate heat
Hi Swagatam
Thanks for your reply and sorry fir troubling you with all these electronic problems.
I did go through the link you sent me about modifying an SMPS, it’s quite a tricky job anyway shall see what I can do or forget about the whole thing, I’ve seen a good LCR meter at Amazon and plan to get it. The inductor in the buck & boost circuit you sent me is a 600 micro henry inductor and should it have a current rating for about 6 to 10 amps, please advise, with this I won’t trouble you till I launch upon some other project
Thanks and all the best to you
Regards
Vee
No problem Vee, I am always glad to help!
yes, modifying an SMPS can be be difficult, that is why the LM317 based SMPS is the best option for you.
An LCR meter will be very handy in this situation, and will help you to build the specified coil easily.
You can use a 1 mm thick wire initially which must be super enameled copper wire, and will give you 2 amp current, for higher current you can take 3nos 1mm wires in parallel and wind them together on the ferrite core.
But first please try with a 1 amp version, if it works then you can change the transistor with TIP147 and go for the 5 amp version.
Hi Swagatam
Thank you for your kind reply, I don’t want to modify the SMPS I have ( that is the 24 volts/ 6A adapter) I want to use this as a power source and want an add on circuit from the out put of the one I have to make it a variable voltage bench supply.
I tried it with an LM317 & a 2N2955 with heat sinks but when regulated to 12 to 12.5 volts and with a load of about 3 amps the 2N2955 heat sink gets very hot, more than 60*C so I switched off the power. If the input voltage is reduced to about 18/20 bolts to the variable circuit the temp is around 45* C Heat sink size 3” x2” with additional fins
Can you suggest any circuit or what to do to reduce the heat ir lower the voltage to about 18 volts
Thanks Swagatam
Regards
Vee
Hi Vee, we discussed this earlier also that only a variable SMPS can be without any heat generation, if you use transistor or IC, then heat will be always generated. It cannot be avoided.
Hi Swagatam
I have a transformer-less power supply (may be smps) of 24vdc 6 amps as input and i want to make the output variable from 3 volts to 24 volts dc . How can i do that, can you please email me a suitable circuit
Thanks
Vee
Hi Vee, modifying SMPS is actually not recommended for newcomers, because they have complex circuitry and is not isolated from mains at the primary side.
Still, for knowledge sake you can visit the following article, which will give some insight regarding how to modify an SMPS
How to Modify SMPS for Adjustable Current and Voltage Output
Hi Swagatam
Thank you for your reply
I have an half inch ferrite rod about 1” in length and about 22 gauge wire how many turns would I need to use to get the 600uH value.
Thanks
Vee
Hi Vee, the inductance value will need to be verified through a inductance meter or a basic oscilloscope, through some trial and error, it is difficult to calculate it using formulas.
Hi Swagatam
Regarding the SMPS Curcuit shiwn here the IN 3880 diode and the 600uH inductor is not available here
They have 220, 470, and 820uH inductors that look like resistances will any of these be ok or is there a current rating too for these and can i use sny other diode in place of the IN 3880 diode
Kindly advise and if possible send me a picture of the type of inductor I need to use with specs
Thanks
Vee
Hi Vee, those inductors are low current inductors and cannot handle more than 300 mA current. For enabling high current above 1 amp or two amps you will have to build it manually by winding the col over a ferrite rod or torroidal core using 1 mm enameled copper wire.
For the diode you can use FR107, or BA159
Hello Swag,
I have a transformerless power supply 24vdc, 10A and wana to use a circuit to regulate the voltage from 3~24Vdc with same rated current of the device(10A). Does it(above circuit) work in this case? If you have something better, kindly introduce it to me.
Thanks
Hello Mah, yes according to the datasheet that should be feasible. However, the PNP may have to be replaced with a TIP36 transistor
hello swagatam
what is the voltage of the tantalum capacitors of 50uf x? and of 100ufx?
hello franz, it can be anything above 60V
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
try the last concept from the following article
https://homemade-circuits.com/how-to-make-current-controlled-12-volt/
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!
Jindro, try a single TIP32 first, if it doesn’t work efficiently then you can upgrade it into a Darlington with a 2N2907 transistor
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
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…
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.
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.
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 – http://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.
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!
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
Hi Jindro, yes you can try TIP127
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
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/
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
Hello Mayokon, you can feel free to share it here through comments, I’ll try to help!
replace Q1 with around 8amp rated device and adjust the pot for getting 4V/5amp
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
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.
Please tell me the wattage of all resistors. Ty
when not specified assume it to be always 1/4 watt rated.
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 ??
Bashir, use a 9V battery with your 12-0-12/320V transformer, that will provide you with approximately 240V at the output
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?
he wants to reduce 320V to 220V that's why I recommended the above method.
hello sir good morning
please I want to know, wher can someone apply the above circuit???
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
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.
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.