The 1. 25V Trick That Gives You Any Voltage You Want
video description
Date: 2026-07-10
Related videos
Comments and reviews: 20
thanhhuynh272
This is a good video with excellent graphics and special condition explainstions, (like protection diodes, but there are some parts that are just incorrect enough to confuse the novice the video is aimed at.
In the grsphic st 17: 18, current is shown originating from residual charge in a capacitor on the adjustment pin to ground. A graphic depicrpts the capacotor discharging through the regulator’s adj pin at circuit power down and going to the POWER SOURCE, this current MUST return to the capacitor where it started.
Similarly, at 17: 51 a similar transient condition current originsying from the output bypass capacitors does not return to them bit to the, just turned off power source. For someone expeirenced like myself, I csn see the errorbut a learner will be VERY confused and it will tske them a very lomg time to see and understand the issue,
All of the description involving the power indicator LED starting st 18: 17 is very flawed with naieve explaination which is in quite stark contrast to the rest of the bideo which is very concise and correct. The resistor in series with a LED is not to keep things efficient, it is essential to limit the current through the LEDwhich in forward bias behaves in a very similar manner to a 1. 8 - 2. 2v zener, as you correctly mention at 18: 10and prevent the magic smoke escaping from the LED and possibly the regulator chip too! 18: 30 The LED behaves as a zener and for any applied voltage greater than 1. 8 - 2. 2 it will pull very large currents snd sttempt to clamp its terminsl voltage. and this the output of thr circuit to that.
Any colour LED can actually be used if a small diode, like a 1N4148 is placed in series with it above its anode and the node between it and the diode above is connected back to the input via a highish value resistor, say 6 -10K ohms, then when the output of the regulator hoes below the LED’s bias-on voltage, the series diode sbove will reverse-bias and the LED will get current from the input side via the highish value resistor back to the input.
Your mention of strapping the wiper of the pot to one of the fixed ends at 8: 28 is a REALLY GOOD move when explaining to beginners, this set up good practice habbits and this is one atea where this video excels.
One last point, not a mistake but an omission. At 1: 38 you state that fixed regulators can only ever give you s fixed output voltage. This is not necessarily true, there are exeptions. Fixed regulators, such as the LM78xx and LM79xx series and the LM340 series can have their output adjustedbut only to voltages greater than their rsted fixed voltsge! Effectively these regulators are very similar to the adjustable LM317 (LM337 neg) regulators the video is about, the only differences are that in the fixed regulators the internal reference zener you mention at 19: 55 is just a higher voltage than the 1. 25v one in the 317 AND the upper divider resistor R1 the 240 ohm you mention at 5: 10 is successivly higher values for higher voltage regs and built inside, it ensures a certain minimum load current which is essential to their operstion, but plscing fixed resistors in series with the gnd pins of fixed regulators will increase their output voltage. So you can get 6v or 7. 5v from s 7805but not from a 7812! You just need to carefully select a resistor value, usually about 180 -560 Ohms and put it between the Lgnd pin and your circuit’s ground reference node.
reply
This is a good video with excellent graphics and special condition explainstions, (like protection diodes, but there are some parts that are just incorrect enough to confuse the novice the video is aimed at.
In the grsphic st 17: 18, current is shown originating from residual charge in a capacitor on the adjustment pin to ground. A graphic depicrpts the capacotor discharging through the regulator’s adj pin at circuit power down and going to the POWER SOURCE, this current MUST return to the capacitor where it started.
Similarly, at 17: 51 a similar transient condition current originsying from the output bypass capacitors does not return to them bit to the, just turned off power source. For someone expeirenced like myself, I csn see the errorbut a learner will be VERY confused and it will tske them a very lomg time to see and understand the issue,
All of the description involving the power indicator LED starting st 18: 17 is very flawed with naieve explaination which is in quite stark contrast to the rest of the bideo which is very concise and correct. The resistor in series with a LED is not to keep things efficient, it is essential to limit the current through the LEDwhich in forward bias behaves in a very similar manner to a 1. 8 - 2. 2v zener, as you correctly mention at 18: 10and prevent the magic smoke escaping from the LED and possibly the regulator chip too! 18: 30 The LED behaves as a zener and for any applied voltage greater than 1. 8 - 2. 2 it will pull very large currents snd sttempt to clamp its terminsl voltage. and this the output of thr circuit to that.
Any colour LED can actually be used if a small diode, like a 1N4148 is placed in series with it above its anode and the node between it and the diode above is connected back to the input via a highish value resistor, say 6 -10K ohms, then when the output of the regulator hoes below the LED’s bias-on voltage, the series diode sbove will reverse-bias and the LED will get current from the input side via the highish value resistor back to the input.
Your mention of strapping the wiper of the pot to one of the fixed ends at 8: 28 is a REALLY GOOD move when explaining to beginners, this set up good practice habbits and this is one atea where this video excels.
One last point, not a mistake but an omission. At 1: 38 you state that fixed regulators can only ever give you s fixed output voltage. This is not necessarily true, there are exeptions. Fixed regulators, such as the LM78xx and LM79xx series and the LM340 series can have their output adjustedbut only to voltages greater than their rsted fixed voltsge! Effectively these regulators are very similar to the adjustable LM317 (LM337 neg) regulators the video is about, the only differences are that in the fixed regulators the internal reference zener you mention at 19: 55 is just a higher voltage than the 1. 25v one in the 317 AND the upper divider resistor R1 the 240 ohm you mention at 5: 10 is successivly higher values for higher voltage regs and built inside, it ensures a certain minimum load current which is essential to their operstion, but plscing fixed resistors in series with the gnd pins of fixed regulators will increase their output voltage. So you can get 6v or 7. 5v from s 7805but not from a 7812! You just need to carefully select a resistor value, usually about 180 -560 Ohms and put it between the Lgnd pin and your circuit’s ground reference node.
reply
engineering_mindset
Very nice video, good job! I loved all the explanations, learned a thing or two today and I have to admit, you designed that circuit very thoughtfully (I'd personally maybe add a decoupling cap or two but would never even think about all the protection diodes; D)
I do have some things I want to note:
Linear regulators are great where the difference between the input and output is small, like you mentioned. However at points you seemed to imply that they can regulate down from rectified mains (350V DC, which, yeah you probably meant the rectified output of a step down transformer (like in a linear supply) but that wasn't very clear.
I'm pretty sure (but not 100%, watching this on a phone and I can't see the exact part numbers on the ICs) the chips shown at 14: 44 are in fact MOSFETs, from a buck/boost converter (which is btw a WAY more efficient way to reduce voltage, just a bit noisy. Actually do you have a video on that already If not absolutely make one, maybe even as a follow up to this one) or a Switch Mode Power Supply. The linear regulators in a supply like this would probably only be used for the control circuitry that doesn't draw a lot of power, like the ones shown earlier at 3: 12
The positive and negative inputs of an opamp are also called inverting and non-inverting, which might be more intuitive to how they work (like you explained around 22: 21)
With all of that said, again, briliant work!
reply
Very nice video, good job! I loved all the explanations, learned a thing or two today and I have to admit, you designed that circuit very thoughtfully (I'd personally maybe add a decoupling cap or two but would never even think about all the protection diodes; D)
I do have some things I want to note:
Linear regulators are great where the difference between the input and output is small, like you mentioned. However at points you seemed to imply that they can regulate down from rectified mains (350V DC, which, yeah you probably meant the rectified output of a step down transformer (like in a linear supply) but that wasn't very clear.
I'm pretty sure (but not 100%, watching this on a phone and I can't see the exact part numbers on the ICs) the chips shown at 14: 44 are in fact MOSFETs, from a buck/boost converter (which is btw a WAY more efficient way to reduce voltage, just a bit noisy. Actually do you have a video on that already If not absolutely make one, maybe even as a follow up to this one) or a Switch Mode Power Supply. The linear regulators in a supply like this would probably only be used for the control circuitry that doesn't draw a lot of power, like the ones shown earlier at 3: 12
The positive and negative inputs of an opamp are also called inverting and non-inverting, which might be more intuitive to how they work (like you explained around 22: 21)
With all of that said, again, briliant work!
reply
jamesg1367
Cheap and reliable. I have myself designed commercial products using variants of the LM317.
Nowadays, the lego approach and the rather weird economics of the Chinese electronics industry have given us some very inexpensive buck converters. They're a greatly superior solution where efficiency is concerned, which can really matter where battery-powered devices are concerned. However, buck converters become expensive if designed for large loads. A linear regulator such as the more robust and higher-voltage versions of the LM317 type regulators can more cheaply handle the heavier loads.
Provided with filtering mechanisms (as illustrated in the video, and especially if these filters are further improved and augmented, the LM317 can deliver a very quiet power supply, very well suited to sensitive sensing and measurement.
reply
Cheap and reliable. I have myself designed commercial products using variants of the LM317.
Nowadays, the lego approach and the rather weird economics of the Chinese electronics industry have given us some very inexpensive buck converters. They're a greatly superior solution where efficiency is concerned, which can really matter where battery-powered devices are concerned. However, buck converters become expensive if designed for large loads. A linear regulator such as the more robust and higher-voltage versions of the LM317 type regulators can more cheaply handle the heavier loads.
Provided with filtering mechanisms (as illustrated in the video, and especially if these filters are further improved and augmented, the LM317 can deliver a very quiet power supply, very well suited to sensitive sensing and measurement.
reply
wtmayhew
This is a nicely done video with a clear explanation. In particular including the reverse polarity protection diode is a very smart design element. Three terminal regulators may be damaged by reverse polarity. How does that happen An example would be connecting a battery to be charged to the output before applying DC to the input. In that case the input filter capacitor is discharged allowing a large current to flow backward from the battery through the regulator chip into the input filter capacitor. A 1N4002 diode connected across the regulator in opposition to the normal current flow would allow the current from the battery to safely bypass around the regulator into the capacitor. When the circuit has its normal DC input, the 1N4002 is reverse biased and has no measurable effect on operation.
reply
This is a nicely done video with a clear explanation. In particular including the reverse polarity protection diode is a very smart design element. Three terminal regulators may be damaged by reverse polarity. How does that happen An example would be connecting a battery to be charged to the output before applying DC to the input. In that case the input filter capacitor is discharged allowing a large current to flow backward from the battery through the regulator chip into the input filter capacitor. A 1N4002 diode connected across the regulator in opposition to the normal current flow would allow the current from the battery to safely bypass around the regulator into the capacitor. When the circuit has its normal DC input, the 1N4002 is reverse biased and has no measurable effect on operation.
reply
Squire_Chris
The only part I've never understood is why a tiny ceramic cap should be faster to react to a large electrolytic in parallel.
I'm guessing that someone will make the argument that the ceramic dielectric behaves differently from the dielectric in the electrolytic. but if we stay with the tank analogy. my 20, 000 litre water tank is going to absorb the strongest gushes during a rainstorm, and sticking a 20 litre bucket in parallel with it isn't going to change the outflow from the 20, 000 litre tank, which will always be steady.
Every time I see this form of decoupling configuration I scratch my head - how could a tiny cap affect a large one in parallel, other than by simply raising the total capacitance by a minuscule, insignificant amount
reply
The only part I've never understood is why a tiny ceramic cap should be faster to react to a large electrolytic in parallel.
I'm guessing that someone will make the argument that the ceramic dielectric behaves differently from the dielectric in the electrolytic. but if we stay with the tank analogy. my 20, 000 litre water tank is going to absorb the strongest gushes during a rainstorm, and sticking a 20 litre bucket in parallel with it isn't going to change the outflow from the 20, 000 litre tank, which will always be steady.
Every time I see this form of decoupling configuration I scratch my head - how could a tiny cap affect a large one in parallel, other than by simply raising the total capacitance by a minuscule, insignificant amount
reply
coriscotupi
In the late 70s at an electronics components supply shop I spotted a introductory kit featuring the all new LM317. The pack had a little printed circuit board, the IC, a few resistors and capacitors and a potentiometer, much like the board shown in the video. But it also came with a booklet explaining how the 317 works, had graphs and tables describing the various operating modes (variable voltage, constant voltage, constant current) and showed practical circuits that could be built with it, such as power supplies and battery chargers. I bought the kit and built my first regulated, variable voltage power supply. Some 48 years later, that power supply still works and is used quite often.
reply
In the late 70s at an electronics components supply shop I spotted a introductory kit featuring the all new LM317. The pack had a little printed circuit board, the IC, a few resistors and capacitors and a potentiometer, much like the board shown in the video. But it also came with a booklet explaining how the 317 works, had graphs and tables describing the various operating modes (variable voltage, constant voltage, constant current) and showed practical circuits that could be built with it, such as power supplies and battery chargers. I bought the kit and built my first regulated, variable voltage power supply. Some 48 years later, that power supply still works and is used quite often.
reply
wendellknicely395
Just one fallacy in the other wise very clear explanation. That is that the Zener regulator once it reaches a certain current is constant voltage. In reality it is never constant voltage but is always a curve going up over current. The 'BIG' trick here is the constant current source. By providing a constant current to the Zener you are essentially at one point on the Zener I/V curve and therefore the voltage is quite stable. I have seen a large number of designs where the assumption is that the Zener always regulates to the value on the package and the designer scratches their head because it varies by as much as /- 25%.
reply
Just one fallacy in the other wise very clear explanation. That is that the Zener regulator once it reaches a certain current is constant voltage. In reality it is never constant voltage but is always a curve going up over current. The 'BIG' trick here is the constant current source. By providing a constant current to the Zener you are essentially at one point on the Zener I/V curve and therefore the voltage is quite stable. I have seen a large number of designs where the assumption is that the Zener always regulates to the value on the package and the designer scratches their head because it varies by as much as /- 25%.
reply
paradiselost9946
yay! magic smoke!
well explained, clear. most of these videos leave me smacking my head.
i would love to see the same sort of video on selecting appropriate gate resistors and what happens when theyre slightly too large and youre really pushing high gate currents. because at first its rather un-intuitive.
and maybe something about motors, generators, load resistances and exactly what happens when you are generating power and also include teh generators resistance. ahem. jacobis law or impedance matching.
rather than the usual handwaving claptrap about coils and magnetic fields. its not the only aspect!
reply
yay! magic smoke!
well explained, clear. most of these videos leave me smacking my head.
i would love to see the same sort of video on selecting appropriate gate resistors and what happens when theyre slightly too large and youre really pushing high gate currents. because at first its rather un-intuitive.
and maybe something about motors, generators, load resistances and exactly what happens when you are generating power and also include teh generators resistance. ahem. jacobis law or impedance matching.
rather than the usual handwaving claptrap about coils and magnetic fields. its not the only aspect!
reply
justinyates1154
This is a pretty cool video, and I like the idea of the kits to build one! I did see a potential future video idea, when you were showing the diagram of the voltage regulator.
I noticed I've seen those same types of electrical components described in the diagram of the voltage regulator IC in through hole designs.
Could you make a video building a voltage regulator using the design for the IC but using through hole components creating an almost exploded view of the internals of the IC
It could also stand as an exaggerated visual representation of how far we have progressed in design and technology
reply
This is a pretty cool video, and I like the idea of the kits to build one! I did see a potential future video idea, when you were showing the diagram of the voltage regulator.
I noticed I've seen those same types of electrical components described in the diagram of the voltage regulator IC in through hole designs.
Could you make a video building a voltage regulator using the design for the IC but using through hole components creating an almost exploded view of the internals of the IC
It could also stand as an exaggerated visual representation of how far we have progressed in design and technology
reply
jonathanwienke512
What about putting a resistor zener diode in parallel with the potentiometer If the zener breakdown is the nominal output voltage, failure of the potentiometer wouldn't cause voltage runaway. The output would just jump to nominal 1. 25V.
That's probably not going to be high enough to cause smoke emission, and you can add some detection logic to trigger a failure warning when that happens. If the leakage current from the regulator is 5mA, you could put an LED in series with the zener to serve as a failure warning indicator.
reply
What about putting a resistor zener diode in parallel with the potentiometer If the zener breakdown is the nominal output voltage, failure of the potentiometer wouldn't cause voltage runaway. The output would just jump to nominal 1. 25V.
That's probably not going to be high enough to cause smoke emission, and you can add some detection logic to trigger a failure warning when that happens. If the leakage current from the regulator is 5mA, you could put an LED in series with the zener to serve as a failure warning indicator.
reply
rh4009
Is Paul Evans still making these videos This script feels AI generated with plenty of typical unnecessary hallucinations, and innapropriate steering wheel analogy. The thought process is laboured and alien. If someone explained the lm317 this way in real life, I would doubt his competence as engineer. There are also errors in the really simple circuit, which a competent engineer will spot at a glance, suggesting that Paul Evins did not write this script, or alternatively that he is not a co petent engineer. So, how was the script obtained
reply
Is Paul Evans still making these videos This script feels AI generated with plenty of typical unnecessary hallucinations, and innapropriate steering wheel analogy. The thought process is laboured and alien. If someone explained the lm317 this way in real life, I would doubt his competence as engineer. There are also errors in the really simple circuit, which a competent engineer will spot at a glance, suggesting that Paul Evins did not write this script, or alternatively that he is not a co petent engineer. So, how was the script obtained
reply
blackpearl1-477
I'm currently looking for a current limiter. The intention is to limit 2 LEDs on my scooter that keeps burning out after a short while.
The LEDs are powered by the stators coils at 12 volts. But I have no idea how to limit the amperage to the LEDs and keep the voltage at 12 volts.
Do you have a recommendation what schematic I could use that is simple to integrate
Originally it had regular 12v 10 Watts light bulbs (T10.
Many thanks for your educational videos.
reply
I'm currently looking for a current limiter. The intention is to limit 2 LEDs on my scooter that keeps burning out after a short while.
The LEDs are powered by the stators coils at 12 volts. But I have no idea how to limit the amperage to the LEDs and keep the voltage at 12 volts.
Do you have a recommendation what schematic I could use that is simple to integrate
Originally it had regular 12v 10 Watts light bulbs (T10.
Many thanks for your educational videos.
reply
D614G_aka_Doug
There is nothing unpredictable about connecting the ADJ pin directly to ground. That will actually provide the most stable output voltage since variation in the ADJ pin current will not be multiplied by series resistance to ground - not that a fixed voltage of 1. 25 is terribly useful.
The ADJ pin can be driven negative with respect to circuit common to allow setting the output to less than 1. 25 volts. This is common in bench supplies using the 317.
reply
There is nothing unpredictable about connecting the ADJ pin directly to ground. That will actually provide the most stable output voltage since variation in the ADJ pin current will not be multiplied by series resistance to ground - not that a fixed voltage of 1. 25 is terribly useful.
The ADJ pin can be driven negative with respect to circuit common to allow setting the output to less than 1. 25 volts. This is common in bench supplies using the 317.
reply
Enigma758
8: 00 To better understand why the voltage runs away look at 20: 20 and imagine that the bottom resistor is gone and there is only a single resistor between the output and the adj input pin. With the zener diode always trying to maintain a 1. 25 higher voltage than the output, it becomes a self reinforcing feedback loop that races to the top (voltage wise. We've lost the ground reference which previously provided stability.
reply
8: 00 To better understand why the voltage runs away look at 20: 20 and imagine that the bottom resistor is gone and there is only a single resistor between the output and the adj input pin. With the zener diode always trying to maintain a 1. 25 higher voltage than the output, it becomes a self reinforcing feedback loop that races to the top (voltage wise. We've lost the ground reference which previously provided stability.
reply
JaenFoster
Another fantastic thing about this little IC is it can also be used as a current regulator. So let's say you had some LEDs, and you wanted to drive them at a fixed 10mA of forward current, you can set the chip up to always provide that regardless of the input voltage. This also means that by adding more LEDs in series, the output voltage will automatically adjust to compensate for the increased forward voltage needed.
reply
Another fantastic thing about this little IC is it can also be used as a current regulator. So let's say you had some LEDs, and you wanted to drive them at a fixed 10mA of forward current, you can set the chip up to always provide that regardless of the input voltage. This also means that by adding more LEDs in series, the output voltage will automatically adjust to compensate for the increased forward voltage needed.
reply
engineering_mindset
This is actually very facinating, I'm going to have to get some sleep in before I can actually think through all the math. My last circuit class was around 9 months ago, so I'm kinda rusty haha.
Question for you sir, how much value do you think the book art of electronics has, I've picked up and have been meaning to get around to reading it, but other passions are using a lot of my time.
reply
This is actually very facinating, I'm going to have to get some sleep in before I can actually think through all the math. My last circuit class was around 9 months ago, so I'm kinda rusty haha.
Question for you sir, how much value do you think the book art of electronics has, I've picked up and have been meaning to get around to reading it, but other passions are using a lot of my time.
reply
saramanbrudesco
can't fixed stabilizers be used to stabilize to different voltages than prescribed
using same resistive voltage dividers.
i had difficulty with that, they oscillated wildly.
if it is not possible, then why, and also why so many publications recommend the use of fixed for different voltage than prescribed.
not that there is lack of bad publications though.
reply
can't fixed stabilizers be used to stabilize to different voltages than prescribed
using same resistive voltage dividers.
i had difficulty with that, they oscillated wildly.
if it is not possible, then why, and also why so many publications recommend the use of fixed for different voltage than prescribed.
not that there is lack of bad publications though.
reply
aleksandarl6975
Great video, inner workings of linear voltage regulator perfectly explained. If i may add, LM350 is a drop in replacement for LM317, but it can handle 3A compared to LM317's 1. 5A, but you have to keep heat dissipation in mind, if your input and output voltages are far apart you will need a much bigger heat sink for LM350 then you do for LM317.
reply
Great video, inner workings of linear voltage regulator perfectly explained. If i may add, LM350 is a drop in replacement for LM317, but it can handle 3A compared to LM317's 1. 5A, but you have to keep heat dissipation in mind, if your input and output voltages are far apart you will need a much bigger heat sink for LM350 then you do for LM317.
reply
pboston6RR
EXCELLENT! The dive into the op amp function is wonderfully clear and a great guide to how an op amp functions.
I really enjoye watching since I have used this kind of power supply for years with both PicAxe and now Arduino projects.
I’ll save this one and watch it again carefully to squeeze the last drop of teaching.
Thanks!
reply
EXCELLENT! The dive into the op amp function is wonderfully clear and a great guide to how an op amp functions.
I really enjoye watching since I have used this kind of power supply for years with both PicAxe and now Arduino projects.
I’ll save this one and watch it again carefully to squeeze the last drop of teaching.
Thanks!
reply
tookitogo
I recently used an LM317L (the low-current version) to generate a simulated sensor signal for an industrial control application where I use a relay to switch between the actual sensor output voltage and a simulated one. :) But it’s a one-off so i just did it on a few square cm of veroboard and shoved it in a 3D-printed DIN rail enclosure.
reply
I recently used an LM317L (the low-current version) to generate a simulated sensor signal for an industrial control application where I use a relay to switch between the actual sensor output voltage and a simulated one. :) But it’s a one-off so i just did it on a few square cm of veroboard and shoved it in a 3D-printed DIN rail enclosure.
reply
Add a review, comment
Other channel videos
