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What is a voltage reference, and why does the LM317 require one?
Posted by
musashibaka
on 21 Feb, 2012 15:43
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Question:
What is a voltage reference, and why does the LM317 require one?
Backstory:
I am currently building my first variable power supply using an LM317 (
http://www.mouser.com/Search/ProductDetail.aspx?R=LM317Tvirtualkey51120000virtualkey511-LM317T). The schematics I am using to build it are from the LM317 spec sheet
and a Sparkfun variable power supply. I could very well build the power supply without understanding
how to LM317 works, but my goal, aside from building it, is to really to understand how the components work.
Thanks in advance.
Musashi
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#1 Reply
Posted by
IanB
on 21 Feb, 2012 16:16
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The LM317 is going to maintain a constant output voltage. How do you think it is going to do that? Don't worry about electronics, just think in simple terms, how would it work?
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From what I understand the LM317 is akin to a voltage divider where the power from the voltage being dropped is transformed
into heat. Yet I still don't quite get what the voltage reference is, or why it is required.
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#3 Reply
Posted by
Bored@Work
on 21 Feb, 2012 16:40
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The LM317 has a build-in voltage reference.
How else should it determine if the output voltage is to high or to low if not with a voltage reference? It compares the output voltage to its reference voltage. Depending on the outcome it decreases or increases the output voltage.
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Thanks for your response BoredAtWork.
The LM317 has a build-in voltage reference.
Why?
How else should it determine if the output voltage is to high or to low if not with a voltage reference?
I am not sure exactly what a voltage reference is nor why exactly a LM317 requires one, hence the question.
It compares the output voltage to its reference voltage. Depending on the outcome it decreases or increases the output voltage.
Why is the voltage difference between Vout and Adjust pin approximately 1.25V? In other words why is the constant voltage reference 1.25V, and
why is required?
Thanks.
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#5 Reply
Posted by
Bored@Work
on 21 Feb, 2012 17:17
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There is a point where the only sensible advice to give is to suggest to pick up another hobby, for example, knitting.
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#6 Reply
Posted by
Torrentula
on 21 Feb, 2012 17:26
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This helped me a lot:
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#7 Reply
Posted by
olsenn
on 21 Feb, 2012 17:33
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Think about a diode; why does it require a fixed voltage across it's terminals in order to operate? You are correct: idealy that 1.25V drop would not exist, you would simply connect its negative/ground terminal to the negative/ground of your source and the positive voltage terminal to the source of power and the pin in between (Vout) would act like a slider on a potentiometer and allow you select ANY voltage as high as the positive rail and as low as ground; however, no semiconductor is 100% efficient, including this one, and there will be a 1.25V drop on this device. That means that if you connect the IC to a 9V battery, you will only be able to select your voltage between 1.25V - 9V, and not 0V - 9V. Also keep in mind that I*V = P = HEAT; this should be taken into consideration to decide whether or not you need a heatsync or if the device is even suitable for your needs.
Don't worry about the terminology, the 1.25V is not a reference for anything, it is simply the MINIMUM voltage that must appear across those terminals.
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#8 Reply
Posted by
olsenn
on 21 Feb, 2012 17:40
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Alternatively, instead of using a LM317, consider this circuit, the voltage buffer:
If you use a high current op-amp (without that resistor in series with the output) that goes rail-to-rail, you can technically achieve better performance than the LM317.
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Awesome!
Thanks Olsenn, that clarifies things a great deal!
Best,
Musashi
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#10 Reply
Posted by
SgtRock
on 21 Feb, 2012 18:22
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Dear Musasibaka:
--I commend you for hanging in there. When you said; "...but my goal... is to really to understand how the components work.", and then got for answers things like; "Don't worry about electronics" and "The LM317 has a build-in [sic] voltage reference.", I had the feeling that if you did not immediately shut up you were going to get the old kiss off, and I was right.
--Now, clearly you asked: "Yet I still don't quite get what the voltage reference is, or why it is required.". And the answer you got: "How else should it determine if the output voltage..." seemed to indicate that you were being argumentative, rather than seeking a rather more full explanation. Finally you were told to take up knitting.
--Please do not let the rude comments dissuade you from continuing to come to our wonderful forum. While DJ is busy trying to get people to join the forum, others are busy trying to run them off. There are lots of people here, who will help you and do not mind if a beginner is a tad ignorant (as I am), so please stay, You are indeed welcome.
--I found Olsenn's explanation, quite helpful as well. Please see below a link to download a PDF of the TI LM317 Data Sheet which has even more schematics, circuits, and explanations, you might find helpful.
http://ebookbrowse.com/lm317-pdf-d136180503--Also, If you would care for a laugh, check the below link, where an inquisitive poster is seeking help, and is advised to turn his design completely upside down, and then is, berated for not saying; "Thank you, Sir, may I have another"
https://www.eevblog.com/forum/projects-designs-and-technical-stuff/creative-ideas-for-fuse-blown-indicator/--In any case, stick around, take your shoes off.
"Does not squirrel crack nuts on bough of oak tree."
Lao Fu 1410 1620
Best Regards
Clear Ether
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#11 Reply
Posted by
Balaur
on 21 Feb, 2012 18:50
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--I commend you for hanging in there. When you said; "...but my goal... is to really to understand how the components work.", and then got for answers things like; "Don't worry about electronics" and "The LM317 has a build-in [sic] voltage reference.", I had the feeling that if you did not immediately shut up you were going to get the old kiss off, and I was right.
Dear Sarge, to be fair, this kind of questions could be perfectly solved by the interested party with just a little bit of self learning. While it's perfectly admirable of this forum's members to kindly provide help and assistance in these rather basic matters, one must encourage excellence in all things, including autodidacticism.
Also, if you please note, IanB's question is fundamental: how such device could work? And if you think in terms of basic building blocks (such as the comparator, etc), electronic knowledge is not even required, the theory could be applied to all kind of systems.
Also BaW response "The LM317 has a build-in voltage reference" answers perfectly a poorly formed question: "What is a voltage reference, and why does the LM317 require one?". LM317 doesn't require one (i.e. as an external component), it has his own or you can even look at it as a high-power voltage reference.
Cheers,
Dan
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#12 Reply
Posted by
Bored@Work
on 21 Feb, 2012 19:12
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If you don't even read the answers you get, but instead continue to childishly respond to every explanation with a stupid "Why?" you should really consider picking up knitting as a hobby.
And that my internet stalker can't keep his piehole shut, well, how surprising.
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#13 Reply
Posted by
olsenn
on 21 Feb, 2012 19:14
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Honestly, I am ashames at the people on this forum! Musashibaka asked a good question for the right reason; because he didn't know the answer and wanted to. There is no need to be assholes and either purposely try to confuse him or insult him for asking electronics questions on an electronics forum.
Secondly, he is actually SMARTER than the bunch of you... when he didn't know the answer he asked, when you didn't know the answer, you guessed... and were WRONG. The LM317 doesn't compensate and it doesn't have any idea of what voltage it is spitting out; it operates on Ohm's Law, and that's it! It has NO VOLTAGE REFERENCE. If you want 5V, you have to hook it up to a multimeter and tune the resistors until you get 5V and hope it never changes! A voltage reference would be used for something like an analog-to-digital converted which needs to compare TWO voltage DIFFERENCES ([Vin - Vref] is five times larger than [Vref - 0], and I know Vref is 1.25V, thus Vin = 5*1.25= 6.25V). The only time you would need to consider the 1.25V in the LM317 as a "Reference" would be to pass it to the Vref of an AtoD converter (if it doesn't already have a built in reference) along with the output, so that it can determine and display what voltage that is being output instead of you having to connect the damn thing to a multimeter to see how much more you need to adjust the damn pot.
musashibaka... please don't let these assholes upset you or persuade you to stop asking questions; I can assure you they will be the ones who end up failing school and panhandling for money!
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#14 Reply
Posted by
SgtRock
on 21 Feb, 2012 19:28
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Dear Dave.S:
--You stated:
"So you are saying Dave Jones is wrong ? He was talking about LM317's Vref in his power supply video ."
--If you happen to remember exactly which power supply video you are referring to, could you please point to it. In the mean while, I will be trying to isolate the correct one, from a number of power supply videos all. Thanks.
--Someone, bring me some popcorn, this is getting good.
“Horse sense is the thing a horse has which keeps it from betting on people."
W. C. Fields (William Claude Dunkenfield) 1880 - 1946
Best Regards
Clear Ether
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#15 Reply
Posted by
olsenn
on 21 Feb, 2012 19:31
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This is directly from the LM317 datasheet from Texas Instruments:
"In operation, the LM117 develops a nominal 1.25V reference
voltage, VREF, between the output and adjustment terminal.
The reference voltage is impressed across program resistor
R1 and, since the voltage is constant, a constant current I1
then flows through the output set resistor R2, giving an output
voltage of..."
Hence Ohm's Law! That's honestly all there is too it. Below is a quote from YOU, and if I was the person asking what the reference terminology means, I wouldn't understand either.
"Without a voltage reference for most linear regulators , they would be essentially useless .
A voltage reference is a feedback loop .
It has to know what voltages you want , how else is it "adjustable" ?"
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#16 Reply
Posted by
Balaur
on 21 Feb, 2012 19:46
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Secondly, he is actually SMARTER than the bunch of you... when he didn't know the answer he asked, when you didn't know the answer, you guessed... and were WRONG. The LM317 doesn't compensate and it doesn't have any idea of what voltage it is spitting out; it operates on Ohm's Law, and that's it! It has NO VOLTAGE REFERENCE. If you want 5V, you have to hook it up to a multimeter and tune the resistors until you get 5V and hope it never changes! A voltage reference would be used for something like an analog-to-digital converted which needs to compare TWO voltage DIFFERENCES ([Vin - Vref] is five times larger than [Vref - 0], and I know Vref is 1.25V, thus Vin = 5*1.25= 6.25V). The only time you would need to consider the 1.25V in the LM317 as a "Reference" would be to pass it to the Vref of an AtoD converter (if it doesn't already have a built in reference) along with the output, so that it can determine and display what voltage that is being output instead of you having to connect the damn thing to a multimeter to see how much more you need to adjust the damn pot.
This is directly from the LM317 datasheet from Texas Instruments:
"In operation, the LM117 develops a nominal 1.25V reference
voltage, VREF, between the output and adjustment terminal.
The reference voltage is impressed across program resistor
R1 and, since the voltage is constant, a constant current I1
then flows through the output set resistor R2, giving an output
voltage of..."
Hence Ohm's Law! That's honestly all there is too it. Below is a quote from YOU, and if I was the person asking what the reference terminology means, I wouldn't understand either.
"Without a voltage reference for most linear regulators , they would be essentially useless .
A voltage reference is a feedback loop .
It has to know what voltages you want , how else is it "adjustable" ?"
This is highly amusing.
Please forgive my crass ignorance and impertinence, but I would like to ask you kind sir, to get a better look at the theory behind all this.
The 1.25V reference voltage means exactly that: a bandgap temperature-independent voltage reference, indifferent (within limits) to whatever supply voltage the device receives.
Please have a look at various documents available online:
ECE 237WikiCheers,
Dan
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#17 Reply
Posted by
alm
on 21 Feb, 2012 20:21
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Honestly, I am ashames at the people on this forum! Musashibaka asked a good question for the right reason; because he didn't know the answer and wanted to. There is no need to be assholes and either purposely try to confuse him or insult him for asking electronics questions on an electronics forum.
In general I think questions like 'what's a voltage reference' are better looked up in a text book (or Wikipedia) than asked on a forum. Asking such questions on a forum will result in some incorrect answers and some self-proclaimed experts arguing about some irrelevant detail.
The LM317 doesn't compensate and it doesn't have any idea of what voltage it is spitting out; it operates on Ohm's Law, and that's it! It has NO VOLTAGE REFERENCE. If you want 5V, you have to hook it up to a multimeter and tune the resistors until you get 5V and hope it never changes! A voltage reference would be used for something like an analog-to-digital converted which needs to compare TWO voltage DIFFERENCES ([Vin - Vref] is five times larger than [Vref - 0], and I know Vref is 1.25V, thus Vin = 5*1.25= 6.25V). The only time you would need to consider the 1.25V in the LM317 as a "Reference" would be to pass it to the Vref of an AtoD converter (if it doesn't already have a built in reference) along with the output, so that it can determine and display what voltage that is being output instead of you having to connect the damn thing to a multimeter to see how much more you need to adjust the damn pot.
You seem to have a strange concept of voltage reference. A voltage reference is commonly considered a component that provides a constant voltage, to be used as reference for something. How would your "just Ohm's law" regulator if Vref (which of course doesn't refer to reference voltage) was a variable voltage, eg. just 20% of the input voltage? Would there be any regulation at all? How is this internal thing which provides 1.25V but is not a voltage reference connected to the internal error amplifier? If it just operates on Ohm's law, what happens if you connect the adjust terminal to ground without any resistors between the output terminal and ground? I assume it just stops working?
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#18 Reply
Posted by
westfw
on 22 Feb, 2012 02:07
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The goal of a voltage regulator is to output some constant voltage output, regardless of load.
I order to do this, the regulator has to have some "reference" to a know voltage that it can compare against; the 'reference voltage.' The regulator circuit could say "I want my output voltage to equal this reference" In the 317 case, the circuit will adjust the "resistance" of a power transistor element in ways that will make the difference between the output voltage and the "adjust" input match an internal 1.25V reference. By putting a resistor divider between the output and the "adjust", you can achieve any output voltage greater than 1.25V (if you wanted 5V, you'd use a voltage divider that divided by 4.) 1.25V is presumably a compromise between what it is easy to fabricate, and what a reasonable "low" output voltage would be...
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#19 Reply
Posted by
Wartex
on 22 Feb, 2012 04:18
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There is a point where the only sensible advice to give is to suggest to pick up another hobby, for example, knitting.
Could someone please take this arrogant dick down a notch, he's fucking personally insulting everyone in 30% of his posts. I would have been crucified by mods for shit like this, yet he gets a free pass.
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#20 Reply
Posted by
SgtRock
on 22 Feb, 2012 04:39
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Dear Wartex:
--While I have not always agreed with you in the past, I do have to say that, usually you saved your wrath for a very few people, with whom you have had disagreements. This guy hates everybody. All he does is chime in, give advice (often not very well thought out advice) and then, when everyone does not say "Jawohl Mein Herr", he starts with the gibbering, slobbering and cursing, and oft times using the most lurid disgusting profanity in personalized attacks, though lately he has improved somewhat, causing some to guess that perhaps he had been issued the yellow card.
"There once was a worker, so bored.
That when his posts were ignored.
He'd strive on inanely,
And most times profanely.
But seldom, if ever he scored."
Gator Dundee 1948 -
Best Regards
Clear Ether
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There is a point where the only sensible advice to give is to suggest to pick up another hobby, for example, knitting.
Could someone please take this arrogant dick down a notch, he's fucking personally insulting everyone in 30% of his posts. I would have been crucified by mods for shit like this, yet he gets a free pass.
i'll take it positively. he just wanted to save some space in this world by avoiding useless arguments such as why V=IR not I=RV, this is not a voltage reference, that is etc etc and what not. someone suggested... read it up in text book, i agree, and internet is vast, thanks to google for managerial effort to collect them. without him i believe the OP will continue to ask why and why indefenitely until why V=IR until this thread will take up space similar to one textbook. BaW did give some advice on what is the Vref (and some others), if the OP still unable to understand and dont make any effort to further investigate it, then there's a problem. you want to help explaining what a votage reference is? then go ahead. we will be glad you are willing to spoonfeed this chap. the OP and the viewer may choose to skip which posts they dont like. but instead of knitting (which i did long time ago and becaming good at) i will suggest for lawn bowl.
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I really appreciate all of the helpful responses I have received.
I would like to address some of the assumptions made by
a few negative posters.
1. I actually do know how to use wikipedia and google.
2. Anyone who has tried either of the above would know that the
quality of information found on topics vary, and sometimes the right
description required to get an intuitive understanding varies per individual.
Not only that, if everything in the world could be explained well by a
google search, this forums sole purpose would be for entertainment.
3. I have read wikipedia page on voltage reference and the lm317.
I have also searched through the three electronics books
Practical Electronics for Inventors, Fundamentals of Analog Circuits, and Beginning
Electronics to name a few. So I sought further advice.
4. I am not an electrical engineer, but I would gather that throughout an engineers
career one must ask questions in order to learn. Excusing rude behavior
under the auspices of abating "spoon feeding" is a weak minded response to
human curiosity.
To those individuals who chose to be rude, or because of their own insecurity
chose to respond with a weak attempt at belittle me, I offer to you the gift of wisdom:
"Any intelligent fool can make things bigger and more complex...
It takes a touch of genius - and a lot of courage to move in the opposite direction."
-- Albert Einstein
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I really appreciate all of helpful responses I received.
I would like to address some of the assumptions made by
some negative posters.
Well said, sir!
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