Author Topic: What is a voltage reference, and why does the LM317 require one?  (Read 8217 times)

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Offline musashibaka

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What is a voltage reference, and why does the LM317 require one?
« on: February 22, 2012, 02:43:11 AM »
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


Online IanB

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #1 on: February 22, 2012, 03:16:33 AM »
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?
I'm not an EE--what am I doing here?

Offline musashibaka

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #2 on: February 22, 2012, 03:30:54 AM »
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.


Offline Bored@Work

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #3 on: February 22, 2012, 03:40:31 AM »
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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Offline musashibaka

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #4 on: February 22, 2012, 04:08:45 AM »
Thanks for your response BoredAtWork.

Quote
The LM317 has a build-in voltage reference.
Why?

Quote
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.

Quote
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.

Offline Bored@Work

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #5 on: February 22, 2012, 04:17:15 AM »
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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Offline olsenn

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #7 on: February 22, 2012, 04:33:18 AM »
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.

Offline olsenn

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #8 on: February 22, 2012, 04:40:07 AM »
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.

Offline musashibaka

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #9 on: February 22, 2012, 04:42:02 AM »
Awesome!

Thanks Olsenn, that clarifies things a great deal!

Best,
Musashi

Offline SgtRock

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #10 on: February 22, 2012, 05:22:24 AM »
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"

http://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

Online Balaur

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #11 on: February 22, 2012, 05:50:23 AM »

--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

Offline Bored@Work

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #12 on: February 22, 2012, 06:12:04 AM »
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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Offline olsenn

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #13 on: February 22, 2012, 06:14:01 AM »
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!

Offline SgtRock

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #14 on: February 22, 2012, 06:28:54 AM »
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.


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W. C. Fields (William Claude Dunkenfield) 1880 - 1946

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Clear Ether


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