Author Topic: What is a voltage reference, and why does the LM317 require one?  (Read 29920 times)

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Offline Zero999

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Offline MarkS

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #26 on: February 22, 2012, 05:52:29 pm »
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.

I do not want to be considered one of the "negative posters", but break down "voltage reference". It is a voltage that can be referred to. What it is is basic common sense. You should be able to extrapolate what it is, and thusly, WHY it is from the two words that make up the description. That is why people have been negative. That and the fact that you seem to skip over the answer and ask why again while quoting the answer. ???

Now, there are questions that deserve asking, like how is it constructed and why is it only 1.25 volts when the output is going to be greater.
 

Offline ciccio

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #27 on: February 22, 2012, 06:20:25 pm »
This thread is very interesting::
1) a "newbie" post a simple question, in clear and plain English. From it's later post we understand that he is not illiterate, and owns and can read engineering books.
2) 30 replies in two days: it's a lot. 
3) more than 1/4 of the replies could be avoided: they did not gave an answer to the original poster, but were (I'll try to be "diplomatic") a little on the "unpolite" side, revealing a selfish and "arrogant" (sorry, I did not find a less crude synonym)  position.
3) another 1/4 of the replies is used to question some other's response.
3) another 1/4 were not giving a correct answer (I mean an answer that is both technically correct and that can be understood by the original poster).
4) the lat 1/4 tries to give, with various success, a response to the original question.
I really appreciated some of the posts in this last group, and could write here my appreciation to the posters, but I want to avoid other conflicts, so I end here.
Fortunately I knew before what a Voltage Reference was, or my ignorance could only increase, being obliged to mediate all these informations.
Best regards
Strenua Nos Exercet Inertia
I'm old enough, I don't repeat mistakes.
I always invent new ones
 

Offline MarkS

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #28 on: February 22, 2012, 07:13:34 pm »
..snip..

I think that most people here have the best intentions. He asked a question and received an answer. He then proceeded to ask the question again, while quoting the answer he just received, as though the answer in front of him wasn't there.

While I hate it when someone is outright told to do a search first, I did a quick search on Google and this was the first reply: Voltage Reference. That leads to a rather useful article here: Understanding Voltage-Reference Topologies and Specifications

I don't think BoredAtWork, for instance, was being rude. This isn't the hobby for everyone. I was able to pick this up from a very early age and am entirely self taught. However, many people will look at what I would consider mundane and their eyes will glaze over. The topic in question is a very simple EE topic. If the OP doesn't understand after the answers that have been given, which were entirely on point, then maybe this isn't the hobby for the OP.
 

Offline SgtRock

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #29 on: February 22, 2012, 07:51:00 pm »
Greetings EEVBees:

--I hope some of you will consider being more tolerant of beginners. They are important to this blog, that is why it seeks them. Now, when someone asks; "just exactly what is a voltage reference, and what does it do", I rather think he is looking for an explanation of some length, and not just, "its a voltage, reference, you fool, how else would you do it." Or "take up knitting".

--I can remember taking courses (Calculus comes to mind) where I just could not get a particular concept. I would search and search and question and question, till finally someone would push the bingo button. So when somebody asks a question and you give a short answer, and he still does not get it, try explaining more fully or reccommending a particular "at length" article. You will have a much better chance of not seeming like arrogant bastards.

"Does not squirrel crack nuts on bough of oak tree."
Lao Fu 1410 1620

Best Regards
Clear Ether
 

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #30 on: February 22, 2012, 08:54:39 pm »
Showing that you already put a lot of effort into it would help getting more constructive responses. Something like:
Quote
I was studying the LM317 but I don't understand what a voltage reference is or why the LM317 needs one. I have studied the datasheet, but they don't define the term voltage reference. I have looked up 'voltage reference' in these three books, and read the Wikipedia page about both voltage references and the LM317, but I still don't understand it. A resistive divider also provides a constant voltage according to chapter 1 of book X. Could someone try to explain it to me in different words?

This saves people from tracking down resources you have already read, and also cuts down on the number of people annoyed by the basic question that should be easy to look up in any EE text book.
 

Online IanB

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #31 on: February 22, 2012, 10:06:59 pm »
I'm honestly still not sure if the correct answer has been understood. It is all about basic concepts of the things around us, nothing specially to do with electronics.

For instance if you want to make a piece of cloth exactly 3 feet long, you need to measure it before you cut it. If you are going to measure it, you need something to measure it against, such as a ruler. It is easy to see that you cannot cut something exactly to length without some way to measure it. In this case your measure, your ruler, is your reference for length. Without a ruler for reference you have no way to know where 3 ft is.

It is the same with everything. If you want to measure and establish exactly 5 volts, say, you need some reference to measure 5 volts against. Without a reference you have no way to know where 5 volts lies. You might say your multimeter is a reference, or "ruler". And it is, in a way. But inside the meter, the meter has to know where 5 volts lies too. The meter does that itself by having some voltage reference, some "ruler" to compare against. Likewise the LM317. It has to have a "ruler" or "measure" to compare voltages against or it cannot function.
 

Offline electrode

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #32 on: February 22, 2012, 10:16:39 pm »
musashibaka,

- The answer is very simple. They way you are thinking of it, is that you have (say) 15V in, and want 5V out, so why can't you just use a resistive voltage divider so that the output is 1/3 of the input, right?

- The answer to "why is a voltage reference needed?" is that your input voltage will change. It may not always be 15V. If it rose to 18V, you would get 6V at the output. So you cannot divide your input voltage to set your output voltage. This is where a voltage reference comes in; it's designed to always give some fixed voltage (eg. 1.25V), so you always set your output to some ratio of that and everything will be stable.
 

Offline amspire

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #33 on: February 22, 2012, 10:58:11 pm »
I suppose there is room for confusion here because some regulator IC's provide a constant current instead of a constant voltage as the reference.

For example, the LT3080 that Dave is using in his power supply can be run in one of two modes. It regulates the output to be equal to the voltage on the set pin, so if you use an external voltage reference to provide 5V to the Set pin, the output will be 5V. Using it in this way, the LT3080 acts as a regulator without an internal reference.

However, if you connect a single resistor from the Set pin to ground, the LT3080 does become a fixed voltage regulator. What is going on? You cannot use the 0V Set to Output differential as a voltage reference (as you can with the LM317) , so how is it possible for the LT3080 to supply a fixed output voltage without an external reference?

The answer is instead of providing a fixed voltage (like 1.25V form the LM317), it provides a constant current from the Set pin instead (10uA).  Thanks to Ohm's Law a constant current into a resistor gives a constant voltage.

This is not the only regulator using a current source output to generate the voltage reference - there are lots of other linear and switching regulators using the same strategy. There is every chance that the LT3080 internally has a voltage reference to generate the constant current out of the Set pin, but it is possible to make constant current devices in silicon without any voltage reference involved.

So basically to get a fixed voltage out of a regulator without any external reference voltage, then a regulator IC must provide either a constant voltage reference or a constant current reference, and thanks to Ohm's Law, either one is equally good. With a single resistor a constant voltage can be turned into a precise current, and a constant current can be turned into a precise voltage.

As IanB explained so well, without some sort of fixed reference, there is just nothing to measure the output voltage against.

Richard.
 

Offline Zero999

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #34 on: February 23, 2012, 06:06:28 pm »
If you look at the LM317 circuit more closely, you'll see that the current flowing though R2 is constant and is only dependant on VREF/R1.
 

Offline Simon

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #35 on: February 27, 2012, 12:24:04 pm »
hoot hoot what a lot of fuss over something so simple.

Why could the first answers not tell the plain and simple. In order for the LM317 to output the correct voltage and know it is correct it compares it to a "known" accurate reference and therefore deduce if the output needs to go down or up ?

I mean common guys just answer the question properly and if you think that because it is such a simple question and therefore below you and for you to be able to answer it you have to complicate matter just MOVE ON
 

Offline Balaur

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #36 on: February 27, 2012, 12:33:47 pm »
In order for the LM317 to output the correct voltage and know it is correct it compares it to a "known" accurate reference and therefore deduce if the output needs to go down or up ?

Why?
 

Offline Simon

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #37 on: February 27, 2012, 12:35:40 pm »
In order for the LM317 to output the correct voltage and know it is correct it compares it to a "known" accurate reference and therefore deduce if the output needs to go down or up ?

Why?

don't you start  ;)
 

Offline metalphreak

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #38 on: February 27, 2012, 06:50:20 pm »


"but why?"

 :P

Offline Armin_Balija

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Re: What is a voltage reference, and why does the LM317 require one?
« Reply #39 on: March 08, 2012, 10:25:56 am »
Basically it's using an internal reference as it's startoff point for it's output. That's why you cannot get an output lower than 1.25V on the LM317 unless you start mucking about with it.

It's 1.25 internal reference can be looked as the regulator's internal "GND" where it's output is based on it's internal "GND" reference point. I use the term "GND" really really loosely obviously.
 


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