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How does the input of LM317 effect the output without R1?

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thidakhaing

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How does the input of LM317 effect the output without R1?
The input is 24V ,40mA
 

It depends on which way the wind is blowing at the time. If R1 (234 Megohms) is aligned north-east to south-west, and the wind blows from the south, the confluence of intergalactic magnetism makes the output higher.
The usual method of stabilizing the input is to use a second resistor, precisely 1.007 Ohms, lying beside the LM317. It is imperative that the second resistor doesn't actually touch the regulator as this would leak photons and unless absolutely level they could roll away and make a nasty mess on the floor.

Brian.
 
I use my special method - a home made, multi-processor, lunar powered (I use the back of solar panels so they don't get blinded by the sunlight) ouija board. I go into a hypnotic trance and teleport myself (sometimes I just use the car) to a point just above Thidakhaing's homework book then draw their mental vibes upward so I can sense their meaning.
Brian.
 
By "R1", are you referring to the feedback resister or the one that goes from the regulator pin to ground?
 
Even considering the standard datasheet application circuit, I can't get the sense of the original question.

6_1284406979.gif
 
Without the R1 for feedback you would have no control. You wouldn't get regulation. I'm not sure whether the output would rise toward the input voltage or drop toward ground or fluctuate, but you wouldn't want to do it that way. Why are you asking this question?
 
**broken link removed**

As shown in the data sheet at the above link:

Vout = 1.25V(1 + R2/R1) + Iadj(R2)

So if R1 -----> infinity,

Vout would be 1.25V + Iadj(R2) where Iadj is typically around 50uA.
 
Riding in on my psychic bicycle...

The question asked how the "input effect the output". Where and what R1 is was never stated. The OP never posted a schematic or referred to a data sheet.
I'll have to do some experimenting, I've got a box of R1s here somewhere.

Brian.
 
Where and what R1 is was never stated.
You're absolute right. I didn't claim to answer an original question. But I learned, that even non-questions can be answered wrong. I still appreciate your first reply and suggest to ask clear questions at edaboard.
 
Hi everyone,
This is schematic for LM317,I want to know the value of Vout and Iout at the specified point.
Thank you very much for all reply
Thida

---------- Post added at 03:09 AM ---------- Previous post was at 02:19 AM ----------

What's the output at ADJ pin?
Thanks and Regards
Thida

---------- Post added at 03:33 AM ---------- Previous post was at 03:09 AM ----------

Refer to ON semiconductor's LM317 data sheet,the Vdiff between Adj and output pins should always be 1.25V regardless of input voltage.But the outputs of both pins are the same as input and it has only a few mA difference.I still don't get any idea about it.
 

It's impossible to understand your observations without knowing the circuit. Sounds like you left Vadj unconnected. The external circuit must provide a current path for Iadj.
 

I just use LM317 as a constant current source but output current is changing with input voltage.

---------- Post added at 08:18 AM ---------- Previous post was at 07:04 AM ----------


Hi FVM
The above is schematic for constant current source and now Iout is 37mA(=1.25/33R)
and what is the output voltage at the same point?
Thanks
Thida
 

The schematic doesn't show on the forum but it does download!
The schematic is a constant current generator, the 317 tries to maintain a constant voltage between it's output and 'adj' pin. Because there is a fixed resistance between them, and it is trying to hold the voltage across it constant, Ohms law says the current must be constant as well. In this case it will be I=V/R I=1.25/33 which makes 0.0378 Amps or 37.8mA.

Referring to your original question, which now makes more sense, provided the input voltage is high enough for the LM317 to start stabilizing and low enough that it isn't damaged, the output current will stay fairly constant. The output voltage will adapt to keep the current at 37.8mA.

Incidentally, the current is not flowing out of the 'adj' pin, it flows out of the 'VOUT' pins and through the resistor, it is being sensed by the 'adj' pin which if anything is an input.

Brian.
(dismounting my psychic bicycle, it was a tough ride.)
 

Hi Brain
If I put a series resistor says 10KR to the current source,then would the voltage impressed across be Io*10KR?
Regards
Thida
 

Obviously, the load resistance must be below about 550 ohm, otherwise the programmed constant current can't be maintained. This isn't a particular effect of LM317, but a general property of a (passive) current source, that don't source energy to the circuit. It's just ohms law. With LM317, the regulator dropout voltage and sense resistor current drop further reduces the available output voltage, respectively maximum load resistor.
 

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