[SOLVED] How to produces 3v output using LM317

[WhyWhy]

Hi guys,

i have some doubt for the LM317 voltage regulator. According to the link below, it's say the reference voltage=1.25V.

1) Where did the 1.25V comes from?
2) Changing the value of the Variable Resistor ( R2 ) can get difference value of output?
3) The formula shown in the picture can be use to calculated the output?

https://www.eleccircuit.com/lm338-lm350-lm317-voltage-regulator-calculator/




Thanks.
Regards,
YY

 

[D.A.(Tony)Stewart]

Which part of the component spec did you not understand?

https://www.fairchildsemi.com/ds/LM/LM317.pdf

LDO's use what is called a Band-Gap ultrastable reference voltage. and Op amp linear gain amplifier and a buffer.

 

[WhyWhy]

I mean the Reference voltage remain 1.25V even i change the input from 6V to 12V ?

And 1 more thing is the R2 got specific value, example: 10k,20k or 100k ohm ?

 

[D.A.(Tony)Stewart]

LDO is like a simple Non-Inverting Op Amp, gain control is 1 + R2/R1, >> Relative to the constant Vref. of 1.25

Input Bias current adds to Vout offset by 50~100uA * R2. according to spec. with a small variation with temp.

So choose R2 small enough accordingly but not too small that it dissipates power. so 1k*50uA= 50mV offset, 10k*50uA = 0.5V offset you can see 10k values have a large offset on this old design (317)

 

[Borber]

Reference voltage is got it's name because it is constant and stable othervice it can not serve as reference.
Zener diode voltage in other circuits is used as reference voltage for example and is not changing with input voltage. Imagine this 1.25V reference voltage as zener diode built in LM317.

 

[Audioguru]

R1 should be 120 ohms for an LM317 and can be 240 ohms for a more expensive LM117.
The 1.25V reference is across R1 so its current is 1.25V/120 ohms= 10.4mA.
R2 also has a current of 10.4mA because it is in series to ground with R1.
Then Ohm's Law is used to calculate the voltage across R2 and/or the resistance of R2.

If you want an output of 3.0V then R2 has a voltage of 3V-1.25V= 1.75V, its current is 10.4mA and its resistance is 1.75V/10.4mA= 168 ohms.
I did not include the bias current in my calculation because the bias current is so small.

 

[crutschow]

I mean the Reference voltage remain 1.25V even i change the input from 6V to 12V ?
...............

The LM317 has an internal reference voltage and circuitry that tries to maintain 1.25V between the output and the reference pin under all conditions. Thus, for example, if R1 were 120Ω and R2 were 240Ω then the LM317 would adjust the output voltage until the voltage across R1 was 1.25V. All current through R1 has to flow through R2 (except for the small reference pin bias current) so the voltage across R2 would be 2.5V. Therefore the output voltage is 1.25V + 2.5V = 3.75V. Thus you can see that the output voltage can change, depending upon the relative values of R1 and R2, while the reference voltage remains a constant 1.25V.

 

[WhyWhy]

Hi all,

I understand the concept right now. Thanks Alot :laugh:

Regards,
YY