How to select resistor values for LM317 regulator

Other Hardware General Electronics
1

Hi Everyone.

I want to experiment with this cct below , does any body know what the resistor values should be?

I want to use 8 bit binary from a Nano with preset buttons and also a UP and DOWN button.

Thanks in advance.

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V_out = 1.25 * ((R2 / R1) + 1)

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3

should be not equal zero.
everything else dependent what you expect to achieve.
and you probably already know, every voltage drop on LDO regulator will convert in heat. actually i mean power. voltage drop * current = power heat dissipation.

4

If you are asking what the values should be to have an 8-bit selector, then all the collector resistors and R2 should have geometric progression values by a factor of 2, and so you would have 256 output voltage levels. You have to keep in mind that to the resistors you have to add the Vcesat of the transistor or so I think. Maybe a MOSFET is a better choice. By the way, if you want such regulation, you can use only 1 pin of the Arduino with PWM controlling the discharge of a capacitor with a resistor.

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5

@flashko
Yes I want to use the full 256 steps , I will play around with the options.

6

Interesting idea but I am afraid C5 will make the transient response of the regulator very very poor. Moving it between Vout and Adj (parallel to R1) may be better.
EDIT: sorry, that was nonsense.

7

@Smajdalf
Interesting but not very practical.

When I start with the high voltage at 30V and use half of that and again half of the new value , I end up with this.

30v > 15v > 7.5v > 3.75 > 1.875 and the lowest is 1.2v

This is only 5 steps and 2^5 = 32
Then 30v/32 = 0.9v this way to high.

8

It's going to have maximum output till one of the 'digital inputs' is taken high
(the greater the ADJ-to-GND resistance, the greater the output).
Consider 0.5V steps.
Outputs < 1.25V (Vref) aren't possible.
(unless the ADJ terminal bias was to -1.25V instead of GND ??)

9

That diagram could be done with a TPIC6B595 shift register.
It has eight open-drain mosfets that can be directly connected to your R2R ladder.
Leo..

10

The posted circuit doesn't show the transistor base current limiting resistors.

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