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# How to calculate Resistors Ratio?

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#### davyzhu

Hello,

I found Resistors Ratio(i.e. R1/R2) is always used in regulator and other voltage translation applications, but choose R1 and R2 manually is not a easy work, is there a software to do this work automatically?

Davy Zhu

#### nicleo

We can fix either R1 or R2. Say we fix R1, then we can use POT as R2. So, we can get the ratio that we want.

#### btbass

In the case of a 317 and similer adjustable regs, R1 is normally fixed at 240R.
It uses a 1.2Volt reference and this value sets the bias current at 5mA. Which is a good compromise for good regulation. Look at the data sheet, it will give you a formula for calculating R2.

#### sinatra

##### Full Member level 4
Hello all.
If you don't want to use a potentiometer so that you will not have to adjust any component during production you can do the following:

1-fix one of the resistors at an available comercial value, for example, fix R1.

2-Choose another commercial value for R2 that will be very near the desired value (the calculated value).

3- If the resistor value chosen in item 2 is smaller that the desired value: choose another very small comercial value resistor that can be put in series with R2 to bring its value to the calculated value.

4-If the resistor value chosen in item 2 is greater than the desired final value then chose a comercial value resistor (probably a large value) that will be put in parallel with the resistor found in item 2 to bring the total resistance of the parallel combination to the desired value.

5-Now you can produce the circuit using available commercial resistors in a series production and you will not need to do any adjust after production....

S.

Points: 2