How to calculate Resistors Ratio?

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

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.

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.

Hello all.
Just to add another point.
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.