I made this chart of all the voltages you'd get using standard 1% resistor values for all the combinations of R1 and R2 where R1 is 120R through 1K and R2 is 120R through 10K. It makes it really easy to choose fixed resistors to get near the voltage you want without having to look up numerous standard values and do each calculation until you find something.
For example if you want to heat a 6.3V tube filament just find your voltage and R1 will be at the top and R2 at the left, in this case 6.25 volts is found at the intersection of 300R (R1) and 1.2K (R2) a nice voltage for the tube just under 6.3. If you wanted to hit it nearly spot on 6.28V is found at R1 = 820 R2 = 3300. Thought it might come in handy for others so contributing it here.
For example if you want to heat a 6.3V tube filament just find your voltage and R1 will be at the top and R2 at the left, in this case 6.25 volts is found at the intersection of 300R (R1) and 1.2K (R2) a nice voltage for the tube just under 6.3. If you wanted to hit it nearly spot on 6.28V is found at R1 = 820 R2 = 3300. Thought it might come in handy for others so contributing it here.
Nice job!
But if you have the ubiquitous LM317 and the like in mind, their internal reference voltage is 1.25V +/-0.05V, i.e. +/-4%. So if you really need to set a precise voltage, a trimmer is always needed.
Furthermore, R1 shouldn't be much higher than 240R to make sure some current is flowing at the output in order to ensure regulation.
It's all in the datasheets, but most of them are written in a very confusing way.
But if you have the ubiquitous LM317 and the like in mind, their internal reference voltage is 1.25V +/-0.05V, i.e. +/-4%. So if you really need to set a precise voltage, a trimmer is always needed.
Furthermore, R1 shouldn't be much higher than 240R to make sure some current is flowing at the output in order to ensure regulation.
It's all in the datasheets, but most of them are written in a very confusing way.
Zung, yes the sheets are sketchy on R1, I've read use 240 but don't go over 1K, but if 240 doesn't hit your voltage a different R1 may work. This is just ballpark if someone doesn't need (or want) a trimmer using just std resistors.