Electronics 2000

Hello. This is a first post from a complete novice about a first circuit. I have made a small board to reduce a DC voltage from a nominal 12v to a nominal 9v using a LM317 voltage regulator. Having made the circuit, with 13.5v going in, I am getting 7.3v coming out. This is without any load other than my meter.

I wonder if there is something about the resistor choice, other than their relative values, that is important. Not knowing that there was anything other than a ratio between the two (from the formula

VOUT = 1.25 * ( 1 + R2/R1 )

I went for 10ohm and 62ohm as this gave me the 6.2 (R2) to 1 (R1) ratio I needed.

In addition to the 2 resistors, there are 2 Tant Capacitors (0.1uf and 1uf) across input and output.

I have since found sites indicating that R1 should not be less than 100Ohms - eg http://www.reuk.co.uk/LM317-Voltage-Calculator.htm
The product information gave no minimum value for R1.

Can anyone help me with this please?

thanks

Coltsfoot

Well I read around a bit more, and found that R1 is usually quoted at more than 100ohm, and often 240ohm. So I substituted in 240 and 1.5kohm resistors for R1 and R2, and the circtuit now does what I need.
thanks
Colstfoot