Other Output Voltages

If the output voltages (20/17.6) produced by the resistor values shown in the parts list aren't quite your personal PR-1's cup of tea, or you just want to experiment a bit, here are a few variations:

1. DIFFERENT VALUES OF FIXED OUTPUTS
The table below tabulates several other combinations of resistors R1, R2 and R3, and the output voltages they produce. Note that some of the resistor values are non-standard; they're created by paralleling two standard values, and are noted with an asterisk.

R1	R2	R3	Vout HI	Vout LO
220	3.3K	15K	20	16.6
220	3.1K*	22K	18.9	16.7
220	3.1K*	15K	18.9	15.8
270	3.6K*	22K	17.9	15.5
220	2.8K*	22K	17.2	15.4
220	2.7K	22K	16.6	14.9
220	2.7K	15K	16.6	14.3
270	3.3K	15K	16.5	13.8
270	3.3K	10K	16.5	12.7

* Parallel Combinations
3.1K = 3.9K in parallel with 15K
3.6K = 5.6K in parallel with 10K
2.8K = 5.6K in parallel with 5.6K

2. CONTINUOUSLY VARIABLE OUTPUT
If you have a good voltmeter, you can simply replace the fixed resistors R2 and R3 with a potentiometer (variable resistor) and set your output voltage to any value you wish. Using the diagram below, substitute a 5K ohm pot (RS 271-1714) paralleled with a 10K ohm fixed resistor (to limit the output voltage to a sane level) for the toggle switch/resistor combo.
Be very careful to never exceed 22 volts!

3. CALCULATE YOUR OWN FIXED OUTPUT VOLTAGE
If you know exactly what output voltage you want and don't wish to be fiddling with adjustments, then just calculate the value of R2 desired from the formula below. I suggest either 220 or 270 ohms for R1, but you can use any value in between if you have a resistor-rich parts bin. Forget about R3 and the toggle switch -- this is the ultimate in simplicity!

R2 = ((Vout*R1)/1.25) - R1
(This formula can also be found on the back of Radio Shack's LM317 blister package)

Updated 7-99