Hobby Electronics Power Supplies Part 1: Wall Warts

Step 4 Variable Voltages

This circuit requires a few parts. R1 is listed as a 240 ohm resistor, which I never seem to have.  Anything close to that will do, but I recommend higher rather than lower (I use 260-270 ohms). I'm told smaller resistors will cause the chip to draw more current.

R2 is listed as a 5k potentiometer. I frequently to use 10k without issue. I like multi-turn pots. Rather than one full rotation, these pots can have 10-15 turns. That enables you to really fine tune your voltage.

The capacitor values listed are very common. Note that in the schematic, C1 is NOT polarized, but C2 IS. Polarized capacitors have a positive and negative terminal and must be connected in the correct orientation. Electrolytic capacitors (the ones in the metal cans that resemble batteries) are polarized. They will have a minus sign printed on them to indicate the negative terminal.

IMPORTANT NOTE! DANGER!!  C2 will have a voltage rating printed on it. This is the MAXIMUM it can withstand before failing violently. Make sure the voltage rating for C2 is HIGHER than the voltage you measured on your wall wart in step 1. When capacitors fail, a rubber plug blows out of one end (potentially becoming a projectile) and small amount of hot electrolyte goo will spray out.