LM7809 Question

[the_random_hero]

I've recently built a power supply to run a few pedals plus my Ruby amp. However, when I'm just running my Ruby alone I'd rather switch the power supply to putting out 12V. Instead of building another 7812 circuit inside the power supply, I was lead to believe that I can just place a resistor from pin 2 to ground to raise the output voltage of the regulator. By my reasoning, as the quiescent current flowing from pin 2 to ground is around 5mA, I want it to drop 3V over the resistor. The value of the resistor equals 3 / 0.005, which = 600.
Is my reasoning correct? I was just going to use a switch to change between the resistor and a wire link to ground.

[Andre]

I don't know about your solution with a resister, but I do know you could also use a zenerdiode to increase the output of a 78xx type voltage regulator.
As far as I know there are no  3V zeners, so you would have to use a 2V7 or 3V3 zener.

André

[Sock Puppet]

Take a look here:

http://www.cpemma.co.uk/diodes.html

The zener version is more common I think.

S.

[dschwartz]

just use a 5.1V zener and run your ruby at 14 volts..it will sound better and a bit louder

[the_random_hero]

just use a 5.1V zener and run your ruby at 14 volts..it will sound better and a bit louder

I don't have sufficient headroom to go much over 12V, only around 15.5V after the filter caps. I'm fairly sure this can be done, but I just want to know if anybody has done it before. I could go the voltage divider route like in the link Sock Puppet posted, but that would involve a bit more playing around on my board - my layout is fairly tight.

[Sock Puppet]

Young Sock puppet thought:

"The resistor R1 is present to produce enough current to make sure the zener is well on, and many regulators will work fine without it (Iq is enough)."

You might just try a zener alone as suggested. with a switch aross it.

S.