naive pedal power question

[m-theory]

Let's say a guy had a pedal power supply with isolated outputs, and he could have two 9v outlets become 1 18v, but what he really wanted to maybe 12v or 15v.  Could he get that simply by adding resistors? 

[iaresee]

Yes. You can build a voltage divider:



If you don't need a lot of current at 12V you could use R.G.'s charge pump to get 18V from your 9V source and then add the voltage divider at the end to tap off 12V from it. See: http://www.geofex.com/circuits/+9_to_33.htm and http://www.diystompboxes.com/smfforum/index.php?topic=71657.0 -- then you'd only have to use one port on your power supply, not two.

[petemoore]

  Depending on how 'stiff' you need that supply voltage to be, how much of a load will be on it.
  A regulator has various advantages, get ~18 going, then regulate down to the chosen voltage.
   

[m-theory]

Cool, it's do-able, then!  I thought so.   

Specifically, I'm looking at either a voodoo labs pedal power +, a BBE supa charger, or a T-rex fuel tank, and I'll be powering a homebrew o/d, distortion, boost, phase, analog chorus, analog delay, and buffer.  I don't know how each circuit will react to higher voltages, and 18v may be excessive for some, so I wanted to know that I could knock 18 down to 12 or 15. 

Thanks!

[ampgeek]

Not sure if you have the urge to build your own power supply or not.
If so, check out this project:

http://www.generalguitargadgets.com/index.php?option=com_content&task=view&id=23&Itemid=26

I stuffed two of them (8 circuits total) into a box and bought a handful of voltage regulators of different outputs so I can easily plug one in to get what ever voltage I desire.

Dave O.

[R.G.]

Cool, it's do-able, then!  I thought so.   

Specifically, I'm looking at either a voodoo labs pedal power +, a BBE supa charger, or a T-rex fuel tank, and I'll be powering a homebrew o/d, distortion, boost, phase, analog chorus, analog delay, and buffer.  I don't know how each circuit will react to higher voltages, and 18v may be excessive for some, so I wanted to know that I could knock 18 down to 12 or 15. 

You do understand that just because the voltage is low, all is not necessarily well, don't you?

With a resistive divider, the voltage wavers around depending on the changes on the load. Unless the load is constant, the power supply voltage sags and rises inversely from the load. If you want a stable lower voltage, you're going to have to regulate it. Something like an LM317, a resistor, and an adjustment pot will do it.

Go read "Power Supplies Basics" at GEO.

[Zben3129]

Cool, it's do-able, then!  I thought so.   

Specifically, I'm looking at either a voodoo labs pedal power +, a BBE supa charger, or a T-rex fuel tank, and I'll be powering a homebrew o/d, distortion, boost, phase, analog chorus, analog delay, and buffer.  I don't know how each circuit will react to higher voltages, and 18v may be excessive for some, so I wanted to know that I could knock 18 down to 12 or 15. 

Thanks!

Make sure your parts are rated for 18v.
Resistors should be fine.
Film/Ceramic caps should be fine.
Check electros, especially anything 10uf and over as it is possible to accidentally use a 16v cap because of its similarity in size to other caps.
Check opamps and transistors on the datasheet. Make sure you get the datasheet of the opamp from the manufacturer you are actually using.

Zach

[m-theory]

With a resistive divider, the voltage wavers around depending on the changes on the load. Unless the load is constant, the power supply voltage sags and rises inversely from the load. If you want a stable lower voltage, you're going to have to regulate it. Something like an LM317, a resistor, and an adjustment pot will do it.

Go read "Power Supplies Basics" at GEO.

Ok, thanks!

Make sure your parts are rated for 18v.

I'll do that.  Electrolytics are fine, because they're all 25v or more.  I'll have to check semiconductor data sheets, though.