Using a switch to toggle DC jacks between + and - grounded.

[aben.eckroth]

I got lucky enough to find an old xbox intercooler power supply that spits out 12v DC at 480mA and I want to use it as a basis for a DC brick.  I'm trying to figure out how I can add toggle switches to each jack to manually switch them between positive or negative grounding.  The supply itself is negative grounded.  Any help or drawings would be greatly appreciated!

Thank!

[aben.eckroth]

Actually, I was way over thinking it.  Sorry for the crude drawing, but this should work correct?  I'm still debating on whether or not I want to use a 3pdt and an RGB led as somewhat of a warning light...but I think a toggle switch pointing the other direction should do the trick.  And not use extra current.  

(and yes, it would be ground....not V-)

[IvIark]

You can't do that, you would need isolation or as soon as you plugged in a positive ground effect the power supply would short and die.  Personally if I wanted to do this I'd use  a charge pump to invert the voltage, then you can use a toggle to switch between with +9V or -9V for the supply and ground can remain 0V for both positive and negative ground effects.

[aben.eckroth]

Hmm...I thought the switch would remove the V+ to ground.  I'm not really looking for V+ and V-, just V+ and ground.  

Am I greatly misunderstanding something about positive grounding? I though it was just a wiring orientation.  Does it actually represent a negative supply voltage?

Thanks for your help BTW!  Learning in progress...

[induction]

First a bit of terminology: V+ and V- refer to the voltage rails, whatever voltage they happen to be. With negative ground stompboxes, V+ is usually +9V, and V- is ground (0V). With positive ground stompboxes, V+ is usually ground, and V- is -9V. With bipolar supplies, V+ is +9V and V- is -9V. For the most part, there is no such thing as a negative or positive ground DC supply. DC supplies give you two leads that are a fixed voltage apart, but neither is tied to a reference voltage, so they can be used for positive or negative ground circuits, but not both at the same time.

That bolded part is the problem for what you want to do. As I said, the DC supply has no absolute voltage. This is true until you connect it to something. Once you attach one of the leads to something with a fixed voltage, then the DC supply inherits a fixed reference. In other words, when you plug your DC brick into a negative ground pedal, the brick's V- is now attached to absolute ground through the ground lead of the guitar cable (the shield), which leads to the actual ground via the ground prong of the amplifier's power cable. If you daisy chain that supply to a positive ground pedal, that same shield will attach +9V to the shield and the psu will attempt to maintain 9V difference between it's two leads while they are connected to each other. This will probably kill the brick.

If you were just building a psu for a single jack that wasn't going to be daisy chained, the switch would probably be ok. But you said you are planning to make a brick with several jacks. You haven't said specifically how you plan to do this, but unless you isolate those jacks from each other (I don't know how you would do that with DC), then any absolute reference you give to any one of those jacks will be inherited by all the rest. So you can flip the polarity of all of them at once, or none of them, but you cannot mix them unless you supply isolation.

Finally, are you planning to regulate that 12V supply down to 9V? If that adapter is switched-mode it will be noisy as hell. A regulator circuit will clean it right up, though.

[IvIark]

Positive ground pedals have 9V connected to ground and 0V where the supply is, so as soon as you put a positive ground pedal in a negative ground chain of pedals it shorts 9V straight to ground in the power supply and smoke will start coming out of it.

Using -9V cures the problem of mixing positive and negative ground pedals.  Ground is still 9V higher than the supply and so it works perfectly well, and because the supply is -9V it means that all grounds can then be 0V and you won't get the problem.  At the end of the day you can either do it like that, or the alternative is to use a power supply with isolated outputs so that you can power positive ground effects totally independently from any negative ground effects you're also using.  The IC you need to convert 9V to -9V will cost you about £0.20 and take you 10 minutes to make.  A power supply brick with isolated outputs will cost you £50+

[IvIark]

Actually just take induction's description, he has done a much better job of explaining it than me

[aben.eckroth]

Thank you guys.  I wasn't really thinking about the issue of mixing polarities - like I said, I'm learning.  Both of your explanations make perfect sense.

At the moment, I wasn't going to regulate the power supply because at the moment i'm only using it for an OCD clone, and the TL082 can handle the 12v.  That and I don't currently have any 9v regulators.  But now that I know I can "solve" my problem with a pump I shall now look into adding one to my supply and moving on with the project.  

Thanks again guys.