I am adding an ATX power supply as a bench power supply to power accessories on my model railroad layout. I prefer to run some lighting accessories at +7v, but obviously a power supply has +3.3v, +5v, +12v and -12v. Someone said getting +7v was as simple as wiring a +5v to a -12v wire. That didn't seem right to me. Is it, and if not, what would I need to use to convert the -12v to positive voltage?
Electronic – Converting negative voltage to positive
atxconverterpowerpower supply
Related Solutions
The PSU's output feedback is based on the +5V rail. You need a load connected always to that rail. I suggest you leave the +12V rail without load (if it works).
simulate this circuit – Schematic created using CircuitLab
A simple npn transistor or n-channel mosfet with appropriate resistors on the power good wire, turning on the 5V relay, which in turn switches the 12V to the amp.
simulate this circuit – Schematic created using CircuitLab
Best Answer
Between the +5V and the -12V outputs, you have a total of 17V, definitely not what you need. Connecting the two together will damage the power supply. Not good either.
The proper way to get +7V is to use a buck regulator to regulate the +12V down to +7V with good efficiency. This is usually easier than to convert from a negative voltage.
The Alibaba web site is choke-full of buck converters of various current ratings that would certainly work just fine.
If your current demand is low (a fraction of an ampere), a linear regulator may work but they tend to get hot unless they are heat sunk. A Buck regulator is easier to implement.
You may need to read in general about buck regulators to understand how they work and apply them effectively.