I doubt 12 vs 15a would be a problem on an XT unless you have it very heavily loaded - you could measure to be sure.

-5 is always almost no current - I sometimes just put a 7905 -5v regulator on -12

So just take the incoming -12V wire and split it into two wires? One going to the -12V input on the motherboard the other passing through a 7905 then going to the -5V input on the motherboard?

Should I use capacitors like the Voltage Blaster does?

That's pretty much it - probably worth measuring the current on the -5, just to see if you need any sort of heat-sink (probably not)

From -12, -5 will "drop" 7v - current will be very low, but even at 10ma if would mean 70mw of disappation - depending the case of the regulator, with no heatsink at all it might get a little warm.

I'm also a big believer in current limiting - over the years I've gotten pretty good a fashioning vert low current "fuses" using bit of super-fine wire... you'll have to decide if it's worth the effort in this case.

What would be the purpose of the fuse? To limit how much the -5V line can draw?

Matrolisk wrote on 2026-08-06, 22:49:

What would be the purpose of the fuse? To limit how much the -5V line can draw?

It's not important - I'm just anal about protecting stuff when I build something that interfaces to it - just in case something goes wrong along the way.
Almost nobody else would bother with it (just take extra care and double check connections a lot 😀 )

This is an interesting thread. Do you have pointers to any visuals or guides so a novice can implement and modify a PSU without existing -5V? Your description is good and sounds simple but some reassurance for newbies is always good thing. Many thanks.

Well... I've not actually done one it quite some time, and I don't think I have anything now to photograph, but here's a quick&dirty image (sorry, it was done in my own "MicroCad" so not excessively high quality - 640x480 original output).

It's just a 7905 05v regulator
"Ground" is any good ground on the system (can take from main power or drive connectors)
"-12vIn" is -12v which you can get by splicing into the main power cable from the PSU.
"-5vOut" is -5v you can send to the system as needed.

"CurrentLimit" is optional as described above ... you can make up a "fuse" using one strand from a fine stranded cable - you could also just use a low value resistor and see if it gets hot - it's going to have a bit less than 7v "dropped" across it (7v minus regulator hidden drop) 10-20Ohms would likely work well (check to be sure the output is -5v not less (more? 😀 - an analog regs typically have somewhere around a 2v internal drop so you'll prob want 7v+ making in to the regulator input - another reason why I tend to go the "super small fuse" route)

Oh, one other thing to keep in mind - it's been so long since I did this (or built a simple PS with a negative output) that I had to lookup the 7905 pinout.

These type of three terminal regulators often have the tab electrically attached to the center terminal - on positive regulators this is ground, but looks like on the negative regulator it's going to be on the input - so just be careful when mounting the regulator - the tab might be hot (electrically) at -12v.