Eudimorphodon
Veteran Member
Wondering if anyone on these forums who dabbles in electronics has encountered some of the "issues" I'm seeing.
Over a year ago I started using a hotwired (just stuck a loop of wire between the power-on signal and ground on the motherboard plug) ATX power supply to power breadboard experiments running at 5v. This seemed to work fine; I sourced the power to the breadboard by sticking wires in one of the 3.5" drive connectors. Recently I bought some 3.3v microcontrollers (Pi Picos) to fool with, and decided I needed to try to formalize this mess to be less jury rigged. As luck would have it I noticed that breakout boards for converting ATX PSUs into bench supplies were all over Amazon; I'd been thinking of laying out a PCB for this on my own, but these preassembled boards are far cheaper than the parts separately so I bought one. Here's the thing I'm running into with an old "Enhance" branded FlexATX power supply (2003-ish vintage?):
The cheapo boards from Amazon don't include any kind of ballast resistors or other dummy load. I remembered from the "old days" that switching power supplies didn't like running with insufficient load, but I hadn't really noticed a problem when I was just using the 5v feed from the supply, so I guess I assumed that "Modern" PSUs didn't have that problem. But what I discovered with the breakout board is while the other voltages (+/-12, 5v) were in the ballpark at least of being right (5v is 5.2v unloaded) the 3.3v feed was only putting out about 2.5v and, worse, if you hung any load off just it the PSU would shut down. An obvious thing to assume would be that the 3.3v output on this supply is bad, but on a hunch I cut some sockets off a string of dead Xmas lights I keep in my junk drawer (it's a convenient source of resistive loads), and experimentation revealed that if I load the +5v output with about half an amp's worth the 3.3v supply starts regulating properly. (Also of note, with a load on it the +5v also starts regulating much closer to a "proper" 5.0v instead of that 5.2v unloaded figure.)
That's... okay, but fiddling around with it some more it kind of seems like that there may still be a limitation that I have to draw as much or more from the +5v feed as from the +3.3v to *keep* its output from falling; IE, with about half an amp +5v I can pull at least around .35a(*) on the +3.3v feed and it's fine, but if I double the draw on +3.3v, IE, make it higher than the draw on the +5v, it goes down to around 3.1v. (Which I can then kick back up by adding more load on the +5v line.)
I guess the long and short of it would boil down to this: does this ATX power supply just suck and I should get a newer one, or are quirks like this likely to be common even in newer supplies? Should I just pile a couple amps of Xmas tree lights (or, you know, a boring old power resistor) on it and call it a day, or buy a new supply?
(* All figures are ballpark because, yeah, I'm using more light bulbs as the test loads because they're here; they run about 9 ohm or so each.)
Over a year ago I started using a hotwired (just stuck a loop of wire between the power-on signal and ground on the motherboard plug) ATX power supply to power breadboard experiments running at 5v. This seemed to work fine; I sourced the power to the breadboard by sticking wires in one of the 3.5" drive connectors. Recently I bought some 3.3v microcontrollers (Pi Picos) to fool with, and decided I needed to try to formalize this mess to be less jury rigged. As luck would have it I noticed that breakout boards for converting ATX PSUs into bench supplies were all over Amazon; I'd been thinking of laying out a PCB for this on my own, but these preassembled boards are far cheaper than the parts separately so I bought one. Here's the thing I'm running into with an old "Enhance" branded FlexATX power supply (2003-ish vintage?):
The cheapo boards from Amazon don't include any kind of ballast resistors or other dummy load. I remembered from the "old days" that switching power supplies didn't like running with insufficient load, but I hadn't really noticed a problem when I was just using the 5v feed from the supply, so I guess I assumed that "Modern" PSUs didn't have that problem. But what I discovered with the breakout board is while the other voltages (+/-12, 5v) were in the ballpark at least of being right (5v is 5.2v unloaded) the 3.3v feed was only putting out about 2.5v and, worse, if you hung any load off just it the PSU would shut down. An obvious thing to assume would be that the 3.3v output on this supply is bad, but on a hunch I cut some sockets off a string of dead Xmas lights I keep in my junk drawer (it's a convenient source of resistive loads), and experimentation revealed that if I load the +5v output with about half an amp's worth the 3.3v supply starts regulating properly. (Also of note, with a load on it the +5v also starts regulating much closer to a "proper" 5.0v instead of that 5.2v unloaded figure.)
That's... okay, but fiddling around with it some more it kind of seems like that there may still be a limitation that I have to draw as much or more from the +5v feed as from the +3.3v to *keep* its output from falling; IE, with about half an amp +5v I can pull at least around .35a(*) on the +3.3v feed and it's fine, but if I double the draw on +3.3v, IE, make it higher than the draw on the +5v, it goes down to around 3.1v. (Which I can then kick back up by adding more load on the +5v line.)
I guess the long and short of it would boil down to this: does this ATX power supply just suck and I should get a newer one, or are quirks like this likely to be common even in newer supplies? Should I just pile a couple amps of Xmas tree lights (or, you know, a boring old power resistor) on it and call it a day, or buy a new supply?
(* All figures are ballpark because, yeah, I'm using more light bulbs as the test loads because they're here; they run about 9 ohm or so each.)