View Full Version : ...somethin wrong here... am I missing something here?
So, I'm playing around with a few of my extra PSUs, right now to power a big DC motor, but tomorrow I'll be starting work on converting one to a bench power supply. So, on to the problem...
For some reason I'm only reading half the voltage I should be getting. Like, the 12V rail is reading as 6V, the 5V rail is reading as 2.5V, and so on.
...any ideas? :? :?
billygoat333
05-01-2010, 02:10 AM
time for fresh batteries in the DMM?
I don't think so, it reads exactly what it's supposed to on some wall warts I've tried it on.
billygoat333
05-01-2010, 02:31 AM
hmm... thats odd.
Ilden
05-03-2010, 01:12 PM
Is the PSU one of those few that are built with two bridged smaller units in one case? One unit died?
Are the voltages exactly half? Could you have tripped the 110/220 switch by accident? Though it's really doubtful that would cut it neatly in half.
Nope, they're both single units. I'll check on the 110/220 switch, but I'm pretty sure they were both in the correct position...one was even taped over by the manufacturer.
Trace
05-03-2010, 04:26 PM
IDK if this would make a difference (I doubt it would) but are you grounding back to a black wire on the PSU or just to something metal in the room?
Oneslowz28
05-05-2010, 03:54 AM
is this one of the PSU that needs a load to fully switch on?
Airbozo
05-05-2010, 10:16 AM
is this one of the PSU that needs a load to fully switch on?
This^. maybe?
Hmm, I wonder...that would make sense...I'll try that when I get home.
Hmmm, still doing it. I checked both of my nice spare PSUs, and they both do it too, so I think I'm missing something. I tried putting a load (hefty DC motor) on the 12V line, and that didn't change anything except for making the voltage go down, just like it's supposed to. Using the PSU I want to convert as a sample case, the 12V line puts out 7.40V, the 5V line puts out 2.93V, and the 3.3V line puts out 2.52V.
billygoat333
05-07-2010, 02:02 AM
have you tried a different power cord to the PSU? just an idea... :think:
No, but the same power cord used with an external adapter gives the correct voltages. I'm thinking it must be something weird with the PSUs that I'm missing.
mDust
05-07-2010, 06:46 PM
I'm thinking it must be something weird with the PSUs that I'm missing.
Have you tried hooking it up to a motherboard and testing the voltages with it running? It may be expecting some of those 24pin connector circuits to be closed. Since you're probably just grounding the green wire it's turning on, but is smart enough to not provide full power unless it has somewhere useful to go.
Hmm, no, I haven't tried that yet...I'll try hooking one up when I get home tonight.
diluzio91
05-07-2010, 07:46 PM
That sounds familiar somehow... a failsafe incase something get plugged in wrong?
Hmm, ok, here's something interesting..
Going off this pinout (http://pinouts.ru/Power/atxpower_pinout.shtml), +5 VDC Standby Voltage rail is outputting the correct 5V. I tried crossing it with ground and 'Power Ok', and neither did anything. Also, 'Power Ok' doesn't show any voltage, so from those two, I'm guessing that the PSU is in standby....now I just have to figure out how to get it out of standby...
Ok, after I finally researched this.. :facepalm: ..I discovered that, yes, most new PSUs need a load on them in order for them to actually power on. It turns out this is usually handled by hooking a 10 Ohm, 10W resistor between the 5V line and ground. I've verified this with two pretty hefty 12V electric motors that I have lying around. With one hooked into the 12V line, it barely turns over, then when I hook the other into the 5V line, suddenly the first bursts to life. Now I just have to get my hands on a 10 Ohm 10A resistor... The best I have right now are some 120 Ohm 3W resistors...and I only have 6 of them, not the 12 I would need to get down to 10 Ohm. I tried with the 6 I have, and it does have a noticeable effect on the motor, but not enough to bring it up to 12V.
billygoat333
05-08-2010, 01:05 AM
eh. probably just needed to sacrifice some goats to the gods of powahh. ;) (and by goats, I dont mean me... )
nevermind1534
05-08-2010, 10:58 AM
Ok, after I finally researched this.. :facepalm: ..I discovered that, yes, most new PSUs need a load on them in order for them to actually power on. It turns out this is usually handled by hooking a 10 Ohm, 10W resistor between the 5V line and ground. I've verified this with two pretty hefty 12V electric motors that I have lying around. With one hooked into the 12V line, it barely turns over, then when I hook the other into the 5V line, suddenly the first bursts to life. Now I just have to get my hands on a 10 Ohm 10A resistor... The best I have right now are some 120 Ohm 3W resistors...and I only have 6 of them, not the 12 I would need to get down to 10 Ohm. I tried with the 6 I have, and it does have a noticeable effect on the motor, but not enough to bring it up to 12V.
If you can wait, I'll add one of those resistors to my Digi-Key back order (they only cost a few cents, but there's one thing on there that won't be in until June or July.
Thanks for the offer, but I want to get this done asap.
...yeah, so much for ASAP. :P I finally got around to ordering some of the necessary resistors off Mouser on Saturday, shipped Tuesday, and I got them today. :D
I went ahead and picked up 10 because, hey, you never know, right?
http://i428.photobucket.com/albums/qq3/x13931x/20100603_002.jpg
and a single one:
http://i428.photobucket.com/albums/qq3/x13931x/20100603_001.jpg
I spent ~10 cents extra each (buying 10, these were $2 each) to get the kinda ones with heatsinks with mounts. That way I can just screw these onto the PSU housing or an old CPU heatsink or something, and keep it nice and cool. I'll do some testing to see how hot it gets. I'd rather just mount it to the PSU case, but I do have some old Pentium-era CPU HS/F's (you know, back when having a fan on your CPU meant you were serious business ;) ) that I can throw it on if need be.
altec
06-03-2010, 11:20 PM
You know you have problems when you can look at those resisters and say "Oh! Cool! I want some!" :P
I have no use for a 10W resister. Haha.
Well, I have no clue what I was doing wrong before..I must have been hooking something up wrong, because it wasn't working... Anyways, today I thought to pull apart my PSU tester to see how it was wired, and...basically exactly what the internet had told me.. 5A 15Ohm resistors on both the 5V and 3.3V lines. Apparently the 3.3V line load requirement is fairly new, but none of my PSUs worked with just the 5V line load.
So, the end result of that is that I finally have it working correctly! I ran it for a bit with some 12V 3A motors that I have acting as a constant load, and the 3.3V resistor seems to be just fine, but the 5V resistor got quite hot after just a short time. Looks like its time to dig through my box of old heatsinks! I knew I was holding onto those things for a reason. :D
blaze15301
06-18-2010, 06:45 AM
hey if you want some smaller heat sinks let me know i have a few laying around id let you have.
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