Hello, I have an ATX power supply. I tested it with a CD ROM drive attached and noticed that the 12V rail gives me a lower voltage when the CD tray is moving (11.79V against the usual 11.87V). Could this be due to bad capacitors?
"Slater"
** Small variations in the 12V and some other rail voltages are quite normal - only the 5V and 3.3V rails are actually regulated with any precision..... Phil
Often only one of the voltages, for example the the 5volt, is regulated, the others are determined by transformer ratio, and are more load dependent. The 12v is usually one of the "raw" voltages, and as such is allowed to vary quite a lot.
Right. All ATX power supplies are identical. Maker and model please? Some power supplies are junk.
If you're getting 10mv accuracy with your DVM, you must have a decent way of calibrating your instrument to that resolution. 100mv accuracy is more reasonable.
See the ATX PS specification at:
Section 4.1.4 specifies: Table 6. Voltage Tolerances Voltage Rail Tolerance +5VDC ± 5 % -5VDC (if used) ± 10 % +12VDC ± 5 % -12VDC ± 10 % +3.3VDC ± 5 % +5VSB ± 5 %
A tolerance of 5% would be anything from 11.4VDC to 12.6VDC. However, if your unspecified maker and model DVM instrument is calibrated to maybe ±3%, your acceptable range might be 11.0 to
13.0VDC.-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
Maker: Whatsup Model: ATX-400W P4
My concern is not how off the voltage is, it's the instability: is it normal? The voltage changes depending on the load. The PSU inside my PC doesnt do this.
Perhaps mine is in that league!
"Jeff Liebermann"
** How pedantic and idiotic !!!The OP is only asking about the small DROP in voltage he measured under load - which NOT dependant on calibration or accuracy of the meter.
The LINEARITY of most DMMs is of a very high order - 1 part in 1000 is typical.
.... Phil
Sorry. I've never heard of that brand. The part number is rather generic but matches an iCute power supply, another brand with which I'm not familiar.
The drop is fairly normal for a power supply going from no load to about 1/2A load at 12v. A drop of 80mv is about what I would expect across the Molex connectors. With a CDROM motor load, that's R = E / I^2 = 0.08V / 0.5^2 = 0.32 ohms internal resistance. That's within what I guess would be normal. Also note that only the 5V and 3.3V lines are actually regulated. The 12V line is not.
I'm not sure why your "other brand" of ATX supply does not show a similar drop. It might be where you're measuring it with your unspecified model DVM (my guess is Radio Shack or worse).
In any case, what you're testing is not very realistic. If you want to test a power supply, use a real load, or a resistor dummy load that simulates full rated load. If there's anything wrong with the power supply, you'll either see it with your unspecified model test equipment, or it will blow up the power supply. I've seen power supplies rated at 300 watts, that will shut down or die at 200 watts (or less).
If you think you have cheap junk or defective capacitors, they will show a drop in output voltage somewhat before full load. That not because the regulation fails. It's because they no longer act as a filter and you'll start to see switching ripple on the output leads. Insufficient filtering will do much the same thing. Attach an oscilloscope to the output leads and you should see the noise level increase dramatically with bad caps.
I've never heard of a Whatsup power supply. Neither has Google. That should offer a clue.
Grab a flashlight and look inside. If you see a crowded pile of electrolytic filter capacitors near the output section, it's probably a good design. If you see fairly small and few filter capacitors, it's probably (cheap) junk.
Good:
Plenty of output filtering.
Junk:
Note the lack of enough output filter caps.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
On 12/4/2010 10:57 PM Phil Allison spake thus:
Wellll ...
Isn't the correct answer here "don't worry about it"?
I mean, the O.P. is reporting a voltage drop of 80 mV. Compared to a reference of 12 volts (nominal), that's ... lessee ... 0.67%. Much less than any *rated* regulated variation of any computer power supply, right?
--
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Pedantic
-adjective
Ummmm... ok. I'm impressed with your ability to read between the lines, especially when there's nothing there to read.
The accuracy of the DVM's ability to measure 12.000000000 VDC is dependent on the calibration. While he could measure the relative voltage drop (80 mv) with considerable accuracy (as you noted), he could not determine whether the output voltage is within the specified
5% ATX tolerance (11.4 to 12.6VDC) unless the meter were calibrated to a tolerance much better than 5%. The question was whether the power supply was defective, which in my opinion is more a matter of keeping the output voltage within the specifications (with any load).Linearity is usually +/- 1 bit or LSB. If it's a 3 1/2 digit cheapo DVM, with a full scale reading of 1999, it probably has 10 bit A/D converter. At full scale, on the 20 VDC scale: 1 bit error = 20.0 V / 2000 = +/- 10 mv error. For an 80 mv difference, there's a 40 mv uncertainty due to non-linearity (+/-10 mv error for the no-load, and another +/-10 mv for the loaded readings). That's worst case and rather improbable for the rather small 80 mv measurement difference.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
"Jeff Liebermann = a lying AUTISTIC ASSHOLE "
** Drop dead you pathetic f****it...... Phil
On 12/5/2010 11:51 AM Phil Allison spake thus:
OK, so my guess is, oh, another week or so before Phil posts a calm, reasonable (and correct) message here.
Guess it takes that long for the meds to take effect ...
--
How To Access Wikileaks
These sites are still up as of 12/3/10:
http://wikileaks.de
http://wikileaks.fi
http://wikileaks.nl
http://wikileaks.eu
http://wikileaks.pl
And these IP addresses can be used:
http://213.251.145.96/
http://88.80.13.160/
Sheesh. I agreed with part of what you said. I thanked you for the correction on the type of A/D converter. But, this is what I get. Did you even read what I scribbled?
Have you EVER thanked or complimented anyone on what they posted? Do you even know how to do that? You know, "thank you for taking the time to explain the problem" or "thank you for the correction". Try it. I think you'll find that admitting a mistake and/or offering a compliment is a fair tension release, that just might produce a beneficial reduction in your vitriolic and venomous comments.
I appreciate your concern for my well being, but I think I can adequately take care of myself. If not, ObamaCare will keep me going until the money runs out. Try not to worry too much about it.
-- Jeff Liebermann jeffl@cruzio.com 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558
"Jeff Liebermann = a lying AUTISTIC ASSHOLE "
** Drop dead you pathetic FUCKWIT !!!..... Phil
On 12/5/2010 6:22 PM Phil Allison spake thus:
So you see, Jeff, it's basically a medical problem, not an etiquette issue.
Too bad. Guy seems pretty sharp on his few good days ...
--
How To Access Wikileaks
These sites are still up as of 12/3/10:
http://wikileaks.de
http://wikileaks.fi
http://wikileaks.nl
http://wikileaks.eu
http://wikileaks.pl
And these IP addresses can be used:
http://213.251.145.96/
http://88.80.13.160/
A baseball bat is quicker.
-- For the last time: I am not a mad scientist, I'm just a very ticked off scientist!!!
Sometimes that doesn't work and you end up like Dave.
-- Live Fast, Die Young and Leave a Pretty Corpse
"Don't try this without a firm grip, and a good follow through..."
-- For the last time: I am not a mad scientist, I'm just a very ticked off scientist!!!
What others told you about only the 5 (or 3.3) being regulated is true. As the load varies on that supply, the other voltages will rise of fall based on the load current on the regulated supply. This is normal and not broken. I first noticed this characteristic around 10 years ago when my power supply fan (runs on the 12 V supply) was changing pitch depending on the processor activity. That machine varied between 50 and 100 Watts.
G=B2
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