Strange PSU question need help, a bit long

I didn't post any questions here for quite sometime now. Anyway, I just put together a new PSU and HSF on my TB1.2GHz to lower the noise down a bit. I got the Antec True430Watts and Zalman 6000cu HSF installed on my rig. However, the Zalman HSF cause a bit problem to my system because the HSF is the flower-like shape and hence cover part of the ATX power connection on my MB (Aopen MK33). I didn't know this problem until I finish installing the HSF and ready to plug the ATX cable in. In the process, I cause a bit scratches on the Antec ATX cable (just very minor scratches, not even deep or anything). Just to play safe, I wrap around the scratched cables with silicone tape (the best electrical tape around rated upto 200C).

Anyway, this solve my problem since everything is nice and fit now. But taking out and puting back the ATX cable cause me problem everytime because of the space problem caused by the Zalman HSF. So I decide to drive out and get an ATX cable extension from a local Microcenter today. It is a simple short ATX extension cable about 6" long. I figured I would leave this extension with my rig at all time and if I want to take out the ATX cable, then I just remove the cable from the extension joint instead of the MB connection. Well, everything went well and I get my system fire up.

Now here is a strange problem I found. The voltage reading from my system using MBM5 change just a bit. The +5v line is more fluctuated (from 4.91v to 4.95v) while the +12v line is more stable (it is now fix at +11.65v and every now and then move to +11.59v). Please note that these readings are from my MB which I suspect that the voltage regulator on my MB is not so good. I also measure the voltage using my multimeter (Fluke 179 if it matter) and read

the +5v as +5.047v to +5.054v
the +12v as +11.94v to +11.96v

The odd thing doesn't end there. The voltage from the multimeter reading is more stable on both +5v and +12v (I mean the fluctuated range is less than when I didn't put the ATX cable in abeit very small differences). So what going on with my system now. To summarize my problem,

After install the ATX extension cable -> MBM5 reading fluctuated range for +5v increases from normally [+4.93v/+4.95] to [+4.91/+4.95].

-> Multimeter reading fluctuated range for the +5v decreases from [+5.046/+5.058] to [+5.046/+5.054].

Is there anything I should worry about? And BTW, how can you measure the +3.3v using multimeter? Is there any ways I can measure the voltage output from the ATX power connection out of the PSU directly? I cannot start the PSU unless I connect it ATX cable to the MB. Thanks.


P.S. The voltage fluctuation only occur relative to CPU usage which seem to be normal to me. I.e., at low cpu usgae the +5v will read +5.046v and at max cpu load, it read +5.058v (using multimeter measured from a molex plug).

Nvidia or ATI, it's all depend on what you like and system optimization :lol:
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More about strange question help long
  1. Such small voltage fluctuations shouldn't cause any problem.

    My PC eats so much money that I'm in 'desperate' need of it to buy PC3500 RAM, help Svol with his OC project!
    --- PM me for information.
  2. I change the ATX extension cable with another new one and get exactly the same result. The wierd thing is the reading from multimeter is actually better with the extension cable (just a bit) while the reading from MBM5 (from MB) is a little worse than without the cable??? I can only assume that my MB voltage regulator is not very good and hence create all this weird problem since I cannot imagine adding a 6" cable will affect the voltage fluctuation at all.

    Now the voltage reading for +12v is much more stable than without the extension cable while the +5v line is a bit more fluctuated.


    Nvidia or ATI, it's all depend on what you like and system optimization :lol:
  3. I thik that as soon as the power comes to the mobo it goes through some capacitators thus changing the voltage a little. But with such readout you don't need to worry at all, +/- 0.1V is very normal.

    My PC eats so much money that I'm in 'desperate' need of it to buy PC3500 RAM, help Svol with his OC project!
    --- PM me for information.
  4. I asked my really old father about this (He was a TV repairman) and he told me this.

    A standard multimeter measures peak voltage rather than true RMS voltage. And in the case of AC this is calabrated to give RMS voltage with a sine wave. If the DC voltage (PSU puts out DC) has AC voltage superimposed on it your normal multimeter will give inaccurate readings. I suggest with your long lines you are getting a high level of AC superimposed on the DC voltage caused by the inductance and the resistance of the wire in association with the high peak currents from the computer. This is causing high peak voltages on the line therefor your multimeter is giving inaccurate readings while the motherboard appears to be indicating closerto the true voltage.

    That is what he said. That sounds like what the problem is. So it is not something that can be fixed and it is not something that is likely to cause a problem either.
    Hope this explains it alright
  5. Thanks everyone. vk2amv, I don't quite understand the superimposed AC thing but my multimeter can read both AC and DC voltages (one of them convert the voltage using the sine curve like you said). Even if I use DC mode of the multimeter, I usually get a very close result to the real voltage (I tested this in my lab using various power supply with known voltages). What do you mean by the superimposed AC on the DC will caused the reading on the multimeter to be inaccurate? This sound very interesting to me since I always think that the reading from a good multimeter or voltmeter is a very accurate one.

    As of now, I take off the ATX extension cable since I don't like the level of fluctuation I got for the +5v line (it fluctuate depending on CPU level too often). Without the extension cable, the MBM read out is more stable. And I believe that my MB (AOpen MK33 about 2 years old) has not so good voltage regulator. I know that few people have problems with other MBs as well when it come to voltage read out and most of the low voltage read out (if the PSU is a good one) is caused by the poor MB voltage regulator.


    Nvidia or ATI, it's all depend on what you like and system optimization :lol:
  6. If think it means that the noise from other components is getting into the longer lines. I dont know if this description will be understandable. You have longer lines so there is more oppourtunity for noise from outside sources to get into the wires. But on the shorter lead it doesnt happen because there is less unshielded lead length. And the AC being superimposed from the outside noise getting into it comfuses the multimeter but the MBM is not getting confused because it is being read after the regulators and everything else so it is easier to read. think that is it. Hope this helps
  7. Yeah I think that is the way to explain it... the electro magnetic interference generates induction flow inside the wires and thus can create fluctuating voltages.

    My PC eats so much money that I'm in 'desperate' need of it to buy PC3500 RAM, help Svol with his OC project!
    --- PM me for information.
  8. I think I understand what you mean. However, I think that a good multimeter reports more accurate voltage (I mean the true voltage that the PSU supplies, not the voltage after the current pass through the MB regulator). This is quite interesting since I've never think about it before. I tried measuring the voltage output from the PSU directly by connection the PSU to a simple PS tester (with about 25W load). I think Antec sell something like this as well. Anyway, I measure the voltages from the PSU directly (the ATX connector) and get the following

    +3.3v -> +3.34v
    +5v -> +5.07v
    +12v -> +11.97v

    which is more or less the same as when I use the multimeter to measure the voltages from the 12v molex (a bit less when the system is running). The reading from both BIOS and MBM seem to be off a lot especially on the +12v line. They reads as follow [idle/full load]

    +3.3v -> [3.23v/3.25v]
    +5v -> [4.93v/4.95v] very few times go to 4.91v
    +12v -> [11.59v/11.65v] very few times go to 11.71v

    I didn't have a chance to test the set up with the extension cable since I already returned it but I think you are right that the longer lead (from the ATX extension cable) caused the fluctuations and voltages deviations. It is amazing that just 6" extension cable can cause wider fluctuation in the voltages that are noticeable.

    Thanks for your helps.


    Nvidia or ATI, it's all depend on what you like and system optimization :lol:
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