Beware the accuracy of the onboard voltage monitors

[BrianT]

I was showing 4.67 V on my 5V rail in the onbaord monitor on my IWILL XP333. So I adjusted my Antec 400W P.S. to crank that rail up to 5V. Fine.

Not. I have a very accurate DMM, so I actually meaured the 5V rail in my system. It was now at 5.32V. I was now overvoltage. When I set the voltage back to *exactly* 5.000V with the meter, my BIOS showed 4.62V. So really, the P.S. was nailed in the first place.

Just a word of caution. Don't trust the BIOS volt meters. Mine was off by nearly .4V. Kind of makes you wonder about Vcore, doesn't it?

Regards,
Brian T

[Ottoman]

where did u measure the 5volt rail with the DMM? straight from the wire?

cuz u might lose some of that to resitant in the board before it hit's where it's supposed to...


but I have noticed a bit of voltage fluctuation between boards and same psu...

[Player0]

BrianT, your motherboard has its own power regulators on board. When you are reading the voltage from onboard (Bios, MBM, etc), you are reading the 5v voltage line from the on board power. If you connect you DMM to those points on the board (near the mosfets) you will read nearly the same thing as the BIOS reads.

Connecting your DMM to the 5v lead right off the PSU *will* give you a higher reading, because you are measuring a different (and often less stressed) power source.

My 5v line on the PSU reads 5.3v right now, onboard is about 5.13v. This is high because I hacked my Enermax to produce more volts. This can help with keeping vcore a little more steady, however most vcore fluctuations are caused by an underrun in current, not volts. Cooling those power mosfets is very important.

[KMS]

Brian, what kind of DVM are you using do you know its impedance ? May turn out to be far less acurate than the onboard reading! Where did u measure those volts out of interest ? on the board before or after the mobo regulation or on the psu ?