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When using a digital voltmeter, voltages in the bottom
quarter of voltage limits are undefined. A ternary world
where we have good, bad, and undefined conditions. It may be
good or it could also be bad - therefore undefined.
That voltage reading is 11.67. Number is within voltage
limits but is in bottom quarter. Now that could be 11.67
volts with a ripple voltage of 50 mv. Or that could be a
voltage that varies between 11.3 and 11.7 because the 12 volt
output has too much ripple voltage - 400 millivolts. Voltage
is still undefined. A meter often tends to read the upper
voltage of that ripple wave. Therefore that 11.67 volts
really might be 11.5 with excessive (out of spec) ripple
voltage. IOW voltage dropping repeatedly to 11.3 volts.
IOW we don't know if that 12 volts is good because at 11.67
volts, it is in the bottom 1/4 of those limits AND because
multimeters don't always read RMS; cannot read higher
frequency ripple voltages.
Again, that 3.3 volts is in limits but suspiciously high.
And that 12 volts is also in a region of suspicion. Yes the
system is often very tolerant of bad 12 V DC. But a system is
often less tolerant of high ripple voltages. 11.67 may be a
symptom of excessive ripple voltage.
kony wrote:
> On Thu, 13 Jan 2005 14:36:18 -0500, w_tom
> <w_tom1@hotmail.com> wrote:
>> Limits for 12 volt supply: 11.4 V to 12.6 V
>> His voltage: 11.67V. Therefore as previously posted:
>>> 12 volts is just above the limit.
>>When taking voltage with a 3.5 digit multimeter, the readings
>>must be in the upper three quarters of those limits. Voltages
>>in the lowest quarter may be due to excessive ripple voltage.
>>This is a failure that has not yet happened. Multimeters
>>cannot measure ripple voltages. Therefore a voltage in the
>>lower quarter of those limits suggests further study may be
>>required using other equipment.
>> ...
>
> Primarily, OP omitted basic critical details of the
> motherboard and power supply. We have no reference of power
> distribution for the system.
>
> I agree that 11.67V is in-spec BUT that a motherboard
> reading cannot be relied upon, in general most motherboards
> do read a lower than actual 12V level when PSU output is
> spot-on 12.0V. Multimeter readings at power supply
> connector to motherboard are crucial for 12V reading.
>
> Having written that, if the multimeter reading is at 11.67,
> it is a sign that the power supply is of insufficient
> capacity, else not appropriate for the system (typically
> using a system with 12V power for CPU but not an ATX v2.03
> power supply, so the PSU has little to no 12V feedback
> weighting for regulation, rather it's regulating based
> mostly or entirely upon 5V reading, which would coincide
> with voltage readings reported. Even so, CPU VRM circuit
> will remain stable far below 11.67V, most would be OK below
> 10V, but this tells nothing of the power supply's other
> rails.