Re: Computer turns on and then just dies

Paul <[email protected]> wrote:

> ...
> 
> 2) Use a clamp-on ammeter, to monitor the +5VSB wire on the main cable.

For typical clamp meters, those rely on alternating current to fluctate
the magnetic field around the conductor; i.e., the common clamp meter
just measures AC.  The 5V standby line is DC, not AC.  With the
20/24-pin connector from PSU pushed onto the mobo (to account for any
load), use a voltmeter to measure DC voltage from the +5VSB line (pin 9,
purple) to a ground pin (black).

I can't think of any AC inside the computer excluding the input circuit
in the PSU.  Power into the PSU is AC.  All lines out of the PSU are DC.

DC also induces a magnetic field around the conductor, but not a
fluctuating magnetic field.  An AC clamp meter using a transformer wound
through the closing fingers of the clamp won't work on a DC wire.  For
DC, a Hall effect sensor can measure the magnetic field: the magnetic
field induces a current in the sensor.  However, those are inaccurate.
When measuring hundreds of amperes passing through a conductor, you
don't care if the actual current is different by several amps.  When
measuring 20A, or especially milliamps, the reading in the meter is so
inaccurate that it amounts to using a continuity tester to see there is
a connection versus how many ohms there are between the 2 points of
contact.  Both AC clamp-on meters and DC Hall effect meters measure by
conversion, not direct readings.  Magnetic field intensity decreases
exponentially by distance, and the huge space inside the fingers of the
clamp means the reading changes if you reposition the wire from the
center of the gap to closer to the wire, and it's never accurate,
anyway.

You and I may have AC and AC/DC clamp meters, but I suspect the OP does
not.  A good AC clamp meter at Home Depot costs $60 to $120.  A good DC
clamp meter greatly hikes the price to $500, and up.

> I try to keep a spare PSU in the house, but it's tough. 

I tried the same using a modular PSU, so I only need to connect enough
cables to the PSU as were needed for the particular build.  But
eventually it got used in the next build.  A $150+ PSU makes for an
expensive parts drawer.  For an emergency, I'll pay for overnight or
2-day delivery rather than buy a PSU that may never get used.  However,
I have other computers to use, so I go with free delivery in 5 days.