Hi Everyone.

My goal is quite simple, but I just could not find what I want to do.
Any help is appreciated.

I have a plain home computer,
Simply, I'd like to measure the currents that are flowing through the rails from the Power Supply Unit (PSU), to measure total power used by the devices, like harddisk.

One simple approach is using the current sensor, like this
http://www.xbitlabs.com/articles/video/display/ati-powercons_2.html

The problem: Each current is different (rails have 12v, 5v and 3v), so I need more than one multimeter to get the reading at the same time.
There seems to be no way around this...

For example, if two rails such as 12v @ 2A and 3v @ 1A for a particular device X, How can I ever measure these 'total current' without using two multimeters ? Or If there is, which multimeters do I use to 'merge' or record separately, and to pass onto (e.g., USB, RS232) to computer? which is recommended?

http://www.hardwaresecrets.com/article/410/2

Indeed, I'd like to buy one of these Current Sensor instrument.. This seems to solve the problems? (e.g., using hall effect sensors)

Problem 2: I have a very weak knowledge in building such PCBs and hooking up sensors.. So where to buy such already-made current sensor? as well as the software that connects with computers for monitoring?

Thanks alot

 

Another problem you have is that if you want to know the total power drawn by a particular device with multiple DC supplies, then the total power is not simply the sum of the individual supply currents.

Rather if you had a device with two DC inputs (say @ 12V and 3V) then the power drawn will be ....

Ptot=(12)*I_12_volt+(3)*I_3_volt

So if you do actually want total power and not just the total of the individual currents DC supply then your problem is compounded.

If you only want a total of the individual currents then I think you can buy current transducers with multiple primary windings which you could use to consolidate the total current measurement.

 

This may not be of much help, but if you can use a small resistors (0.1Ω) in line with your power supply then it becomes and issue of using a switchable voltage measurement, and leave the resistor in place. If the resistance is too large you can drop it down another order of magnitude.

 

my $0.02
If you add a data acquisition (DAQ) device to your system (such as this USB DAQ made by National Instruments), you can read the analog output current sensors with it. DAQ card is essentially a bank of analog to digital converters (ADC).