7. AC socket for power measuring. 自製實用可量度電源功率座 16 May 2009
When I study in technical college, the current transformer is interesting. 

Looking at the blue and grown color coil, each one have 10 turns. There are total 20 turns, it is 20 times sensitive to measure the current flow.

 
Look at the LCD display, that is 5.4A current flow. The real value is 5.4 / 20 =0.27 A because this system is 1:20 turns current transformer.

The power On PC machine is not light, because the power consume is 0.27A x 220V =60W.

So the advice is turn Off the machine when no using.

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