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Old 03-28-14, 01:12 AM   #1
NiHaoMike
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Default Hacking a Kill-a-Watt for very low power measurements

Measuring standby power - Page 1
Quote:
One of the more intriguing suggestions, which I have yet to try is to modify a Kill-A-Watt by removing the 2mOhm (or so) current shunt and replacing it with a 200 mOhm (or so) current shunt. Device should read approximiately 100x more sensitive than it is. Obviously, you don't want to plug something that draws 15 A into it after modification, but it should be good for generating low power measurements. Probably very inaccurate, but better than nothing.
I modified one of mine to do just that, but I also added a pair of inverse diodes to provide protection against inrush and a switch so it can be used in "normal" mode. (There are lots of gotchas to adding a switch if you want anything resembling an accurate measurement!)

What I found is that most switching wall warts draw very little power - usually 0.3W or less. (I found that a recent Samsung phone charger used so little in standby that even a 100x more sensitive Kill-a-Watt would not register it - that's less than 10mW!) Linear wall warts vary all over the place, from a few watts for the inefficient ones to around 0.3W for the good ones, just like many switching wall warts.

BTW, don't expect that mod to be particularly useful for actual savings since 1W left on 24/7 is 0.72kWh/month - way down into the noise for even a very efficient household. (In an alternative energy system, just use a multimeter.) It's purely for bragging rights!

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