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First of all, I'm a noob. I just like to tinker around. I would like to find how efficient my little 12v/1a switching power supply is (and many others like it).

From some research, it may appear things aren't so simple on the 120v input side. It sounds like power factor and delay of voltage/amperage waves make it harder than simply multiplying voltage by amperage.

AC input power can't be calculated simply as RMS input voltage times RMS input current because of the differences in phasing between the voltage and the current in an AC system, designated as ϕ and seen in figure 1. If we did this, it would yield a result that's too high because the value would include both the power converted by the power supply (P) and the power that's returned to the AC source (Q). --> Source

I just ordered the Kaiweets HT206D (6000 count) meter which measures AC amperage, though it's a clamp meter. Probably don't want to use it due to lack of precision at that low of current. (It has settings for 60A and 600A.) I also ordered a cheap multimeter that measures AC current (AstroAI M4KOR - 4000 count - settings for 400ma / 10A). For the record, I also have a Kill-a-Watt that will measure power factor.

Here's the question: Can I use the power factor reading from the Kill-a-Watt combined with the voltage and amperage measured by the multimeter to gauge the wattage the power supply consumes on the input side and then pit that against the output DC amperage & voltage to assess efficiency?

Thanks for your input!

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  • \$\begingroup\$ The clamp meter should work, just make several loops around the sensor, it will multiply the reading by the number of turns. 10 turns equals 10X. \$\endgroup\$
    – Gil
    Jul 29, 2021 at 1:48

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If your serious about getting accurate effiency readings I would recommend you purchase a dedicated power meter that can do the efffiency measurement. When it comes to power supply eff every .1% is a huge battle to get. Im guessing if you do a tolerance analysis on all of the equipment you have you will start to see the challenges.

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  • \$\begingroup\$ Thanks for that info. Any example equipment models that you could give me that could set me in the right direction? \$\endgroup\$
    – pkSML
    Jul 29, 2021 at 1:45
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    \$\begingroup\$ Yokogawa wt series are good \$\endgroup\$
    – ISEMI
    Jul 29, 2021 at 1:57
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Most small mains in SMPS will run on DC. Arranging the DC is easy and in the absence of a good real power meter is an option. I have used a Variac with an isolation transformer feeding a basic bridge rectifier and 450VDC electrolytic filter cap sized for low ripple. Now I measure the DC input volts and DC input amps with a standard DVM to get power in.

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  • \$\begingroup\$ That's an interesting thought. As a noob, a lot of this goes over my head, but maybe it'll make sense some day :) It does make sense that DC-in would make measurements easier. \$\endgroup\$
    – pkSML
    Jul 29, 2021 at 11:45
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    \$\begingroup\$ I guess using DC instead of mains would alter the efficiency. Most SMPS even have different efficiency plots for 110VAC in vs 230VAC in. \$\endgroup\$
    – tobalt
    Jul 29, 2021 at 12:50
  • \$\begingroup\$ @tobalt Yes. I suppose you lose the AC—>DC conversion losses when you run dc in. \$\endgroup\$
    – pkSML
    Jul 29, 2021 at 17:16
  • \$\begingroup\$ If "small" includes wall warts, most of these will not run off DC unmodified. This is because they drop voltage through a capacitor. \$\endgroup\$ Jul 29, 2021 at 20:52

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