Re: Let's build a power meter for five bucks. It's too bad TechTV is promoting non-sense like this

>> Haha why spend $100 on a true wattmeter when a $5 resistor shunt and a
>> cheapo DMM will do the samething?
>>
>>

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>> article mentions
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>> ndex&req=viewarticle&artid=19&page=1
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>> It only cost $5, because it doesn't give out any meaningful result >when
>> you're measuring anything more complex than Christmas lights and table >> lamps.
>>
>> Measure computer or refrigerator's power consumption? yeah right. >It's
>> sad TechTV is supporting this non-sense.
>>
>> Some people just don't get that things aren't as simple as basic P=I*V >>
>> First of all, they're totally ignoring power factor and when you're
>> dealing with a load with a PF of 0.5 to 0.6 and decide to ignore it, >you
>> might as well not measure it.
>>
>> You can't measure the current draw of most computer power supplies >with a
>> cheap DMM either in A mode or V mode with a shunt. Except with a true
>> DMM multimeter, something cheapskate is unlikely to have, the readings
>> from measuring a peculiar current waveform of a computer power supply >is
>> incorrect.
>>
>>
>> Test result for my CRT(which has a switch mode power supply just like >a
>> PC and I'm not going to unplug my PC right now):
>>
>> Line voltage is 123.3V
>>
>> True power meter reports:
>> 1.29A
>> PF 0.63
>> 100W
>>
>> Regular DMM reports:
>> 0.67A
>> 123.3*0.67A= 83W
>>
>> Damn talk about way off.
>
>I had the same problem when I tried to measure the current and power of
>a $5 Intermatic timer, the one with the dial with the pegs on it on the
>front. It measured more than 2.5W with the DMM, but it was probably
>less than 1.8W when the PF was taken into consideration. The motor has
>a lot of reactance.
>

You could take a simultaneous voltage [Not the voltage drop across the series (shunt) resistor, but the SOURCE voltage] reading, integrate the two instantaneous values over a period of 2*pi radians, and do a vector sum. That would give you true RMS power, no?

-- -john wide-open at throttle dot info

~~~~~~~~ "The first step in intelligent tinkering is to save all the parts." - Aldo Leopold ~~~~~~~~