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Watt meter concept

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You can get these resistors for fifty cents on Digikey, and they ship all around the world.
 
Mainly the fact that it's AC. R15 is a "shunt resistor", and this is fine for DC circuits, but for AC it gets a little more tricky. You need to amplify the sine wave on it first, then rectify and filter it to measure it.


You can't rectify and filter the current signal (and the voltage signal) to find power.

If you do that you will get U x I = Aparent power (in VA), instead of Active (or "real") power.

To find Active power you must multiply the instant values of the voltage and the current many times in each cycle, and then average the values of those products.
 
Yes, that is a very good point. For actual wattage, simply multiplying RMS values isn't enough because of the power factor. You really need to read it many times per cycle and average like ecerfoglio says.
 
my project

duffy....u seems to know how to measure the power usage using the nts-15p current transducer.

i've been doing some project on to measure the power using a voltage and current sensor n currently using the nts-15p.

the problem is the current and the voltage i've been applying is in ac n i dunno hw to interface the sensor to my microcontroller. cn u help me here??

another question is dat, is true, microcontroller only accept a sensor with a range of 0-5volts for the adc??

need help so much from u

thanx
 
duffy....u seems to know how to measure the power usage using the nts-15p current transducer.

i've been doing some project on to measure the power using a voltage and current sensor n currently using the nts-15p.

the problem is the current and the voltage i've been applying is in ac n i dunno hw to interface the sensor to my microcontroller. cn u help me here??

another question is dat, is true, microcontroller only accept a sensor with a range of 0-5volts for the adc??

need help so much from u

thanx

hi,
On the 'ac' output of you 'nts' you require a 'precision rectifier', smooth the full wave output using a Res/Cap filter [dc].

The PIC's adc input will accept a 'dc' voltage in the range of 0V to +5V.
 

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hi,
On the 'ac' output of you 'nts' you require a 'precision rectifier', smooth the full wave output using a Res/Cap filter [dc].

The PIC's adc input will accept a 'dc' voltage in the range of 0V to +5V.

yeah thanx..

but however, i need to calculate the ac power, not dc.. is there anything i cn do with the rectified signal so that i cn obtain the actual ac power??

i mean if i get a value from 0-5volts from the adc, then, it is a discrete dc value rite??

sorry coz im a little bit confused when it comes to ac power
 
yeah thanx..

but however, i need to calculate the ac power, not dc.. is there anything i cn do with the rectified signal so that i cn obtain the actual ac power??

i mean if i get a value from 0-5volts from the adc, then, it is a discrete dc value rite??

sorry coz im a little bit confused when it comes to ac power

If its a 'good' sinewave then the RMS value after precision rectification will be 0.707 of the peak value of the rectified and smoothed signal.

You will see in the 'prec rect' circuit that the peak value is reduced to half , ie 50%.

Say you amplified this rectified/smoothed signal to bring it back to 100% this when multiplied by 0.707 would give the RMS value.

Example: say the sensor gave 2Volts peak to peak 'ac' signal

after the prec rect the signal would be 0.5V from 0V to its peak value, assume that we amplify this signal by 2, so it becomes 1V, from 0V to its peak.
Its now the same amplitude as the original, but full wave rectified.
Lets now assume that we smooth the signal so that a capacitor charges to the peak voltage of 1V.
The adc will read this as 1V, the program could multiply this value by 0.707 which would give the original RMS value, the value you need to calculate the power.

So you can see that by rectifying the signal, smoothing it its possible to adjust the amplification to give the RMS or peak value of the sinewave input to the PIC's adc.

Do you follow that OK.?
 
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yup.. thanx.. i follow wat u taught and i get it right!!

thank you..

now, wat im missing is on how to calculate the power factor of an ac power..

thank you so much!!
 
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