Sensing Power

Edson Brusque askes

You know how to calculate the RMS power based on a series of (above 0) samples from an AC input? I could simply note the peak value (say, 3V) and calculate it's RMS (3V * 0,7 = 2.1V, IIRC). But this assume the input is a perfect sine, and maybe it's no true.

Harold Hallikainen replies:

Square each sample as it comes in (resulting in a 16 bit number), add up 256 of the squared samples (24 bit result), divide by 256 (back to 16 bit), then take the square root (back to 8 bit).

Power@ Tools@ Current (Amps)

Note: Utility companys do NOT measure of the amount of energy delivered. They charge you as if all phases were delivering the same amount of power. If all your energy comes through one phase (because you have thing plugged into the breaker box in an unbalanced manner) then you will be billed the same as if ALL phases had that much power coming through them. Get an electrician to visit your house and ask them to check the balance of your phases and move circuits around accordingly.

see also

• http://members.tripod.com/~edward_cheung/automa/power.htm a PIC house meter
• http://www.analog.com/industry/energymeter/solutions/ic_solutions.html

Questions:

• ~NOSPAM~joel.finkel at @spam@asu.edu asks:

  I have been looking high and low on the Internet for information on how to calculate the average current in an MCU-based circuit. I have an app note from Microchip on the subject, but it only shows the battery life computation after average current is known. Can someone please explain to me how to compute average current on an I/O pin? As an easy example, let's say that Vdd is +4.5VDC, and the pin fires 8 40kHz square pulses through a load of 2KOhms. Also, what is duty cycle and does it have an effect on average current? Thanks for your help.

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Archive:

• PICList post "Pic based wattmeter (I forgot the attachment !)" +

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