# Design a power meter / wattmeter

Discussion in 'Homework Help' started by Sebyo, Nov 29, 2007.

1. ### Sebyo Thread Starter New Member

Nov 29, 2007
3
0
Hello..

The aim of my project is to design a power meter to measure the power in "stand-by" or in "sleep mode". The problem is I have no idea about the biginning of this project, that's why i'm here, I would like to find some information and some idea about that.

(Sorry for my vocabulary, I'm french)

Thank you

2. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Power is the product of voltage and current. Use an ammeter to measure standby current, and multiply that by the voltage. The current is going to be low, so a microammeter might be more to the point.

A small value resistor can go in series with the power lead. Use .01 ohms, so as to disturb the current minimally. Apply a differential amp to amplify the voltage across the resistor to give a voltage proportional to the current through the resistor.

If you can get the current reading, a four quadrant multiplier should be able to output the analog wattage. Otherwise, digitize the voltage and current and use a mocroprocessor to obtain the wattage.

3. ### Sebyo Thread Starter New Member

Nov 29, 2007
3
0
Thank you for your help, I think if I use a microcontroller (PIC16F86A) I can write voltage value.. U = 230V...
What do you think, it's right or wrong?
Do you think I need a decoder between the microcontroller and the digital display?
thank you

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
If the supply voltage is 230 VACRMS, then that is the voltage term to use.

Can't answer about the display - not without knowing the microprocessor (they aren't all alike).

5. ### Sebyo Thread Starter New Member

Nov 29, 2007
3
0
hey,

I don't know microprocessor that I need to use, but after my microcontroller I will be in digital and you said "If you can get the current reading, a four quadrant multiplier should be able to output the analog wattage".

The microprocessor is my decoder? Can you help me again about that?

Thank you very much

6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
283
Given that your voltage term is known and probably invariant, the multiplier is a needless elaboration. You simply need to get the current reading into the microprocessor by way of an analog to digital converter.

I may be guilty of sloppy nomenclature here - a microcontroller should be the term to use. It should be able to digitize the current input, adjust the reading into an engineering unit, multiply that by the voltage, and run the display.