I'm working on a Digital Wattmeter project, where the wattmeter is plugged to the mains, and an appliance is plugged to the wattmeter.

My question is, is there a way to measure Power Factor accurately?

EDIT:
I have heard of a Power Factor IC, which conveniently measures the power factor when connected. However, I could not seem to come across it.

 

You need to measure both the voltage waveform and current waveform (since you are making a wattmeter you would be doing this anyway) and compare the difference in phase. The power factor is the proportional difference between the two.

By the way - you can buy a wattmeter that works like that and measures power factor for $20. The "Kill-a-watt".

 

Since I cannot use an osilloscope, what would you recommend I use though, to measure the difference in phase?

And sadly, where I live, there is no "kill-a-watt".

 

You can often find a kill-a-watt on eBay. Many sellers ship worldwide. You haven't indicated your location so it's hard to say.

__________________
Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com/

 

There we go.

Anyway, the purpose is building a digital wattmeter, sort of one like the "kill-a-watt" meter. It would be fantastic if the schematic of said meter is readily available, that would really help with information.

If not, I would still like to know how to accurately measure Power Factor without the help of an oscilloscope.

 

Going to be difficult without a scope. Are you using a microcontroller?

 

Yes, a PIC16F877A.

I will be interfacing the ACS712 Hall Effect Sensor with it to obtain the Voltage and Current reading.

 

Besides the phase method, you can measure the apparent power going to the load, which is the product of volts * amps ... and then measure the watts to the load, and take the ratio of these two quantities. .... If you are capable of taking these measurements.

 

I've used the ACS712 before, it's great for reading current, but it doesn't read voltage.

 

You could use an energy measurement IC such as one of these Energy Measurement | Analog to Digital Converters | Analog Devices.

__________________
Carl
Curmudgeon Elektroniker

 

I might be wrong, but isn't the product of (Volts * Amps) True Power?


Isn't it the other way round?
i.e. the ACS712 receives current and outputs voltage?



Once again, I might be wrong.


I'll have a look up on that.
Thank you.

 

It OUPUTS voltage but it READS current.

So it doesn't READ voltage.

Yes, but you said you were trying to OBTAIN voltage, which that doesn't do.

 

For DC. For AC the product of volts times amps is apparent power. The product of volts times amps times the cosine of the phase angle between them is true power. For example a capacitor will draw AC current when placed across an AC source but it will draw no real power from the source since the current and voltage are 90 degrees out of phase.

__________________
Carl
Curmudgeon Elektroniker

 

I see, thanks for clearing it up crutschow.

Ah, I'm terribly sorry, I misunderstood how the ACS712 functions.

With that note, is there a way to measure voltage from the mains connecting to an appliance?

 

A voltmeter.

__________________
Carl
Curmudgeon Elektroniker