Power Monitor

[matt1eg]

Hi

Am new to the forum but reading over peoples posts everyone seems very helpful so i thought i would see if i could get in on the action.

I'm trying to design a contactless power monitor. Planning to use some sort of hall sensor clipped around the mains supply and then use this output to calculate the power being consumed.

I know i could buy one of these for fairly cheap but am trying to 'improve' ( more like get) an understanding of electronics more.

What I am thinking is that I can use a hall sensors ouput (an ac voltage representative to the current flowing through the wire?) and a pic to calculate the power.

I wasn't planning on taking a voltage reading as I assume that this doesn't fluctuate much around 230V (uk) so was just going to use this static value for the calculations.

Anyway my main query is what do i have to do with the ac signal from the hall to get it into the pic. I know i will need to convert it to DC and that some sort of rectification will be needed to do this.

I'm assuming that the currents i will be reading will about 80A and i want to measure fairy accurately from 0A-80A.

I have been looking around and have found a circuit that says it does something like this but i dont understand it much.

The first 2 sections look like(according to google searches) low pass filters and then some sort of rectifier and voltage follower. As for the other section and the 'float' voltages i have no idea whats going on.

Could anyone give me a clue as to why/what is being filtered and how the rectifier is working? Also any ideas as to other section would be greatly appreciated.

 

[mneary]

Your circuit measures current. Power requires understanding power factor (phase).

 

[Diver300]

As mneary said, you need to measure voltage as well as current to find the power.

If you are using a PIC, you do not need to rectify the signals. You can read the current and voltage signals plenty fast enough with the A/D converters to follow the 50 Hz mains.

Use a current transformer like RS 537-4536. Put the output into a suitable load resistor and measure the voltage.

The mains voltage can be safely and accurately measured through a small transformer with no load. You just need two mains transformers, one to power your circuit and one to read the voltage.

For both current and voltage you need to offset the voltage to mid rail (2.5V if you are running on 5V) so that the A/D can read positive and negative.

You just read the voltage, read the current, multiply the results, repeat at least 500 times a second and average the results and you have a true RMS power meter.

If you are prepared to guess that the voltage is 230V (its typically about 245 V in the UK even if the nominal is 230 V) then you can read the phase from the mains transformer that is feeding your circuit, but your calculations become more complicated.

BTW, running power meters from voltage and current transformers is common practice. Where I used to work, there was a 3MW, 11kV motor. The energy used by it was measured buy a meter that looked much like the ones in houses, but it was fed from 110V from a transformer from the 11kV. The current was transformed down as well, mainly to keep the 11kV away from everything than to get the current down, but that probably had 100:1 ratio as well.

The result was that the power measured by the meter was 10,000 times less so the meter was just a small unit. That is the sort of thing that you will be doing.

 

[matt1eg]

Thanks for your help and advice and it does sound liek a good way to go. I was thinking that I would have it powered by battery like this one Efergy Wireless Monitor from HomeEnergySaving.co.uk. How do they get accurate results?

 

[matt1eg]

My relpy seems to be lost in moderation but in the meantime out of interest, in the circuit given what is the purpose of the filters?