Measuring mains load

[Freman]

G'day folks.

I'm a little stuck, I want to measure mains load - I don't need it overly accurate, just enough to work out if something (say a tv/fan/etc) is on or off.

I'm looking to do it with the least number of cheapest components I can get my hands on (or hopefully already have)

I've looked at the safer sure fire options of using a current transformer or a hall effect based sensors (ACS712 for example) but they're either really expensive, or not very easy to obtain.

I know I can measure current drawn with a multimeter and a 10watt 0.1 ohm resistor across the lead - but how do I get that to a PIC?

[Acetronics2]

Hi,

Do not tell me what you had intended ...

http://www.conrad.fr/compteur_de_consommation_electrique_digital_monito r_3000_p_19103_19143_360561_356451

you won't do better ... and no need to mention about the cost !!!

Alain

[T.Jackson]

I think the safest way is any method that is completely "contact-less" I thought about offering some advice on a shunt-based schema before, but have since decided not to. Too dangerous. Out of interest -- it's completely illegal in Australia now for unqualified people to mess around with 240VAC. It's the way it should be too. If you're on here asking questions about 240 then I'm sorry to say that you don't know what you're doing. There's no second chances with electricity.

Trent Jackson

[Freman]

T.Jackson - Sure there are, I've had several already :)

While I still have trouble with some electrical principles, I'm actually a fairly safe person around higher voltages (now - I learned my lessons young)

The projector controller I've been playing with has unmarked hot areas - and I didn't even make it.

I originally looked at the current transformers because they were contact less, I'd just have to wrap a couple of active windings around it to measure load - however, I've only found one place here to buy them, and they don't carry any of the other components I want - so to buy 2 it costs me $20, most of that shipping.

Acetronics - I'm afraid that's not what I'm after, I have 2 of what I think they are already. My aim is to sense whether a device is powered up, or in standby/off. I can already do it via hardware hacks (looking for 1.1v on composite out, physically altering my amp, etc) but I'd like to build a more generic method.

The ACS712 is a really nice chip - absolute no brainer to use, plenty of isolation - http://www.farnell.com/datasheets/94750.pdf - it's just getting them, while they are available from my preferred stockist they're still so expensive here.

Maybe it's worth getting ACS712's so that I can be better isolated and not have to deal with it.

[Melanie]

R6,R7,D7 together detect FS1 blown. Detect incoming AC pulse in software.

Remaining components detect Loading. Designed for 40W approx. Change R2 to suit your application to give you less than 1v at your desired load. Use ADC or Comparator to measure if device is ON, OFF or sideways. Do not design above 2v input to PIC as protection Zener will start to conduct and distort measurement.

Caution, PICs 0v rail is common with supply Neutral.

[Freman]

Thank you Melanie.

Now I couple that with a transformer-less PSU and an opto-coupler, and I have an isolated sensor.

Just one question. Is that meant to be GP0 or VSS?

[Melanie]

In this example the junction of R3/R4 is connected to GP0 - it doesn't have to be - it can be any PIC pin with ADC or Comparator analog input.

VSS and VDD are shown not connected - it is assumed you will know what to do with them. VSS would be connected to the Neutral (0v) line - that's the SUPPLY Neutral and NOT the LOAD Netural. VDD is connected to your +5v Supply which if derived from a transformerless source is ideal (see elsewhere on this forum for examples of that).