Home Job 230V Kill-a-Watt

Jonathan

Just thought of this earlier and was wondering would any of the electrical heads around here find any fault with it (more the theory end, rather than the safety end). I have a multimeter but I'm weary of trying to measure mains currents with it.


So I had an idea of getting an extension cable, chopping it to about 0.25m and replacing the neutral cable with a set of chopped probe cables which would be insulated and slot directly into the multimeter.The multimeter has a 10A internal fuse, so by putting a 7A fuse in the plug, I would never be able to overload the multimeter.

In theory, the probe cables should be safe to touch provided that there is nothing plugged into the extension side regardless of whether the wall side is plugged in or not?


Would this be a workable, safer option?
I know I should just go out and buy a proper wattmeter, but being a student kinda puts an end to that, and being an engineer, I want a little project :P

Seanieke

Not 100% sure what your getting at but if your connecting the live to the multimeter & are using the other lead to measure against live your Multimeter will just read Zero because it just measures the potential difference between the values.
Under normal circumstances your measuring between live & neutral/earth giving you a potential of 230v, but if you go between live & live you'll just get zero as there's no difference.

Maybe this'll help, maybe you know it already & are trying to go deeper than this! End of the day it's only 230v, it'll go more bang than wallop if it goes wrong!

Jonathan

I'm trying to measure the current and hence the power used.

Seanieke

Ah!! Well then erm.. Best of luck with that one!! maplin have clamp on ammeters for e25 at the moment if it doesn't work out!!

docmol

In theory, the probe cables should be safe to touch provided that there is nothing plugged into the extension side regardless of whether the wall side is plugged in or not?

I think you could still get a back blast from the neutral if anything else on that circuit was live. If we don't hear back from you in the next few days I'll assume I was right

Jonathan

Seanieke wrote: »

Ah!! Well then erm.. Best of luck with that one!! maplin have clamp on ammeters for e25 at the moment if it doesn't work out!!

Not great for small currents tho.

Seanieke

You'd prob have to throw something like this into the circuit so if your going small... This could be a big job yet!!

http://www.edn.com/contents/images/6434367f5.pdf

Jonathan

Seanieke wrote: »

You'd prob have to throw something like this into the circuit so if your going small... This could be a big job yet!!

http://www.edn.com/contents/images/6434367f5.pdf

I meant about about 3-4 amps.

I find that my clamp multimeter is not accurate at all for currents that small.


I highly doubt there is a need for an opamp

Jonathan

For anyone that cares, it works perfectly.

I was going around the house yesterday measuring how much power various things use. :pac:

Seanieke

Good to know you didn't short yourself out anyway!!

Pete67

Glad it worked out, you probably know already that you are measuring VA which equals power only if the load is resistive. For any capacitive or inductive loads where the power factor is not unity, VA will be higher than the true power consumption in watts.

Jonathan

Pete67 wrote: »

Glad it worked out, you probably know already that you are measuring VA which equals power only if the load is resistive. For any capacitive or inductive loads where the power factor is not unity, VA will be higher than the true power consumption in watts.

I'm not measuring any big motors so its fine :P

2011

I was going around the house yesterday measuring how much power various things use.

Hi Jonathan. I see what you are trying to do. To calculate the power used the formula is:

Voltage, (volts) x Current, (amps) x Power factor, (no unit) = Power, (Watts)

For resistive loads the power factor = 1, but for other loads it will be less than 1.

I am pointing this out because I see that you were not keen on using a clamp on ammeter.

You can buy reconditioned ESB meters for less than €40. If you were to connect on to a load for a few days with this you could work out the everage power used. This would be good for something that switches on and off like a fridge.

I would think that the multimeter you have is not rated to carry the load current for very long.

Out of interest why are you doing this??? Many appliances have their power consumption written on them.

Good to see it worked for you.

EDIT: Sorry I am half sleeping I see Pete67 pointed the PF out already!

Jonathan

2011 wrote: »

Out of interest why are you doing this??? Many appliances have their power consumption written on them.

Good to see it worked for you.

I forgot to reply to this.

I was just doing it for the laugh to fill time during the summer tbh. And also something vary based on what the appliance is doing. PCs etc

Stoner

Jonathan wrote: »

For anyone that cares, it works perfectly.

I was going around the house yesterday measuring how much power various things use. :pac:

I'm glad it works, thanks for sharing. Just for those who should avoid wiring, there are safer methods, small units that you can plug into a socket and the appliance plugs into it. They will measure the power usage on household items with 13Amp plugs.