wattmeter

[rudivanstaden]

Hi there to all

Can someone possibly help me?

I want to design a wattmeter by using a picaxe?
What picaxe should I use. I am playing with the 14m at this moment.
And is it possible to measure the voltage and current, and then multiply it to give me a wattage. Should I use the adc connections?

I am clueless at this moment.

thanx

 

[Texy]

I guess it depends on how much watts/voltage/current you wish to measure. I also have a wattmeter on my to-do list for a picaxe project, for electric RC car/plane use. I have managed to blag a few hall-effect current sensors from allegro distributors here in the UK. I have, from memory, 50A and 100A rated ACS 750 or 752's, but the project is a little way off yet. There is a little bit of info on these forums if you search for ACS750.

Texy

 

[Dippy]

Actually we're clueless at the moment too as to whether you're talking about a low voltage DC wattmeter or something for measuring watts at mains voltages.

For low voltage DC it is as simple as VI as you asked.
To measure I you can use a sensing resistor method or a hall-effect based sensor - all of which have been covered well on this Forum.

Mains AC electric can be more difficult and potentially (no pun intended) very dangerous.

So, to save polishing our crystal balls down to the size of marbles can you be more specific?

Thanx

 

[rudivanstaden]

Sorry about the weak description.....

Yes I want to measure AC .

Want to see how much power some of my home appliances consumes.

Came across a "hall effect watt transducer" that can maybe help me with the voltage and current calculation, but is it possible to feed that signal into the picaxe and then display the value ???

 

[ciseco]

See this thread

http://www.picaxeforum.co.uk/showthread.php?t=10482

Cheers

Miles
________
Chevrolet Monza

 

[premelec]

Please be more specific about the hall effect unit you are considering - all you are looking for is in phase volts X amps and hall effect works well for this - however different units have different characteristics - the PICAXE mainly can count events or read voltages [from 0 to supply voltage] and then process this information further. If you can accurately read the in phase volts and amps as voltages then the PICAXE can multiply these and get watts... The trick in AC circuits is the phase as the voltage and amps can be out of phase with each other over the AC cycle.

 

[rodmcm]

As stated by others to measure AC power you need to know the cosine of the angle between the current and voltage. A simple technigue to do this is to use the rising edge of say the voltage wave form to set a bistable and the rising edge of the current (inverted) to reset the bistable. The pulse on to off is proportional to the angle. By inverting one signal at unity pF the on pulse length is the same as the pulse off. The sensing can be done digitally or by smoothing the pulse then by analog means, but this is less accurate.

 

[Dippy]

Rudi,

All this has been covered in recent and ancient threads. The info above is correct.

To make this accurate (i.e. incorporating Power Factors/Phase Angles) involves direct connections too which are potentially lethal. An experienced person would know about the care required and careful selection of components, the design and safe construction.

The questions you ask imply 'Newbie Status' and I would strongly suggest that you DON'T DO THIS. Just go and buy a cheap fairly-accurate add-on. Maybe a teacher or other adult could explain to you the dangers (and maybe the legal/insurance aspect as well) of fiddling with mains.

 

[ciseco]

Agree with dippy, not a simple task by any chalk and can be lethal. Yep insurance might be invalidated, would need testing to be safe, UL, EMC, CE all cost thousands, trust me I'm doing it now.

Miles
________
marijuana vaporizers

 

[RogerTango]

I am a ham myself, and a PICAXE RF WATTMETER is one of the projects I want to build, looking at the inside of a wattmeter you can see there is not much to it... the concept I have is to donate a wattmeter and interface it with a PICAXE and output the data via serial port. In concept at least.

Andrew

 

[premelec]

An RF wattmeter is quite a bit different than 50 or 60 Hz wattmeters - often times you just look for heating power in a small thermistor with and without the RF input applied keeping the thermistor at constant temperature by substituting DC power to the thermistor which is 'easy' to measure... see "bolometer". I think perhaps Linear Technology had a chip set up for it. IF you can get _accurate_ RF voltage and current it's like DC . OR if you know impedance and accurate current then I*I*R does it... and for relative power tuning up into a fixed load [antenna or cantenna...] then tune for maxium current gives maximum power - RF ammeters usually have a little thermocouple inside that's very easy to damage [I've done it!]. Good luck... [a ham for almost 60 years...] . The PICAXE can be a help in modulating DC heating power for a bolometric power meter and calculations....