I am interested in this project from an education point of view, rather than as a specific project.
I have built the CT sensor circuit as described here: http://openenergymonitor.org/emon/buildingblocks/ct-sensors-interface
Due to just being components i had lying around i have used a 47 ohm burden, 100k ohm divider resistors, and a 47uF Cap.
as far as i can see, the CT clamp outputs "a voltage" which conforms to the AC wave form of the supply (UK 240V)
How does the burden resistor affect the output to the arduino? (my max monitored amps is 75A, but does this affect the reading given to the analog pin?
What form is the output to arduino analog pin if you dont use the OEM library? a very fast waveform? too fast for the arduino?
if i have hypothesized this correctly, the library must look for the maximum of recorded peaks over a timeframe and present them as the actual current?
I am hoping to be able to write my own sketch to read the current draw, but am struggling to see what i am looking at (due currently to limited understanding i guess)
Hope this all makes sense. thanks
Ol
Re: CT clamp data output?
The clue is in the name - current transformer. It acts as a current source, the output is a current, not a voltage. No matter what value the burden resistor (within limits), the same current always flows in the secondary circuit. That current is of course generated by the changing flux in the core which has in turn been generated by the primary current. So the secondary current should be an exact replica of the primary current - the current you're trying to measure.
From that, it's a simple step via Ohm's Law to see that the burden resistor converts the current into a voltage. We need a voltage because the ADC input has a very high impedance and it draws almost no current. Nor can you force a current into it - that's what 'high impedance' means.
The limits I mentioned are imposed by the construction of the c.t. Like any other transformer, it has a maximum power (or VA) rating and if that is exceeded, you overload it. So maximum power is with a high value of load (burden) resistor and minimum power is with zero value burden - a short circuit. I said a c.t. was a current source, this is the exact opposite to a normal voltage transformer.
The OEM library is just a collection of software. It's not necessary to use it (but it's very convenient!) You can do exactly the same things without it. What you need to do is measure the voltage in some way. What we choose to do is sample the voltage many times (about 50) during each cycle of mains and use the numbers to calculate the voltage, then knowing the various conversion constants (like the transformer ratio and burden value) we can convert that into a current. So your hypothesis is wrong - we don't just measure the peaks, we measure all of the wave. If I carry on, I'll be repeating a lot that's already written in Building Blocks.