Open Energy Monitor - homegrown emonPi Construction (part 2)

Today I looked at connecting the CT - Current Transformer? This is a 2000 turn secondary, clipped around the 1 turn of the incoming house mains cable. It thus provides a maximum RMS current of 20mA, given a maximum house current of 100A. This is measured using a load resistor with a value that makes the voltage drop across it match the input range of the Analog pins of the Arduino. This is all discussed on the OEM website.

I'm using the Arduino at 3.3V, so the "burden" resistor value is 

R = V/I

where

V = AREF/2 (AREF is Arduino Reference)

I = PeakSecCurrent

=> R = (AREF/2)/PeakSecCurrent

Substituting 

PeakSecCurrent = PeakPriCurrent/Turns

=> R = (AREF/2)/(PeakPriCurrent/Turns)

Substituting

PeakPriCurrent = (RMSPriCurrent * SQRT(2))

=> R = (AREF/2)/((RMSPriCurrent*SQRT(2))/Turns)

Substituting

AREF = 3.3

RMSPriCurrent = 100

SQRT(2) = 1.414

Turns = 2000

(3.3/2)/((100*1.414)/2000) = 23.34 Ohms 

Closest I can get to this value is by pairing some resistors in parallel, probably 100 and 30 ohms:

Rt = 1/((1/R1)+(1/R2))

R1 = 100, R2 = 30

Rt = 1/((1/100)+(1/30)) = 1/(0.04333) = 23.07 Ohms

or about 1% too small, and apparently best to err on the lower side, otherwise V > AREF, muy bad.

Rather enthusiastically, I've soldered all the relevant parts into a piece of Veroboard, which was a mistake because I messed up the calculation the first time! Fixed it though, several spare slots left :-).

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