Hello, i would need help running this CT. I bought it thinking it would be better for my solar pv, but i get wrong values, how do i modify the config file of the emonpi to get it running?
Many thanks in advance
You get the wrong values because you have two burden resistors, one inside the c.t. itself and another inside the emonPi. In effect, you have made your c.t. a 133 A one instead of a 30 A one.
You can change the numbers in emonhub.conf, but it is by far not the best solution. What follows won’t be totally accurate, because I’m assuming the SCT-013-030 is a standard SCT-013-000 with an internal burden, when I know it has a slightly different number of turns in the secondary winding.
The current ratio is (approx) 100 A : 50 mA, the internal burden is sized to give 1 V at 30 A, therefore its value is (approx) 66.6 Ω. You have a 22 Ω burden in parallel inside the emonPi, making the effective burden 16.54 Ω.
At 30 A, you will only have 0.248 V, when the emonPi’s ADC is expecting 1.1 V (at 100 A) for best resolution. So you will be using only the bottom quarter of the scale (approximately).
You can get close to the correct values by changing the numbers in the line
``scales = …" for Node 5 in your emonhub.conf file. Change the first three values to 1.33:
[[5]]
nodename = emonpi
[[[rx]]]
names = power1,power2,power1pluspower2,vrms,t1,t2,t3,t4,t5,t6,pulsecount
datacodes = h, h, h, h, h, h, h, h, h, h, L
scales = 1.33,1.33,1.33,0.01,0.1,0.1,0.1,0.1,0.1,0.1,1
units = W,W,W,V,C,C,C,C,C,C,p
If you are prepared to dismantle your emonPi, and know how to work on printed circuit boards, the best way will be to remove the burden resistor from the circuit board. It is the big SMT component immediately behind the jack socket, and it has holes for a wire-ended resistor at each end. That will give you much better measurements. In that case, the numbers in scales = ... must be 0.3
Unfortunately, I cannot find the accurate data for the SCT-013-030. Therefore, none of those numbers will be accurate, but they should be within 10%. I suggest you calibrate it against one of your meters and adjust the scale factor until it reads correctly.
I also bought some SCT-013-030 sensors and reverse engineered them. Their spec is 1V for 30A. When opening it up the internal burden R is 62 Ohm. This means this CT has 1860 turns:
30A / 1860 * 62 Ohm = 1.00 V
I also directly checked the turns ratio with instrument grade current probe measuring input and output current into a short. That measurement verified the 1860:1 current ratio.
So, if the emonPi internal burden R is 22 Ohm that means the net burden is 62 in parallel with 22 or net 16.24 Ohm.
Welcome, George, to the OEM forum.
Approximately - we know the manufacturer will adjust this slightly to achieve the best performance over the specified range. An old (2011) data sheet I have specifies 1800 turns on the secondary, so it’s quite likely that something like a change of core material or a “rationalization” has meant a small change to the number of turns.
But that’s nevertheless useful information and I’ve made a note of that internal burden value.
Thank you.