Hi everybody,
I have just received my emonTx V3 fully assembled with several SCT-013-000. Straight away I have done several current measurements. According to what I am looking for, and after testing CT1-3 and CT4 over the same line to check their differences. The extra sensitivity and accuracy of channel 4 is exactly what I am looking for.
I am new in this area and I am a bit lost in electronics (did some many years ago). So, after some reading, my question is: Could I transform the other CTs (CT1-3) into extra sensitive? To do so, will I need to do something else apart from modifying the burden resistor in CT1, CT2 and CT3? Will then these behave as currently CT4 does? Is there anything else I need to have into consideration when trying to make these channels more sensitive?
Therefore, to sum up: I want to modify CT1, CT2 and CT3 in order to make them as sensitive as CT4. Looking at the PCB layout and Schematic found on the wiki (http://wiki.openenergymonitor.org/images/EmonTxV3.2_brd_dimentions.png) will I be able to achieve this by replacing R7 R8 and R9 for another resistance which same value as R18?
Thank you.
Adrien P.
Re: Transform CT1-3 into CT4
Could I transform the other CTs (CT1-3) into extra sensitive? Yes
To do so, will I need to do something else apart from modifying the burden resistor in CT1, CT2 and CT3? No
Will then these behave as currently CT4 does? Yes - but you'll need to adjust the calibration.
Is there anything else I need to have into consideration when trying to make these channels more sensitive? In the sketch, you need to adjust the calibration. Use the same current calibration constant as CT4 (16.6).
Therefore, to sum up: I want to modify CT1, CT2 and CT3 in order to make them as sensitive as CT4. Looking at the PCB layout and Schematic found on the wiki (http://wiki.openenergymonitor.org/images/EmonTxV3.2_brd_dimentions.png) will I be able to achieve this by replacing R7 R8 and R9 for another resistance which same value as R18? Yes, you need to remove the SMT resistors, and you will find holes ready to accept wire-ended ones in their place. They are in fact shown as R3, R5 & R22 on the schematic. I recommend 120 Ω 1% metal film.
Re: Transform CT1-3 into CT4
I wonder how much of an improvement the extra sensitivity for CT4 really makes when measuring smaller AC currents?
I've just been testing a system that's sized at around 18kW, yet a 40W signal can still be picked out without any difficulty. When voltage and current are both being sampled, it's amazing how the maths can distinguish a correlated 50Hz signal from the background noise.
Re: Transform CT1-3 into CT4
What got me into the low current measurement business was the desire to monitor individual circuits so I could see where the power was being used. My biggest breaker is 20A, so I never really need to measure more than about 5kW. If I want totals it's easy enough to add them together in the back-end.
Once you have that sort of resolution, you can start asking "how much difference will it make if I turn X off". If the rest of the house is steady-state, I can plug my phone into its 5VDC 1A USB wall-wart, and see the power dial go up by 5.5W. 40W is a double-decker bus from where I stand ;-).
Re: Transform CT1-3 into CT4
Hi all,
Sorry to use this post to make a question but I am in a similar situation, I am measuring currents of around 200mA and when adding several turns CT4 seems to keep linearity better that the other CTs reason why I thought about change the burden resistors.
Besides, I have observed that CT4 reacts faster to changes in current. If I change the burden resistor, will all the CTs have the same reaction speed?
Cheers
Regards
Alejandro
Re: Transform CT1-3 into CT4
I don't believe the 4th channel reacts any differently to the others. All the processing is identical, apart from the multiplication by the scale factor. You might be seeing a difference due to smoothing that has been applied, for example in emonCMS.
All CTs will exhibit a degree of nonlinearity. At the bottom (low current) end, it is due to the losses in the core becoming large in relation to the magnetising flux. Good manufacturers will always specify a range over which the claimed accuracy is maintained.