Tips to design a analog automatic switch selection circuit for CT (Current Trasformers)

Hello dear forum. 
I've been beating my brains out in a circuit I attached a figure below. Well I have 2 current transformers(CT). First from current on secondary 100A:50mA and second from voltage on secondary 100A:1V. In my first CT, I need to add burden resistance to measure current on secondary and add an offset of +2.5V because of my ADS1115 (16 bit ADC) Im going to read on its analog input, however for second CT I just need to add offset. About this circuit its done and you can find more info to get update about my following problem: 
https://learn.openenergymonitor.org/electricity-monitoring/ct-sensors/interface-with-arduino
I want to know if is possible to have 1 KRE 2 connector on the board im going to design and when I connect 1º CT (100A:50mA) on my KRE 2 connector I should select its circuit (with burden resistance) and then go to my A0 ADS1115 input while my 2ºCT keep disconnect of course. However, if I wanna plug my 2ºCT on my KRE 2 connector, I need to switch for its circuit and then go to ADS1115. Signal track for ADS gonna have 2.5V because my offset. I just would like to know how to do it automatically if possible just using analog components. Any doubt, let me know
Thanks 

Hi Didan-The KRE 2 is a 2-position terminal block, right?Can you clarify why you need to plug in either of the CT's and have it automatically detect them?The ADS1115 has four inputs, plus a programmable gain amplifier which will allow you to adjust the scaling without changing component values. Can you set up one input for one CT and another input for the other type of CT?-Rick

One problem I can see...your Burden resistances are going to have to be grounded at one end for a return path.  So adding your offset will not be that simple - your voltage divider will be pulled down by the burden resistance.  You could get round this by having a capacitor between the CT output and the offset divider.  Just make sure that the Burden resistor is ALWAYS connected across the CT.Alternatively you could do some signal processing between the CT and the ADC - an op amp or two would let you do the offset, maybe a bit of amplification, and even detecting if the CT is connected, but if you do this build in LOTS of protection against spikes and the like.

Hi, i need to measure 3 phase energy. I will have 3 CTs going to A0, A1 and A2 from ADS1115. A3 I should hang a voltage sense. I need to have it automatic because it prevents against mistakes at the time I just unplug ct with current measure (which I know I need burden resistance parallel to output and its grounded on my circuit) as well I have all circuit done. I still have second ct with voltage measure on secondary which I dont need burden cause its built inside it. I also need it automatic because its an end application for a customer and im not supposing hes going to know how to deal with circuit or change ct manually with a pin header for instance. Any add information let me know, so you can help me with a circuit ?

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I am afraid I am going to have to disagree with David. A voltage is developed across the burden and the current flows in a closed loop. The resistive divider provides a DC offset to move the complete AC waveform into the positive region. Whilst I have never used this approach, I believe it is valid. However there is a gotcha (at least it gotched me)  CSA (Canada Standards Authority) require (I am not sure how hard and fast it is) that one side of a CT be tied to  ground.  I don't know what the industrial standards in other countries are.
At times I have worked on the theory that the wave is symmetrical about the time axis, and only measured the positive half of the waveform. Other times I have applied a level shift with an op-amp and on other occasions I have generated a DC voltage proportional to the average or RMS value using analog methods.Didan , I would like caution you to be extremely careful when working with 100A as there is likely to be high voltage associated. Your health would be at risk. And I would expect many strange micro crashes and possibly even hard failures as a result of radiated noise especially if you are working with a commercial Arduino board. I would be inclined to develop with a much smaller CT, 1A (5A is much easier to find) in the primary if you can find it, as it will be far easier to control the current with a variac and some load. Depending on your (and your employer) commitment to testing this both in the lab and in production, you may want to look at my article on generating AC currents without voltage Part 1, part 2, part 3 and part 4 .There are a few commercially available (but expensive) AC current generators that I believe work on the same principle.Now on to your question- any kind of detection that I can think of to switch automatically ends up in a chicken and egg situation where any measurement you make requires the signal to be plugged in and operational before the switch can be decided. I do like Rick's solution and would probably be my approach.

Hi, thanks for asnwer. Thats it what u answer but I cant welder 100 burden on each CT because Im going to do it on large volume and someone is going to install it for end user, so I need something easier thats why automatic without need changes once im going to buy like 100 CT and send for assembly product and its gonna make very expensive manually modification on CT. Imagine a panel installed and just with KRE 2 connector outside. What I need is just plug CT (being it with burden built inside or not)and i dont think theres a problem if during 5 seconds after I plug my CT, for instance (100A:50mA) which need burden resistance wait for circuit switch, should have a delay because of course I will need to plug my CT first on KRE 2 and then my circuit detection be able to answer if CT need burden or not and then switch for right circuit as I attached.
Well, let me clarify better what I need again. Im going to measure 3 phase, so I will use A0,A1 and A2 analog input from ADS1115 cause I have better resolution of course than use Arduino ADC or ESP adc thats why external ADC and thats wy ADS1115 (I get 15 bits up to 32768 samples (1bit for start)) and I will leave A3 open (no connect) its for especific application, which is just measure 3 phase current. Im designing a board which gonna work as a shield for main board in my company thats just it.  I should hang a voltage sense on A3 but i dont need it.
The idea is I plug ct on KRE 2 (terminal clock yeah, there are blue one and green one) and after it a circuit to detect if the tc have burden inside or not to switch for the right circuit because ct1 (100A:50mA) theres no burden inside, then I need a burden to have a closed loop. However, if its ct2 (100A:1V) it already have burden built inside then I just need offset circuit for ads. Thats what I need. I was thinking about comparator but i dont know if gonna work. Thats where I need help. I need something analogic to switch between SW1 or SW2 depending on which CT I plugged on KRE2. Thats what I need help, please.
If someone have a better idea for circuit topology let me know too, if I can reduce components, for instance.

Didan
There are still many questions:1. Is the current always 100A AC or does it vary from 0 t0 100A AC?.2. What are you trying to measure- instantaneous current, average current or     RMS current? Are you looking to sync some event with a zero crossing?3,. Given the accuracy of the CT and the noise you are going to see on the line, why exactly do you need 15 bits of resolution?4. My impression is that you are using an Arduino processor. Keep in mind that the frequency of the signal is 50 or 60Hz. At 50 Hz you are going to have to sample at least at 1mS (in my experience) per channel to detect a peak/zero crossing or take sufficient readings to measure average or RMS. For 3 channels you have to sample at 3 times that rate. Can your processor and/or ADC execute at sufficient speed?5. When you plug the CT into your PCB, is the 100A current already flowing?

Aubrey - have to disagree - as per below the CT/Burden is sitting in mid-air as shown with only one connection to the offset circuit:
Sure there is a cap to ground but that would be more for catching spikes etc.I'd envisage something like this (with your Canadian grounding

Bu all means have a smaller cap to ground as before...Or have I missed something?

David
Not shown on the diag, but the input to the ADC taken from t’other side of the burden(the side not connected to the offset).
Take a look at the link in the original post.I prefer a low impedance reference- an op-amp buffering the divided voltage. There is aTI part that does exactly that, but I can’t remember the number right now.

Hi Didan-A couple suggestions:  First, use a 4-pin connector on the CT.  Two pins for the connection to the CT, and the other two pins would be jumpered for one type of CT and not jumpered for the other. This would allow your circuit to detect which CT is attached and change the math (or switch in different analog circuitry). Second, put the burden resistance in the connector so it will stay with the CT.-Rick 

Guys I just wanna to measure AC current, anything else. No power, nor voltage, only current with sct013. Aubrey current gonna be the value it needs to be when clip it around wire to measure current. sct013 measure ac current from electrical network, then i suppose RMS? Here in Brazil its 220V most of states and 110V some few states. I think 15 bits from ADS is for better resolution, its firmware goal team. We are using a esp8266 but we power board and there is a voltage regulator to power whole board. 

what should happen if I put a burden resistance for ct which already have burden built in? Gonna be ok? For ct with no burden even If i connect it without burden theres a voltage on secondary because diodes internally?  We cant leave open circuit on secondary but what gonna happen for sct013 with no burden ?

Didanwhat should happen if I put a burden resistance for ct which already have burden built in? It will be like 2 resistors in parallel so the effective resistance is lower and so the resultant output voltage will be lower.  It is still possible to calculate the output voltage relationship to the input current.For ct with no burden even If i connect it without burden theres a voltage on secondary because diodes internally? I don’r Think I have ever seen internal diodes in a CT. But if so, how are they connected? I guess back to back zeners might work if there was enough internal resistance to limit the current. We cant leave open circuit on secondary but what gonna happen for sct013 with no burden ?
With no burden the voltage on the output of the CT can get really high. In fact as a result of this,  Canadian standards require that there is a shorting bar when a burden is removed. Connector manufacturers make special products to implement this shorting bar approach ( I am not in the office till Tuesday, but if you want details let me know and I will post then). You will need to provide current limiting resistors and clamping diodes and/or zeners at the input of the SC1013 to protect it. If there are indeed internal diodes on the CT then perhaps your input protection doesn’t have to be quite as robust.

Good luck!
You may want to look at my blog   RMS Measurement  does cover measuring RMS with a micro

So if I wanna to avoid make a "magic switching circuit" which tell me which ct is connected and then switch for which one I wanna. I will leave my idea drawed for you really inderstand and another option I thought, maybe you can help me to consider which one is better and safe. Well, thats original idea on picture below. Second idea is dont use any automatic circuit just place burden resistance and if someone plug ct with burden inside, I just make a voltage multiplier to correct it

How can I simulate it on ltspice? You guys can help me to understand secondary of this ct? Its because its weird 0.72R I thought only when u have voltage source u need a series resistance and for current source need a parallel resistance to take measurements for a non-ideal sources. So you have 5A current source on secondary, this is what means 5A on diagrama? Another point is why (6.3V 8A) on secondary inductor of ct, where this value came from? Its like if you have sct013-100 (100A:1V) Which difference between yhdc (100A:1V) and (100A:0.33V) in terms of calculations?

Didan
Second idea is dont use any automatic circuit just place burden resistance and if someone plug ct with burden inside, I just make a voltage multiplier to correct it
I think this is a good idea.

Didan
To me this looks like the variac is driving a step-down isolation voltage-transformer, stepping down 240vac to 6.3vac. The 0R72 limits the maximum current to 6.3/(0.72 + 0.33) which is about 6A.  With the resistance of the CT the maximum current is around 5A. I believe the 5A is an ammeter. The 8A on the transformer is the secondary maximum current rating (as determined by the wire gauge on the secondary). Adjusting the variac will change the current through the secondary circuit from 0A AC to the maximum of about 5A AC.The other 0R33 looks like a shunt to measure the current as an alternative to the CT.That is my reading- perhaps it is entirely wrong and you need someone who know LTSpice to answer your questions.

In an earlier post I wrote about testing this. Have you thought about it?  With 240V and 100A you need to dissipate 24KW in your test gear, unless you are going to develop with the real equipment. 

But how should I know the difference between each ct ? Ct1(100A:0.33V) ; Ct2(100A:1V) ; CT3 (100A:5A) ? For instance if I have a conductor with 50A should I have 0.6V on ct1 secondary, but if I have conductor with 10A I should measure the same 0.6V on Ct2, how can I know difference? Maybe Im misleading about it too. What u think

But how should I know the difference between each ct ?

I don’t know. I’m sorry.