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

Thread Starter

xDidan

Joined May 23, 2019
6
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
 

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ebeowulf17

Joined Aug 12, 2014
2,945
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
It sounds like the only thing you want to have change automatically is the connection to a burden resistor which is needed for one transformer, but not needed for the other.

If I'm understanding this correctly, I think you should permanently connect the appropriate burden resistor to your transformer that doesn't have one built in, and then you can hook either one up to your board at any time with no problems.

In addition to avoiding the need for a magic detection circuit, it protects your transformer from damage when it's not connected to board. A key fact about current transformers is that they should NEVER, EVER have power run through their primary if there's no burden resistor connected to the secondary. That's because they'll "try" to generate the appropriate current at the secondary output, and since there's no path to complete the circuit, they'll "try" to create infinite voltage to overcome the infinite resistance they face.

In reality, they'll reach a saturation point beyond which they can't generate higher voltage. If you're lucky, the peak voltage won't destroy anything. If you're unlucky, it could leak through the thin insulation in the windings, rapidly shorting and destroying them. Or shock you. Or destroy sensitive components when you initially try to connect it to another circuit. Or...
 

Thread Starter

xDidan

Joined May 23, 2019
6
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.
 

ebeowulf17

Joined Aug 12, 2014
2,945
Doing it with a microcontroller seems like something I could probably figure out if I tried, but doing it with just analog circuits and logic chips is beyond me. Sounds like an interesting challenge, but not one I'm up for. Sorry.

Also, just to double check, these transformers without burden resistors that you'll be plugging and unplugging - are they always connected to the lines they'll be measuring? If so, re-read my earlier comments about over voltage risks, etc.

Are you buying new transformers, or building circuits to work with existing transformers? If you're buying new ones, why not just buy ones with the burden resistance you need?
 

dendad

Joined Feb 20, 2016
3,070
As ebeonwulf17 says, you DO need to have the burden resistor permanently connected to each current transformer, then switch the resulting voltage. Get transformers with built in resistors, or use Hall effect versions.
Doing otherwise is just asking for trouble.
You come here for advice and my advice is to take it ;)
The burden resistor is the cheapest part, a lot cheaper than the current transformer. If you are worried about the cost of the resistor, then I think you need to drop the project.
 
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