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Energy Metering system using CT (Current Transformer)

Didan Ribeiro
Prodigy 30 points
Other Parts Discussed in Thread: ADS1115, INA225

Hello. I would like to know more how I should get a PGA (Programmable Gain Amplifier) to work with my CT (Current Transformer) (Cuttof Frequency of system: 1Khz, so i just need pga for a 2khz bandwitch or 1khz its ok) . Im developing a new design product in my company to measure current and need to interface it using an ADC (ADS1115). I would like to know part numbers and how I should get in touch with specialists in this for any doubts and implementation. We wanna to produce over than 1000 boards and if it goes well we are going to use TI Ci, thats should be a good deal for both too.

The idea of my project is plug any kind of CTs (voltage or current output). Im going to place a shunt a fixed shunt resistor of 0.1R 10W after connector to convert in the case of CT(current output like 100A:5A) current to voltage. So if I have CT(100A:5A) for a 0.1R shunt I will have 700mV on my output. So i will need to set a gain to amplify this signal from 700mV to 2.5V for instace. My ADC range is 0-4.096V. However, if I plug CT (100A:1V) it has shunt built inside then after plug it on my circuit I will have lower voltage because parallel resistance. For instance for 100A I will get 100mV instead 1V, so I will need to amplify this signal to get 2.5V. That voltage 2.5V should be my reference for my ADC measurament as instance. So  after search a little I found PGA which I saw I can interface it using SPI with my uController like my ESP-12 and set the gain. As well I will need to set offset too because my uController dont read negative voltage ... But in generall thats the system which will going to use as energy metering. IN this specific occasion I wanna measure phase current and my doubt is which PGA to select and how to interface it with my uC, adjust offset and gain. I wanna to operate up to 1kHz bandwidth... If what I said looks confuse, let me know, all I want is some help to design that system =D

  • 0
    TI__Mastermind 32395 points

    Hey Didan,

    Welcome to the forum.

    Where are you getting 700mV from? Is this presuming 7A coil current? When you say the CT is 100A:5A, I believe you mean that at 100A of primary current (Ip), the secondary coil current (Is) will be 5A. Correct?

    The speed of the secondary current/primary current will determine how much bandwidth you need from your PGA. If the current is moving at 1kHz, you should be able to sense this with a PGA that has 2kHz closed-loop bandwidth.

    Since you want to design your system for current and voltage transformers you need to sense a large range of currents because the voltage transformer has a built-in resistor to convert the voltage to current. Am I understanding this correctly?

    If you could provide all of the possible secondary currents you will have to measure and at what common-mode input voltages (Vcm), then that will be helpful in narrowing the scope of your design. I believe you will need some current return path for the secondary coil. The range of Vcm the PGA input pins will see is going to be important as not all devices can operate when VCM = 0V, VCM<0V, or VCM>Vs (here VS = PGA's differential voltage supply). If you are able to supply a +- dual supply to your PGA that will certainly broaden your possible options. For example, if you have a +5V and -5V supplies on your board, then a PGA or INA (instrumentation amplifier) could operate a secondary coil that has -5V<Vcm<+5V.

    Hope this helps, but please provide more information on what currents you need to sense and at what Vcm and at what error.

    Best,
    Peter

  • 0 in reply to Peter Iliya
    Prodigy 30 points
    Hello, Peter. Thanks for reply. CT 100A:5A its right, secondary coil current 5A. Peak corrent: 5A*sqrt2 = 7.07A Im going to sense AC network, then 60Hz but I need 1Khz bandwidth because I wanna harmonics currents too. I just wanna to design for current transformers, not voltage transformers. I just mentioned that have CTs with burden built inside which give me voltage output. However CTs without burden give me only current on secondarym like 100A:5A. If I have a CT 100A:1V, it means this CT already have a burden built inside which when I connect it, I will have voltage because resistor built inside convert it to voltage.
    What I need is a CI like PGA with automatic gain select by firmware and offset adjustment. Why? If I plug 100A:5A I will have a circuit with a fixed shunt resistor anyway. So I will be able to convert current to voltage depending on my load I will have different voltage. For instance, if I set 2.5V
    offset for my ADC read it, I will have resolution of :2.5V/100A = 0.025 volts / ampere. So if i have 50A load my CT gonna read 1.25V. So by firmware I need to read 50A at 60Hz precisely. However, if I plug CT 100A:1V its resolutoin is: 1V/100A = 0.01V so If I have 50A I will read 0.5V my firmware need to read it at 60Hz and about offset need to correct for 1V offset but I will do it remotely so If I wanna to plug a CT I will know which CT type it is and then by firmware set the appropriate offset and gain. I dont know how offset works on PGA, but I will only wanna to fix 2.5V offset, for my 100A:5A example. If I plug any other CT like 100A:1V and for 50A get 0.5V I will amplify it for 5, and I will get 0.5V * 5 = 2.5V, then I just can read it correctly by firmware. If I should set different offset for each CT should be better I think, but anyway I will do it remotely thats what I need.
  • 0 in reply to Didan Ribeiro
    TI__Mastermind 32395 points

    Hey Didan,

    Have you looked into the INA225? This is a bi-directional, programmable gain current sense amplifier. So it has a reference pin (REF) which allows its output voltage to be centered about 2.5V. You can also adjust the gain between 25 V/V, 50 V/V, 100 V/V, and 200 V/V. INA225 can be powered with a single supply source (VS) ranging from 2.7V to 36V. Regardless of VS it can operate with a common-mode voltage up to 36V.

    There are other programmable gain current sense amplifiers, but they do not have a voltage output (OUT) pin for the ADC to measure the voltage. These are digital products that report the current values over a digital bus.

    http://www.ti.com/amplifier-circuit/current-sense/power-current-monitors/products.html

    If this device does not work with your system requirements, then you will have to look into a programmable-gain amplifier (PGA) or variable-gain amplifier (VGA), but I do not know these products very well.

    http://www.ti.com/amplifier-circuit/pga-vga/products.html#

    Best,

    Peter Iliya