using CT (current transformer)

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Dear friends

Hi and good day

I have never used a CT yet and now im going to use the AC1030 CT which I have attached the datasheet .the aim is to use the ct in an energy meter (AC current measure)

I have some question about it .

1- it has used a 100 ohm resistor in the output , can I use other resistors too ? for example a 10 ohm ? 

2- how can I change the voltage which I read from the micro controller (adc) to the real current ?

3-does it have a logarytmic pattern (As I see in the voltage -current figure or it is linear ? (could you please tell me the exact way via example)

regards

 

 

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Last Edited: Mon. Mar 28, 2016 - 07:15 PM
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A quick Google search for "ct rms schematic" uncovers many pertinent hits.  One of the first directs you to

openenergymonitor.orga/emon/buildingblocks/ct-sensors-introduction

http://electronics.stackexchange...

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A few answers:

 

1. You can use other than 100 ohm, but it is designed for 100 ohm load. Odds are, the calibration constant will not be correct unless you use 100 ohm. Why would you want to use another value?

 

2. The voltage at the output of the CT is related to the current by the turns ratio and the load resistor.  The spec sheet says 100mV/Amp (primary current, with 100 ohms load). This is an AC voltage! To read this with the ADC, you have two choices: precision rectify the AC output and feed that in to the ADC input, or sample fast enough (say higher than 10X the frequency of the power signal) and "rectify" in software. To do the latter, you might want to bias the CT at Vcc/2 and use Vref = Vcc; that eliminates the need for capacitor coupling.

 

3. Absolutely linear, up to saturation. 

 

Jim

 

 

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ka7ehk wrote:

1. You can use other than 100 ohm, but it is designed for 100 ohm load. Odds are, the calibration constant will not be correct unless you use 100 ohm. Why would you want to use another value?

Thanks a lot sir for your kind reply

because I am using ADE7753 energymeter IC and the maximum input voltage for the current channel is 0.5 . so I need a lower resistor for the CT to have a lower max voltage 

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theusch wrote:

A quick Google search for "ct rms schematic" uncovers many pertinent hits.  One of the first directs you to

Thank you very much as you help me always ...

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You need 1/6 of 3V, so use a resistor divider whilst retaining the 100Ohm burden resistor.

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Kartman wrote:
You need 1/6 of 3V, so use a resistor divider whilst retaining the 100Ohm burden resistor.

if I use resistor divider , then the equal resistor that the CT sees would be less than 100 ohm 

am I right ?

Last Edited: Tue. Mar 29, 2016 - 04:02 AM
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High-ohm divider resistors so minimal impact on 100 ohm load.

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To build the resistor divider, we can use the following two equations:

 

R1 + R2 = 100
R2 / ( R1 + R2 ) = K

 

where:
R1 the resistor connected between a CT terminal (the other CT terminal is grounded) and R2.
R2 the resistor connected between the desired output node (with R1) and ground.
K  the required voltage attenuation, 0 < K < 1

 

Therefore:

R2 = 100*K
and
R1 = 100*(1-K)

These are the theoretical (ideal) values of R1 and R2 for a desired K.

 

Of course we can also use a high-ohm divider in parallel with a 100R resistor.

In this case, the 100 (100R) in the 1st equation above could be substituted with 10000 (10K), for example.

 

The equivalent resistance seen by the ADC input pin would be very close to R2 // R1:

Req = R1 * R2 / ( R1 + R2 )

 

Last Edited: Tue. Mar 29, 2016 - 05:26 AM