Some doubts on current transformer

[premkumar9]

Hi
I need to sense AC current up to 30A. Planning to use a current transformer to convert this current to voltage, then rectify it to DC and measure the DC using uC.
CT (toroidal)with 2500 turns and 20A primary rating is available in the market. Just want check whether the following statements are true.

1) When the primary (single turn) current is 20A, secondary short circuit current will be 20000/2500 = 8mA.

2) A 600Ω resistor connected to the secondary will give 600 x 8 = 4800mV.


3) 30A through the primary can cause core saturation and distorted secondary O/P

 

[#12]

Are you having trouble reading the datasheet? The one you did not provide for us to read?
Why should I know more about this unknown size/brand/manufacturer/part than you?

 

[carmusic]

i would just use ct like similar to that https://www.electro-tech-online.com/custompdfs/2011/01/api_ctx-ac_ctx-acr.pdf

 

[Reloadron]

If you read through this thread which extends into actually two threads you will find a wealth of information on using a CT to measure current. There are considerations you have to think about when using a current transformer. Additionally this link is a good read on some CT basics.

Also, my choice for an application like you mention would be as carmusic suggest. They work well.

Ron

 

[premkumar9]

Thank you

If you read through this thread which extends into actually two threads you will find a wealth of information on using a CT to measure current. There are considerations you have to think about when using a current transformer. Additionally this link is a good read on some CT basics.

Also, my choice for an application like you mention would be as carmusic suggest. They work well.

Ron

Thank you. After going through the thread and data sheet you suggested, it seems that my previous statements are correct.

 

[premkumar9]

Are you having trouble reading the datasheet? The one you did not provide for us to read?
Why should I know more about this unknown size/brand/manufacturer/part than you?

Here is the data sheet. Is this enough to say whether my statements are true?
https://www.electro-tech-online.com/custompdfs/2011/01/CT1231_02201911.pdf

 

[SABorn]

If you have access to some discarded computer power supplies, than you will normally find a CT in them for free.

Visual appearance is the same as the ones in your data sheet.

I have used them long ago and dont remember the specs for them now.

 

[Reloadron]

Thank you. After going through the thread and data sheet you suggested, it seems that my previous statements are correct.

Yes, pretty much. Something you have to watch is when you drive a 30 ohm load using your CT remember you have an AC signal proportional to your AC current. When you rectify it as you plan, the rectified DC will not be a flat line but will have bumps like the output of any full wave bridge sans filtering. Just means you will need to include some signal conditioning.

@ SABorn, I will have to look at a few of my junk ATX power supplies. Good idea to harvest some CTs.

Ron

 

[premkumar9]

If you have access to some discarded computer power supplies, than you will normally find a CT in them for free.

Visual appearance is the same as the ones in your data sheet.

I have used them long ago and dont remember the specs for them now.

I need more numbers as this is for the requirement of a customer. Anyway thanks for the suggestion.

 

[premkumar9]

Yes, pretty much. Something you have to watch is when you drive a 30 ohm load using your CT remember you have an AC signal proportional to your AC current. When you rectify it as you plan, the rectified DC will not be a flat line but will have bumps like the output of any full wave bridge sans filtering. Just means you will need to include some signal conditioning.

@ SABorn, I will have to look at a few of my junk ATX power supplies. Good idea to harvest some CTs.

Ron

In fact I used current transformers for this purpose earlier. I used 30A/5A one (big ones)bought from the market and adjusted the primary windings to get the required voltage after rectification. Those pieces are working well now. But now I see smaller CTs readily available which may suit for my purpose.

 

[Reloadron]

In fact I used current transformers for this purpose earlier. I used 30A/5A one (big ones)bought from the market and adjusted the primary windings to get the required voltage after rectification. Those pieces are working well now. But now I see smaller CTs readily available which may suit for my purpose.

Well, as can be seen, there is a number of ways to get where you want to go. For a commercial application you face a decision of cost. The methods you suggest will work fine, especially since you have used these methods and done the subsequent calibration. Still given a choice and finance, I would consider one of these units if finances allow.

Ron

 

[premkumar9]

Well, as can be seen, there is a number of ways to get where you want to go. For a commercial application you face a decision of cost. The methods you suggest will work fine, especially since you have used these methods and done the subsequent calibration. Still given a choice and finance, I would consider one of these units if finances allow.

Ron

Thanks for the information. I can definitely consider using this type of sensors for projects which can afford to have these (where more measuring accuracy is required).

 

[MikeMl]

Your problem is likely how you are converting the voltage across the 30Ω burden resistor to a DC level. You really should be using a "true RMS" circuit, not a simple peak detector.

 

[premkumar9]

Your problem is likely how you are converting the voltage across the 30Ω burden resistor to a DC level. You really should be using a "true RMS" circuit, not a simple peak detector.

Earlier I had to sense only normal working, over current and no current states only (by measuring the current). That could be achieved with peak detection.

 

[premkumar9]

Could you please suggest a method for true RMS voltage measurement of line voltage (typical 230V AC)?

 

[crutschow]

Could you please suggest a method for true RMS voltage measurement of line voltage (typical 230V AC)?

I assume you want something besides true RMS voltmeters, which are readily available.

Analog Devices and other make chips that output a DC voltage proportional to the RMS value of an AC voltage. You have to scale it with a resistive divider or use a transformer to match the input voltage range of the chip.