CT sele

[preeti.kr]

Hello,

I am looking for options to measure current ~50mA using CT. Came to know about SCT 010 20A: 20mA. Still studying the datasheet. Want to know how to identify the minimum current it can measure. Couldnt find anywhere in datasheet. And also the accuracy .
Do we use CT for small currents like 50mA ?

 

[KlausST]

Hi,

What about posting a link to the datasheet?

CT means you want to measure AC. No DC. So what waveform is the signal and what frequency?
A CT may work ... but it depends on your applications requirements whether a CT is useful.

Why are you focussed on CT?

*****
I´m doing electronics design for decades now.
Your task is to measure current. But you don´t give any requirement - besides 50mA.
Using a CT is a part of the solution.
And now you look for specifications
So you do it this way:
Task --> solution --> specification

My way is:
Task --> decide specification/requirements --> find solution

I guess the signal flow is:
Source (50mA) --> CT --> ?? --> measurement device --> ?? --> the value you are after

Examples:
* Source (50mA) --> CT --> amplifier, filter --> ADC, microcontoller --> RMS calculation --> RMS current
or
* Source (50mA) --> CT --> RMS-to-DC converter --> ADC, microcontroller --> LSB to current calculation --> Display RMS current
Just two basic ways to measure RMS current using a microcontroller. Rather different, with different ways to process data. With different requirements.

We don´t know whether you are after RMS value or not, whether it is mains frequency or RF, whether you expect 5mA accuracy or 50uA accuracy.
Whether one has to expect pulses...

Klaus

 

[preeti.kr]

The aim is to measure Air conditioner unit's Fan Only mode current(AC ) consumption. I tried measuring it using A Clamp meter which was around 50mA.
The solution needed is non invasive that doesn't change existing wiring so CT is the option I have.
Datasheet of CT is here : https://drive.google.com/file/d/1zZdHNJsWK5P2fDLHKsVX4NufP2MUM7OW/view?usp=sharing

The maximum current to be measured is 5Amp.

I was curious about how small current (RMS) CT's can measure. or Is there any limit to the minimum current(RMS) .
Accuracy is not much important here so +-10mA will work.
Just started with CT analysis, so will be checking output wrt to the input current on oscilloscope. Further the signal will be given to adc of ESP8266 for measurement.
Adding the ref circuit:

 

[FvM]

For AC current measurement, minimum current is only limited by circuit noise. 50 mA should be no problem.

Primary:secondary current ratio is 1000:1. Datasheet specisfies 10 ohm burden, with higher burden, e.g. 470 ohm you get probably large error due to secondary magnetizing inductance, but the datasheet doesn't specify.

Best option is an OP current-to-voltage converter, Can use large conversion resistor, e.g. 10 k for high sensitivity.

 

[preeti.kr]

I started with 56E. But unable to find stable values i.e. amplitude on scope below 100 watt loads (460mA). So i increased it to 470 E where i m able to get stable voltage on scope for 60watt(260mA).
will check current to voltage conversion.

 

[KlausST]

Hi,

If I understand correctly:
* load current is 5A (what is this? RMS? Sine waveform?... While the CT maybe is in RMS and maybe you want the result to be RMS, you still need to calculate with-peak-to-peak for the ADC and the limiter)

* what is the ESP8266 decodable ADC input voltage range? (min/max)

If you´re interested in the low currents only, you could use diodes / zeners to limit the voltage across the burden.
Then maybe use an amplifier to adjust ranges.

***
You feed the instantaneous value to the ADC, This means you need to perform continous sampling with a fixed sampling rate to calculate RMS value. I like it this way. Just want to ask whether you know how to write this software. As alternative you may do this in analog hardware, so that the ADC just needs to read a DC value.
It depnds on what you prefer: analog or digital processing)

***
Btw: Why no link to the datasheet at the manufacturer. I don´t open files of unknown source / type / content.

Klaus

 

[dick_freebird]

CT-1 and CT-2 are both fine for 50mA. You'd get 50mV or 250mV
for that, at a 50-ohm-terminated 'scope input. Take 8-bit vertical
resolution (there are some better, if you have the

,,​

$) and that
says 0.2mA or 1mA resolution. What other error accrues, don't know.

I think I recall Tek had some newer ones, but I've always been content
with the basic CT-1.

 

[BradtheRad]

The aim is to measure Air conditioner unit's Fan Only mode current(AC ) consumption.... The solution needed is non invasive that doesn't change existing wiring....

Can you get at the wall plug to hook up jumper clips? Then you can install a sense resistor. Non-invasive. Read AC voltage across it with your ordinary voltmeter. Isn't that simple and straightforward (in contrast to a CT)?

The series resistor should be high enough ohms to obtain a usable reading but low enough ohms that it doesn't heat up excessively. Example, to read a few Amperes through your appliance, 333 mOhm is reasonable. Made from three 1 ohm resistors in parallel. Take a reading quickly before they're hot.

Link to above circuit running in Falstad's animated interactive simulator:
The schematic loads on his simulator and runs on your computer.

tinyurl.com/yqk3573y