 # Current Transformer to Arduino

Hello...

If i have current transformer which convert 300 amp to 1 amp ( 300 / 1 ) turn ratio . My secondary current varies between mili ampere up to 20 ampere ( maximum current and that happen when fault occur at High side cable )

I want to be able to read all these ranges of current between a few mili ampere up to 20 ampere and rectify them. The problem is ;-

1- at few mili ampere diodes used for rectification can not turned on ( because it needs 0.7 volt and that far away because i just have mili volt)

I reasearched and find a precision rectifier using op amp.

My quesion is can i use op amp even if my input signal is 0.0001 volt and rectify it ( i have this volt when i have few mA ) ?

Or this value is too low ?! I can not amplify it because some times i get 20 amper.

How to deal with this varing range ?

How can i solve this problem ?

300 amp primary is 1 amp secondary…

and 20 amp secondary is therefore 6,000 amp primary… is this a homework theory kind of thing.?

no opening for Arduino I fear.

I think you will find those kind of current trannies are for use with expanded scale meters probably starting at 150 amp or so.

Just be extremely careful playing with those things as an open circuit secondary can be lethal.

How do you transform the secondary current into a voltage?

For a huge range multiple ADC can be used, for low and high ranges. Find out which ADC is not overdriven and use that value and related scale factor.

Find out yourself how low the AC signal can be for your precision rectifier. Amplify very low voltage before rectification.

But why rectify at all? The amplitude of an AC signal is hard to catch, I'd go for a peak hold circuit instead.

300A on the primary and 1A on the secondary so 1:300 turns ratio. So if it were a perfect transformer then N mV on the primary and 300 N mV on the secondary. The secondary will need to be loaded so that the 1A becomes some mV. Something of the order of 1 ohms so that you get 1V rms output but you need to look at the transformer specification to see what loading is recommended. A precision rectifier (the web is full of circuits to do this) with a low pass filter and feed the result into an analogue input.

skyvan:
300A on the primary and 1A on the secondary so 1:300 turns ratio. So if it were a perfect transformer then N mV on the primary and 300 N mV on the secondary. The secondary will need to be loaded so that the 1A becomes some mV .Something of the order of 1 ohms so that you get 1V rms output but you need to look at the transformer specification to see what loading is recommended. A precision rectifier (the web is full of circuits to do this) with a low pass filter and feed the result into an analogue input.

Yes BUT the problem comes when the Op wants to get a "precision reading" at a 1 mA secondary.

Seems a very weird arrangement to me overall.

As i said i want to measure current ( rms value specifically ) , so i need to rectify the signal then filtering it.

First , i put shunt resistance 0.25 ohm to make voltage drop at resistor ( Now my voltage drop at maximum current is 5 volt so arduino can deal with it )

Suppose seconday current is 1 ampere , i will get 0.25 Volt . NOW How can i rectify it ? Diode has forward biased at 0.7 v ( will not rectify )

You might said amplify it and rectify it .. that is right but if build my circuit to amplify signal and voltage drop was 5 volt ( amplifying here makes problem with arduino ).

Dealing with mili ampere to 20 ampere makes some problems

bluejets:
Yes BUT the problem comes when the Op wants to get a "precision reading" at a 1 mA secondary.

Seems a very weird arrangement to me overall.

Then at low ranges change the load from 1 ohm to 1K ohm to get 1V rms again ! I hope the OP is not trying to do auto-ranging. That would be difficult.

r5emy:
As i said i want to measure current ( rms value specifically ) , so i need to rectify the signal then filtering it.

First , i put shunt resistance 0.25 ohm to make voltage drop at resistor ( Now my voltage drop at maximum current is 5 volt so arduino can deal with it )

Suppose seconday current is 1 ampere , i will get 0.25 Volt . NOW How can i rectify it ? Diode has forward biased at 0.7 v ( will not rectify )

As has been said - use a precision rectifier. See Precision rectifier - Wikipedia .

You might said amplify it and rectify it .. that is right but if build my circuit to amplify signal and voltage drop was 5 volt ( amplifying here makes problem with arduino ).

Change the load resistor to change the transformer output voltage !

Dealing with mili ampere to 20 ampere makes some problems

Are you trying to make this auto-ranging?

We would actually need to see the specifications for your CT. Things like type, accuracy class and VA burden are quite important.

If i have current transformer which convert 300 amp to 1 amp ( 300 / 1 ) turn ratio.

Note that 300:1 turns ratio could mean that its a 1500:5 CT.

My secondary current varies between mili ampere up to 20 ampere ( maximum current and that happen when fault occur at High side cable)

Note that if this is a 300A to 1A CT, then any measurement taken less than (for example) 10% of rating (100mA), the waveform becomes progressively distorted. At a few milliamp, it wouldn't even look like a sinewave anymore. The same thing happens for currents greater than (for example) 150% of rating (1.5A secondary).

How can i solve this problem?

Provide specifications?

I would highly recommend not using a standard transformer but rather a CT aka current transformer.
1,000 amps sensed yields microamps output because there is no primary [in the way a standard transformer has a shared Ferris core ]. The shunt resistor used to get the desired voltage is not carrying much current.

A standard transformer is designed to transfer power, a CT is designed to transfer a signal. That is why you typically use 1/4 watt resistors.

Study auto-range multi-meter circuits
The answer you seek is in a hardware circuit design