# 200A Current Transducer and Arduino UNO

Hello, masters! Has any of you worked with a 200A current sensor before?

I'll be using LEM's Current Transducer LF 205S/SP3. Datasheet: http://docs-asia.electrocomponents.com/webdocs/121e/0900766b8121e1c6.pdf

Specifications: Frequency Bandwidth DC up to 100 kHz Input Current Range 0 → 200 A Output Current Range 100mArms Sensor Type Closed Loop Hall Effect Supply Voltage 12 → 15 V

The arduino can only handle current up to 20mA(?) 40mA. Could you teach me on what to do to be able to use it with an Arduino UNO?

Arduino doesn't read current, only analogue meters could truely be said to use current.

Arduino does however read voltage, so pass the current through the appropriate value resistor to give you a 0 to 5V range and you will be able to read it.

so if max current = 200mA

R = V / I

V=5 I = 0.2

R=25

use a 22 Ohm resistor, between the output pin and GND this will give you 4.4V on the output pin for max current i.e 200mA

this will read as approx 1023 x 4.4 / 5 = 900

BTW, The Arduino only has a 10 bit ADC, so your resolution on 200A will be 200A / 1024 = 190mA

rogerClark: Arduino doesn't read current, only analogue meters could truely be said to use current.

Arduino does however read voltage, so pass the current through the appropriate value resistor to give you a 0 to 5V range and you will be able to read it.

so if max current = 200mA

R = V / I

V=5 I = 0.2

R=25

use a 22 Ohm resistor, between the output pin and GND this will give you 4.4V on the output pin for max current i.e 200mA

this will read as approx 1023 x 4.4 / 5 = 900

BTW, The Arduino only has a 10 bit ADC, so your resolution on 200A will be 200A / 1024 = 190mA

Im afraid its not that simple. Output current is AC, a resistor wont be enough because you will get an ac voltage output maybe between 5 and -5 volt depending on the resistor. Then you have to adapt the signal to change between 0 and 5 volt there are various methods you can rectify the signal to get a proportional dc signal or you can use opamps to set the offset at 2.5 volts and adjust opamps gain to set the signal at 0-5 volts. Then you can read the ac signal and calculate the rms value through arduino sampling.

Thank you for the replies!

JohnLincoln: What current do you actually want to measure? Is it : 0 to +200A -200A to +200A or some other value? Is it DC or AC?

0 to +200A, AC

arduinoTime:
Thank you for the replies!

JohnLincoln:
What current do you actually want to measure?
Is it :
0 to +200A
-200A to +200A
or some other value?
Is it DC or AC?

0 to +200A, AC

Ac current is negative at some point, as most ac balanced signal with ground reference do. I guess it measures 200 A rms, so it can change between -200sqrt(2) and +200sqrt(2) A.

And i guess you can damage the analog port if the signal goes below 0 volts .

mart256:
… or you can use opamps to set the offset at 2.5 volts and adjust opamps gain to set the signal at 0-5 volts. Then you can read the ac signal and calculate the rms value through arduino sampling.

Sir, what kind of op amp do you suggest I use?

arduinoTime:

mart256: ... or you can use opamps to set the offset at 2.5 volts and adjust opamps gain to set the signal at 0-5 volts. Then you can read the ac signal and calculate the rms value through arduino sampling.

Sir, what kind of op amp do you suggest I use?

I dont have the model right know sadly. There are tons of opamps which works between +-15 volt. You should get the ones that are for signal and precision rated to work at +5 volt with no negative supply. Look at Digikey for this kind of opamp.

Texas Instrument offers good opamp precision models.

I'd advise you model this in LTSpice and then build it.

JohnLincoln: You need a dual supply for the sensor, the same supply can power the op-amp.

Sorry, sir, but I don't understand. What supply? 5v or 15 v?

Do you mean 'Voltage - Supply, Single/Dual (±)'?

This is the link the technical support gave me at digikey: http://www.digikey.com/short/04rcn

JohnLincoln: There is no need to select an op-amp that doesn't require a negative supply , like mart25 suggested.

You need a dual supply for the sensor, the same supply can power the op-amp.

I think single supply is safer. You know an opamp can have an output as high as the supply offers, then it will saturate higher signals, which is good if your tranducer reads a failure current which are really high. For example if you use opamp powered at +-15 volt and you calibrate gain resistance for 0-5 volt on normal operation, at failure current the output may reach +-10 volt for example and you can blow arduino. If you use a single supply opamp powered at 5 volt you know your analog arduino port wont get a voltage lower than 0 volt and neither higher than 5 volts even if reading failure currents on transducer (you shouldnt worry about electronics because most transducer do have isulation)

A friend of mine suggested once OPA343 from Texas Instrument for analog reading maybe it can work for you too. Altough he worked on 3.3 volt..

Sorry, did I understand this correctly?

Setup: Current Transducer --> Op Amp --> Arduino

The current transducer will have an external power supply of 15V while the op amp will be supplied with arduino's 5V.

Sir, how can I adjust op amps gain to set the signal at 0-5 volts?

It may be worthwhile to look at the Allegro Systems ACS758 family of current transducers. They range up to 200A with minimal power loss and an easy interface to Arduino. Sample code for a 50A version is available from DFROBOT.
Refer http://www.allegromicro.com/en/Products/Current-Sensor-ICs/Fifty-To-Two-Hundred-Amp-Integrated-Conductor-Sensor-ICs/ACS758.aspx
and
http://www.dfrobot.com/wiki/index.php/50A_Current_Sensor(SKU:SEN0098)

arduinoTime: Sir, how can I adjust op amps gain to set the signal at 0-5 volts?

Hmm thats not a short answer. You better get a book or some article about opamps and linear circuits.

Sir, I worked with a 5A [ACS712] current sensor before and read AC current but I haven't worked with an op amp. How could I determine if it can read up to 200A? I really don't know what I'm looking for =(

Current - Input Bias ? Voltage - Input Offset Ideal is zero? Current - Supply ? Current - Output / Channel ? Voltage - Supply, Single/Dual (±) 5V

I think there may be another way to do this.

If you run the transducer from a separate isolated supply from the Arduino.

Then create a 2.5V reference voltage with respect to the Arduino GND (you can do this by just using a resistor divider network from the Arduino 5V rail, or something more complex)

Then if you attach the GND of the isolated supply of the tranducer to the 2.5V reference, Use a 10ohm resistor across the output of the transducer to convert its current output to voltage Then connect the output of the transducer to the analogue input.

This is the basic principal used by the current clamp metering input circuit on the OpenEnergy Monitor project, except their transducer is passive, i.e just a coil.

However I must admit at the moment the only way I can see to have a separate supply is to use a separate mains to USB charger / PSU unit. I'm sure you can get 5V to 5V isolating PSU's but I suspect a separate PSU would be easier and cheaper.

If this doesnt make any sense, let me know and I'll try to find the time to draw a diagram

I'd be tempted to use a full-wave rectifier and feed the output to either an analog input directly (adjusting for voltage), or through a filter to an analog input.

I'd be tempted to use a full-wave rectifier and feed the output to either an analog input directly (adjusting for voltage), or through a filter to an analog input.

I think this method could have issues because of the clipping of the waveform below the forward conduction voltage on the diodes.

i.e normally they don't conduct until supplied with 0.5V, so you'd get flat areas in the sine wave which could probably mess up any calculations.

Sir rogerClark, like this?

What does 0v means in the current transducer connection?

@arduinoTime

Fantastic diagram …

Yes. Almost

I think there needs to be a resistor across the output of the transducer, as the OP said the spec was the transducer output current was related to the input current, so to convert it to a voltage for the Arduino to read you need to put a resistor across the output, (not sure what value, possibly 10 ohms).

I have also sometimes seen a capacitor from the center point of Z1 Z2 to GND which I didn’t mention before (sorry I just remembered)

Thanks

Roger