Understanding the datasheet for a LEM current tranducer

[gnif]

Hi All,

I am recycling some LEM LA-125 P modules for current sensing power generation on my solar system, however I am a little lost and need some basic help on how to make best use of the device. My requirements are to measure up to 100A DC, in one direction, I do not care about reverse current flow. This module seems perfectly suited as it has a nominal range of 125A.

Datasheet: https://www.lem.com/sites/default/files/products_datasheets/la%20125-p%20sp4.pdf

I intend to measure the value from the module using a STM32F103C8T6's ADC, so the output needs to be a voltage from 0-3.3V. From what I understand however the output of this module is current, not voltage. So to convert to a voltage I am simply using a 56 ohm resistor to ground, and a 3.6V zener to clamp the voltage to protect the MCU (schematic attached).

This seems to work on the bench, but I am unsure of the math to scale/convert the voltage output to the current flowing that the LEM module is sensing, or if I am even approaching this correctly at all.

[PartialDischarge]

Correct, the conversion that you have is 56/2000 V/A. I would also add a capacitor in parallel with the 56ohm resistor.

[coppercone2]

i think if you read about 4/20mA to voltage signal conversion in a industrial setting it will answer your questions

[Berni]

Yep that sounds correct. As per datasheet the transducer gives you a current that is 1/2000th the current trough the loop. Then the 56 Ohm converts that to 56V per 1A so you get to 56/2000 Volts per Amp

Also keep in mind that this transducer is capable of measuring negative current too. In that case you will get the output go negative. For monitoring solar panel current it doesn't matter, but if you want to measure the battery current or the AC side inverter current then you will need the negative part too.

[gnif]

Thanks all, I will see if I can figure this out

As for reverse flow, I have no desire to measure it as I have several of these and the plan is to put one on the MPPT output, and one on the load input using both in the forward direction. This will also increase my resolution as I won't have to devote 1/2 of the ADC range to reverse current flow.

I would also add a capacitor in parallel with the 56ohm resistor.

Is this to reduce the noise or average the value? And what value would you suggest?

[PartialDischarge]

Yes, I’d use  1 to 4.7uF and a series resistor out of the sensor of 33ohm for example

[gnif]

Like attached?

Sorry I am not at all great with any of this, I am more into the digital side of things, but I would like to understand. What is the purpose of this additional resistor? And isn't it now in series with R1 increasing the resistance to 89ohm?

[Berni]

He likely meant the resistor between the 56 Ohm resistor and 4.7uF capacitor.

The capacitor serves two purposes. One is low pass filtering the output of the sensor, since the sensor is very fast and can actually capture the switching ripple of your MPPT controller. Since you are interested in the average current you want to filter that down to a clean DC signal. The other benifit is that the ADC has a internal sample and hold circuit, this circuit connects a tiny sampling capacitor to the ADC input in order to capture the voltage then hold it for the ADC to do the conversion. Charging up this internal sample and hold capacitor takes some current, so having a capacitor close to the ADC input helps charge it up faster, making the ADC provide cleaner and more accurate results when running fast.

[gnif]

Thanks, that makes total sense.

One more question, as I intend to have this on a cable, at most 30cm from the MCU, would it be better to have this capacitor at the MCU? or (which I think is more correct) have a 2nd additional capacitor, say 0.1uF near the ADC input?