Sensor for Low current usage detection on 15-18V AC

Hi,

I'm working on a miniature train project where I need to detect current usage in individual track zones, for train detection.
I know it has been done many times before, but I do no see any DIY solution or simple sensor I can add in between the power source and the track section.

As an example, if there is any current usage I want a digital input to the Arduino, the current load will not exceed 1A. The voltage is 15-18V AC. (DCC System / NEM / MOROP)

Any suggestion of a sensor or electronic components I can put together and create a sensor my self. ?

  • I tried to use the ACS712 5A module, but the current load was too low to be detected, and will also cost an Analog input.. Some times the load is as low as 20mA.

So basically what I want the sensor to do is to give a 5V digital signal when there is Load, and no signal when there is No Load.

Thanks in advance

Use a shunt resistor(s) on a section power rail and measure voltage drop there by ADC.

In your case you can use any 0.5 Ohm/ 1/2 Watt resistor. And set an ADC to use 1.1V as a reference

https://www.mouser.com/pdfdocs/bourns_n1702_current_sense_accurate_measurement_appnote.pdf.

image_2020-12-09_162920.png

Or you can use a current transformer module with amplifier - ZMCT103C 5A Range Single Phase AC Active Output Onboard Precision Micro Current Transformer Module Current Sensor|Integrated Circuits| - AliExpress

image_2020-12-09_162920.png

Thanks for the drawing, I will test the shunt resistor solution first, could be the solution I have been looking for.

Sure, but be aware that voltage on ADC pin is constantly changing. So you should do around 200 readings in a row and calculate an RMS.

rram:
The voltage is 15-18V AC.

Surely the track voltage is DC? If AC, how does an engine reverse?

johndg:
Surely the track voltage is DC? If AC, how does an engine reverse?

Well, the system is running DCC. Frequency is 8-10kHz. 15-18V on my tracks.

Mention of that, and perhaps a link to NMRA Standard S-9.1, would have been well-placed in your OP. Or is it NEM?

johndg:
Mention of that, and perhaps a link to NMRA Standard S-9.1, would have been well-placed in your OP. Or is it NEM?

That would be NEM (MOROP), but the DCC system should follow the same standard.

What's wrong with an old fashioned Hall current sensor?
Honeywell CSLW series: http://www.farnell.com/datasheets/1686062.pdf
LEM CTSR 1-P: https://www.lem.com/sites/default/files/products_datasheets/ctsr_1-p.pdf

I know LEM in particular do a massive range, although mostly high current.

The basic idea is put a hall sensor in a thin gap in a toroidal core, and feedback a signal to
keep the primary and secondary MMF's cancelling out, so that the current in the secondary is
proportional to the primary but much smaller and easy to measure - like a current transformer
but able to work at DC too.

The sensitivity and signal/noise ratio can be much better than an ACS715 style chip as the
core magnifies the field considerably, and reduces interference from nearby magnetic components.

Your OP mentioned making the sensor yourself.

You can make it yourself by using any mains to low voltage secondary transformer and letting it serve as your current transformer. Connect the transformer secondary in series with the wire with the current your want to measure. Bridge rectify the now secondary winding(originally the transformer primary) and use a small value filter capacitor.
The voltage from the bridge rectifier is now a function of the current in the transformer's new primary winding.

Calibration will be required.

Paul