# Product selection help: Data logging for 74 volt locomotive wiring

**URL:** <https://community.sparkfun.com/t/product-selection-help-data-logging-for-74-volt-locomotive-wiring/68784>\
**Category:** Qwiic System\
**Created:** [May 18, 2026, 3:33am UTC](https://community.sparkfun.com/t/product-selection-help-data-logging-for-74-volt-locomotive-wiring/68784 "2026-05-18T03:33:07Z")\
**Posts on this page:** 1\
**Showing post:** 2

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**Author:** ![PaulZC](https://yyz1.discourse-cdn.com/sparkfun/user_avatar/community.sparkfun.com/paulzc/32/67_2.png) [@PaulZC](https://community.sparkfun.com/u/PaulZC)\
**Post date:** [May 18, 2026, 8:09am UTC](https://community.sparkfun.com/t/product-selection-help-data-logging-for-74-volt-locomotive-wiring/68784/2 "2026-05-18T08:09:55Z")

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Hi @HeavyIron ,

Welcome!

The ACS37800 is designed to measure high load currents in power systems. It may not be the best choice or may be overkill if you only want to measure voltage.

For the voltage measurement, it depends if you are using GND or LO as the reference. With GND as the reference, a RSENSE of 8.2K delivers 245mV with an input of 60V: 60 \* 8200 / 2008200.

 ![image](https://canada1.discourse-cdn.com/sparkfun/original/2X/2/2969c7a270e8ad3344aec3b6be8c71ad11bddd29.png)

 ![image](https://canada1.discourse-cdn.com/sparkfun/original/2X/a/a4e7a6aca6a46284cdb77be6f8e146da7e6ff0dd.png)

The LO pin and extra 2 x 1M resistors are only needed if you need additional isolation:

 ![image](https://canada1.discourse-cdn.com/sparkfun/original/2X/e/e8f66bf19991be5f3c68f04fbdaa8ebad3360acd.png)

Maybe, for your application, you do need the extra isolation the LO pin would give you? I’m not sure… If you do use the LO pin as your ground reference, then, yes, you could measure voltages up to 120VDC. But the SparkFun product is not designed to used above 60VDC. Just to be clear:

![image](https://canada1.discourse-cdn.com/sparkfun/original/2X/b/b010eeb58d4419fbd128c5098151ff7d656bb28f.png)

It is not clear to me if you are measuring true DC, or actually rectified AC? Half wave rectified, or full wave? That said, I’m wondering if a simple 4-channel ADC could work better in your application? Would you be happy adding your own voltage divider resistors? If you are, then you could use a single ADS1015 ADC plus a DataLogger to log four voltages simultaneously. But please be careful with your grounds. It is not clear to me if it would be safe to have all four voltages plus the DataLogger sharing a common ground. Maybe you need optical isolation?

I will include a link to a similar discussion about voltage measurement below. I hope it helps.

Best wishes,  
Paul

> [@How to monitor the health of a relay and it's contacts](https://community.sparkfun.com/t/how-to-monitor-the-health-of-a-relay-and-its-contacts/67610):
>
> Hi, I have a challenge and hope someone has and idea how to solve it. I want to monitor the health of a relay by monitoring the state of relay contacts in relation with the state of the coil. I want to see if the contacts are open or closed when the coil is energized or not energized. I want to do this with an Arduino. It must be something like you see in the drawing below. Can you tell me if there is a component or sensor that can do this. Let me know what idea you have how this can be done. Thanks

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