Reading Voltage Input on ADC

Question

I'm thinking of using the Waveshare ADC board with a RaspPi 3 to monitor the current driving a DC brushless motor.

I've run through the waveshare examples, and I can handle reading inputs programmatically.

I did an initial bench test without the pi, using the circuit in this photo

Everything works splendidly there. My meter reads 0.3v at rest and 0.6v when the motor's running just as expected (There's a screen and some other circuitry connected to the power supply that's not shown).

My plan is to run measurements using the ADC inputs on the Waveshare board, like this:

Anything else I should be considering here?

The motor circuitry has back-emf protection, and I'll be adding a filter capacitor to the bridge rectifier for a cleaner DC output for sure.

Accuracy isn't a major concern here, if I'm off by 20% I'm still doing much better than I am now using commercial current sensors at this measurement range.

Alternatively, does anyone have a good lead on low value current sensors (< 1A)?

Answer 1

Update 2019apr08hkt1109

On second thought, the question can be laser cut to focus on a very specific topic:

"How to measure the magnitude of AC current in the range of 0 ~100mA, with a generous tolerance of 20%?"

I think we can begin our feasibility study with two current sensors: （1） AC712, (2) INA12x/22x, ...

ACS712 (or newer ACS723) 5A full scale version can measure +-5V, with a sensitivity 200mV/A. We can use an ADC with 10/12/24 bit resolution to suit any tolerances, ...

ACS712 Datasheet - Allergo MicroSystems

INA12x measures DC current only, but can be very precise by using an appropriate external gain resistor, ...

INA12x Datasheet - TI

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/ to continue, ...

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The following paragraphs contains errors, and would be modified and/or deleted later.

The question

How to measure the analog AC voltage across a 0.5 Ohm (errata- should read 10 ohm) shunt resistor in the range of 0.3V ~ 0.6V (I = V/I ~= 0.5/10 = 0.05 ~= 50mA)

The answer

/ to be continued, ...

The following quick and dirty, long winded, messy draft notes is to be deleted after project is completed.

Project Goal

Using the Waveshare ADC board with a RaspPi 3 to monitor the current driving a DC brushless motor.

Progress so far

Completed Rpi program controlling Waveshare ADC to read analog signal. Completed bench test using digital volt meter to measure motor on and off analog readings 0.3VAC and 0.6VAC. 

User requirements

Accuracy isn't a major concern here, off by 20% is still doing much better than using commercial current sensors at this measurement range (0.3VAC ~ 0.6VAC). Low value current sensors < 1A AC

User constraints

Cannot easily access circuits at the DC motor side, therefore need indirectly measure AC current at the AC mains side.

Because the motor is too small, current transformers which are usually for large current does not work here.

Current sensors and ADCs to explore

[ACS712, INA226, ADS1015, ADS1256,AD7190] Raspberry Pi Current & Voltage Sensors (10A rating and ~40V respectively)

[ADS1256]Waveshare ADC Board

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Answer 2

This is not an answer but a warning!

Your circuitry shows a direct connection to the mains active - this this is potentially LETHAL.

Incidentally the first circuit you posted would do nothing (unless some of the diodes are faulty) - there is no potential difference across the meter.