Measure load current of & with a Pico

Support Product Questions
1

Hi all.

While my decades old Fluke 77 does measure DC mA its voltage burden means I can’t use it to measure the load on my Pico projects.

Q1: Can someone recommend a DMM that would be suitable for measuring the current load of a Pico or other MCU based project?

Q2: I see Core carry numerous current modules. Is there a Micropython module that I can use in circuit with a Pico project?

Cheers.

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Hi @Mark285907

Most of the sensors from the Current Sensors category will work with a Pico (or a MCU of your choosing) the biggest thing to pay attention to will be the logic voltage, as some modules will only work with 5V while others like ACS711EX Current Sensor Carrier -31A to +31A | Buy in Australia | POLOLU-2453 | Pololu | Core Electronics are more flexible and are fine to use with either 3.3V or 5V.

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Hi Mark
Does your Fluke 77 have a 10A or higher range. This should get you down to mA resolution without a voltage burden problem (very low).
I have 3 instruments
Multicomp DMM MP730624A
UNI-T DMM UT71B
UNI-T Clamp meter UT210E

All give mA resolution on 10 / 20A range
Less messy than sensors and mostly you will have to rig up an Arduino or RPi and be limited to ADC resolution.
Cheers Bob

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Thanks Bob.

Here’s a table from the terrible manual for the Fluke 77. Yes there’s a 10A socket that’ll read DC mA with a resolution of 10mA (fine for my purposes) and if I’m reading this correctly, the voltage burden is 1% of that in the 320mA range?

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Hi Mark

Yes, 0.8% actually. At 50mV per A that should not worry you too much.
If you can live with 10mA resolution you should be OK. Looks like DMMs have improved in this regard a bit over the years. The Fluke instruments used to be considered the holy grail years ago. In fact most of the others were calibration checked using a Fluke Calibrator.

I can remember when the AVO 8 was pretty much the bench mark with the mirror scale (to minimise parallax error). Even then it had to be laying on its back to balance the meter movement. When digital devices came along with their 5V limit the first thing you had to do was remove the 22.5V (I think) battery in case you accidentally destroyed everything by doing continuity checks on the wrong range.
I still have an analog meter somewhere. Such a meter is still useful if you are adjusting tuned circuits in a transmitter or such. A digital meter is hopelessly slow for such tasks. But even the 20kΩ/V meters are a bit slow for this application.
Cheers Bob

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Thanks Bob.

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Hi Mark,

Just to confirm, are you looking to measure DC current or transients? i.e. the current consumption of the Pico itself.

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8

Liam,

What I was after was a module or method to monitor the load current of a pico project whole. ie my weather station and the load on the output of the Adafruit Universal USB / DC / Solar Lithium Ion/Polymer charger - bq24074 | Buy in Australia | ADA4755 | Core Electronics

Cheers.

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9

Hi Mark,

Understood, the most basic way to measure that directly would be as Bob mentioned, a DMM with high resolution (some meters have a function to capture the max meaurement - giving you a worst case, most drawn number).
I think that UNI-T clamp meter ought to have it(I got one on a whim and it has been good so far).

The most advanced is with a power measurement device like the Otii Arc: Qoitech Otii Arc Pro | Buy in Australia | CE09636 | Core Electronics

It captures the fluctuations as the processor is running. (other options are available, but I’ve used the Otii with amazing success).

Both of these options give a calibrated, repeatable way to estimate battery life.

Boards like this will allow you to read the battery remaining: Adafruit LC709203F LiPoly / LiIon Fuel Gauge and Battery Monitor - STEMMA JST PH & QT / Qwiic | Buy in Australia | ADA4712 | Core Electronics
I recall one also offered current measurement but I wasn’t able to track it down

There have been some interesting topics recently on this. Donald (Connect solar panel to ESP32's 5V IN pin - #4 by Liam) has a good summary in recent times (it is also worth taking a read through some of the regulars posts over the years).

If you do go the route of an external (Pico attached device) imo it would be worth using an I2C device or similar that offers readings like average, max current readings.

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