Posts: 2
Joined: Sun Nov 03, 2019 11:17 am

Messure AC current

Sun Nov 03, 2019 11:35 am

At home I have a 230V AC water pump and I would like to messure how much power the pump is drawing.
So that I can log it with my raspberry zero.

I have the following pump ... 3&skat=956

The pump description tells that the pump is drawing between 5 W and 25 W.
My Idea was to messure the current and compute the power with it.
But if the pumps descriptions is exact than the current would be between 20 mA and 108mA and I don't know how to messure such small currents.

Has anybody an idea how I could messure the current ?

Posts: 667
Joined: Sat Feb 24, 2018 4:09 am
Location: Buffalo, NY, USA

Re: Messure AC current

Sun Nov 03, 2019 4:44 pm

I don't think that you will find a current transducer that will give you reasonably precise measurements at these low currents, so you will probably need to put a low value resistor (e.g. 200 Ohm 5W) in the circuit and measure the AC voltage across it. Even though the currents you are measuring are low, you are dealing with mains voltages, so you should not embark on this unless you understand how to do it safely.

Posts: 267
Joined: Fri Jul 06, 2018 2:13 am
Location: USA

Re: Messure AC current

Sun Nov 03, 2019 8:58 pm

When working with mains voltages, please observe all the applicable precautions first.......

To measure anything off the mains, you should really use a transformer for isolation.
For measuring current, you could go with something like this: ... 83&sr=8-11
The transformer will provide the needed isolation. However, the output is an analog value of up to 1 VAC (fs input/output across 200 ohms). For the Pi, two things need to happen first - the input to the Pi needs to be limited to a maximum voltage of 3.3 volts, and the analog value needs to be converted to something you can send to the Pi. With this sensor, you will need an A-D converter. Either use an A to D converter with an I2C output to talk to the Pi, or use an integrator that will provide a variable speed pulse train output depending on the input. I would opt for the A to D converter for simplicity.

Because you're wanting to measure power, you're also going to have to sense the voltage. This is easily accomplished with a small step down transformer whose primary is connected across the mains feeding the pump, and the secondary voltage (which will be a known ratio of the primary) can be sent to the Pi. The math can be done in software.
Last edited by JohnsUPS on Sun Nov 10, 2019 3:39 am, edited 1 time in total.

Posts: 12402
Joined: Wed Jan 25, 2012 6:39 pm

Re: Messure AC current

Mon Nov 04, 2019 8:24 am

And don't overlook the small issue that whatever you put together may need to be calibrated.
Quis custodiet ipsos custodes?

Posts: 2
Joined: Sun Nov 03, 2019 11:17 am

Re: Messure AC current

Sat Nov 09, 2019 9:22 am

Thanks for the answers.

I will try this one ... 83&sr=8-11.

Posts: 26
Joined: Mon Jan 14, 2019 6:29 pm

Re: Messure AC current

Sat Nov 09, 2019 2:38 pm

Bear in mind that will only tell you the current, not the power as the power factor is likely to be somewhat less than unity.

If you are happy with the process of re-flashing the firmware you might be better with a Sonoff POW ( or other places)) flashed with Tasmota firmware connected to a MQTT broker (eg mosquitto) on your pi. This can then report all consumption details (volts, current, power, apparent power, reactive power, power factor) on whatever schedule you wish, and post changes if required.

You will need to have the kit for reflashing the device (only needs to be done once) and be happy with wiring up mains power (**NOT** while reflashing) but will provide a more permanent safe solution.

Chris K

Return to “Other projects”