home project to non invasively measure current - Page 1
Forum section: electronics / projects

author: msknight
author_rank: Regular Contributor
timestamp: February 24, 2025, 10:02:19 pm
content: I've been going round in circles. I believe it is possible to determine whether current is flowing through an AC cable without having to cut any insulation, but I want to do this without cutting the outer sheath of the cable either... so that the live, neutral and earth are still bound together in the cable.My aim is to have a 5v DC current reflecting the strength of the flow, to a small ESP32 board which will transmit the result back to home assistant... but my ultimate goal is not really accuracy... I just want to know whether there is power flowing or not... to detect a power cut in a cable.Obviously I've tried a clamp meter but AC current, when neutral and live are alongside each other, doesn't work.I know that there are non-invasive sensor, "pens," out there so this should be possible. But no matter how hard I search on the internet, I'm just finding videos and instructions which have, as their first step... separating the live from the neutral...Grateful for any thoughts and advice please.

author: Benta
author_rank: Super Contributor
timestamp: February 24, 2025, 10:06:16 pm
content: Good luck with defying physics.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: February 24, 2025, 10:11:38 pm
content: Did you try linear hall effect parts like DRV5057 ?The wires are not in the same physical location, so a probe with maybe 4 (or more) of those sensors around the cable circumference, may pick up enough differential signal from thenearest wire.It would need to be locked in place.Addit :AI says thisA miniature linear Hall sensor used for current sensing in a wire measures the magnetic field generated by the current flowing through the wire, and outputs a voltage proportional to the magnetic field strength, which can then be converted to the corresponding current value using the equation: V_out = K * B = K * (μ₀ * I / 2πr), where V_out is the output voltage, K is the sensor's sensitivity constant, B is the magnetic field strength, μ₀ is the permeability of free space, I is the current in the wire, and r is the distance from the wire to the sensor's sensing element.so it looks like a simple inverse distance plot, where the nearest wire will dominate.It will depend on the cable moulding, but a 2:1 distance will give a 2:1 field ratio, so you measure 50% of the current. Easy to calibrate, as it is fixed.The analog version DRV5055 mentions a 20kHz bandwidth, the digital PWM version DRV5057 says 2kHz carrier, so that might sense 50-100Hz ok (40 samples in 20ms)

author: Zero999
author_rank: Super Contributor
timestamp: February 24, 2025, 10:20:44 pm
content: Do you want to detect whether current is actually flowing or if there's a voltage present?As you've discovered, it's not easy to detect whether current is flowing, because the magnetic fields generated from the live and neutral conductors cancel one another out.Voltage can be detected using capacitive sensing, which is how those pens work. You just need an amplifier with a high input impedance. You could try a J-FET such as the J112.

author: Geoff-AU
author_rank: Frequent Contributor
timestamp: February 24, 2025, 10:32:16 pm
content: As PCB.Wiz said, hall effect sensor placed closer to one conductor than the others. The opposing currents won't cancel out equally because the distances are different.

author: ledtester
author_rank: Super Contributor
timestamp: February 24, 2025, 10:41:41 pm
content: Can you use a line splitter, eg.:https://www.amazon.co.uk/Kethvoz-Conductor-Dismantling-Multimeters-Accessory/dp/B0C6JST6BQIt allows you to use a clamp meter without modifying the cord.

author: Someone
author_rank: Super Contributor
timestamp: February 24, 2025, 11:12:46 pm
content: Quote from: Benta on February 24, 2025, 10:06:16 pmWell done having no imagination:https://docs.rs-online.com/99fc/0900766b8053ce85.pdfMMC850Quote from: msknight on February 24, 2025, 10:02:19 pmWell you can in theory do it the way you propose but the cost would be off into "are you really really sure that is they way you need to do it?" sort of range. Or to put it another way "sure we can do that for you, please put down a six figure deposit". 99.9999% likelyhood there is some other routine/common way to solve the actual problem that electricians already use in your locale.

author: msknight
author_rank: Regular Contributor
timestamp: February 25, 2025, 06:45:03 am
content: Quote from: Zero999 on February 24, 2025, 10:20:44 pmI actually need to do both.My first position would be as the input to my UPS. In event of a power cut, there would be nothing to the UPS input. In that case it's just if there is voltage present.My second position is to a heating element with an internal thermostat. So voltage will be present but I need to know when it's demanding current.The power line splitter... good idea. I didn't know they existed. That's a safe way to do it.There is an experience electrician in my area who I've started talking with. I'll put these past him and see what he says.In case you're asking why don't I just ask the UPS, I did connect it to USB on the raspberry Pi that is running home assistant, but there is a spike of EMF that wipes out the Pi even with a suppressor at either end of the USB cable. And it's not a cheap UPS either, it's a £500+ sine wave 1600 Schneider Electric unit. Biggest I could get for domestic UK use. As the Pi is running the automation, I can't risk the UPS throwing it out.

author: Smokey
author_rank: Super Contributor
timestamp: February 25, 2025, 08:25:28 am
content: USB isolator?

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 25, 2025, 08:32:46 am
content: I would also try Hall effect sensors. Possibly several on a circle around the cable, so that whatever the orientation of the conductors inside the cable, at least one of the sensors should pick up something.

author: tggzzz
author_rank: Super Contributor
timestamp: February 25, 2025, 09:19:05 am
content: If it is a sinewave inverter, you might want to check the current from the batteries when there is no load. It might be higher than you would hope. Alternatively just leave the UPS running, and see how long the batteries last.

author: msknight
author_rank: Regular Contributor
timestamp: February 25, 2025, 09:38:37 am
content: Thanks.I'll try all that with the UPS.That only leaves the element which I need to tell when it's running. I've had no joy finding a 13a cable splitter... I'm obviously using the wrong search terms as I'm finding literally split cables that power two devices off the one plug. I've also contacted the manufacturers of the 13a fused connection unit and they have no plans to introduce a unit that reports back the current being drawn. So I'm up against a wall on that one, reduced to monitoring the temperature around the element to see when the temperature is rising.I'll look up the hall effect sensors to see what I can achieve with them.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: February 25, 2025, 10:02:40 am
content: Quote from: msknight on February 25, 2025, 06:45:03 ami'd be looking for thefaultconditioncausing that spike. start off with a multimeter set to AC volts and connected between the shell of the USB socket on the UPS and the shell of the USB connector on the RPi. then check for a DC voltage difference. if the RPi is being powered from a 230vAC outlet on the UPS, i'd also be ensuring there is continuity between MAINS ground and OUTPUT ground on the UPS. i am sure others can suggest further checks to perform to better understand what is going on.cheers,rob :-)

author: PGPG
author_rank: Frequent Contributor
timestamp: February 25, 2025, 10:41:44 am
content: Quote from: SiliconWizard on February 25, 2025, 08:32:46 amMe too.But I would certainly be trying to use only one and find around the cable the right position.