Inductive current measuring using Raspberry Pi

back in may i experimented with using temperature sensors like this taped to the house wiring to see if various high current appliances are turned on and off and that works as long as the thing has been on for a few minutes and it's drawing at least four amperes otherwise the temperature difference isn't enough so i had the idea of just using a little wire loop like this tape to the wire and inductively pick up any current that's running through the wire so with this space heater drawing 11 amperes through this wire and when i hold that loop around the wire i get a signal but it's a pretty noisy one and unfortunately it blinks a lot because of the way the camera works so looking at romex wire we have the two current carrying conductors on the outside plus a ground in the middle and the way this works is the current traveling through these two conductors cause a magnetic field and the conductors of my sense coil being along the wire or in cross section here will intersect some of these flux lines and basically induce a small voltage for any change in current in here so this works a bit like an electrical transformer but very inefficient but that's okay i just want a signal but with only five turns on this short coil my signal is very weak so i made another coil a little bit longer and it's got 10 turns on it so this one picks up a fairly stronger signal but it's still an awful signal because when you look at this this is just high frequency noise the 60 hertz component is actually much weaker and although mains power is a 60 hertz sine wave there's a lot of higher frequency components on top mostly from lots of electronics and whatever other things that switch on and off and of course those higher frequency components have a much higher rate of change and so the sense coil picks up that noise much better than the signal itself and that's because a coil like that only picks up changes in the magnetic field so essentially i'm picking up the rate of change of magnetic field so i figure no problem i'll just connect a small condenser across the coil to dampen some of the high frequency components but if you look at the scope this is without and this is with without width so the condenser positively makes things worse and then i realized of course it makes it worse because with the inductance of my sense coil plus the condenser in parallel to it that turns into a resonant circuit and so that just resonates with some of the high frequency components so now with the condenser across the scope probes but a resistor in series with the sense coil that prevents any resonance and also acts as a filter and i get this nice sine wave whereas if i bypass the resistor i get this mess but i also found i was capacitively picking up some of the voltage in here and that was distorting my signal a little bit so i tried a piece of tin foil between the coil and the wire and that cleaned it up quite a lot but it's a bit awkward so then i made a sense coil this one with 20 turns of magnet wire and i wrapped that in tape then wrapped it in tin foil and then wrapped it in tape again to make a shielded sense coil and here's my shielded sense coil taped to the wire and this coil gives me a much stronger signal so right now i've got this hooked up to my cheap digital oscilloscope on the laptop computer and you can see the signal is not very big and now with my 20 turn sense coil i get a stronger signal but this is turned up as far as i can turn it this analog scope is just a lot quicker and more sensitive and it doesn't get these little glitches like that so the analog scope is much better for this sort of thing it just doesn't show up very well on video and now without the resistor just the condenser on the sense coil you can clearly see how it resonates and now with it paused zoomed in and two cursors placed it tells me i have a 1.72 kilohertz uh resonance on here which is just curiosity because the resistor gets rid of the resonance and i really don't want that there because that screws up any a d readings and of course there's these uh clamp on current transformers that do pretty much what this does but a much better job much more sensitive all i have to do is clamp it around one of the conductors and it does a really good job if i put it around both conductors like this they just cancel each other out and this thing picks up nothing all i have to do is split the wire but that's exactly what i'm trying to avoid now to get the readings into the computer i ordered one of these ads 1115 edc converters 16 bits with a programmable gain converter very useful and i hooked it up to a raspberry pi to get the readings and here's my condenser and resistor to filter this thing and i also have a similar setup of resistors and condensers actually capacitors here for the power supply for this thing because it turns out i was getting a lot of noise on the 80d converter depending on what i was doing on the raspberry pi because the power supply on this thing is very noisy when it's doing stuff and i realized that the noise was not the afd converter but the pi when i saw that depending on what else i had running on the pi the noisiness of the a d readings got better or worse and i wrote a whole bunch of code to read the entity converter clean up the signal and graph the result so here it's taking several readings per second and if i turn on the space heater to low i get a reading next up next up so that's about 11 and a half amperes right there but it can pick up smaller currents too like turning on the 60 watt light it shows it clearly right here i can safely detect currents down to about 100 milliamperes because for every ampere going through the wire my sense coil puts out about one millivolt worth of voltage and with the gain on here cranked up to max and averaging a whole lot of readings correlating to a sine wave i get a pretty decent signal but it turns out this a d converter is only made out of the finest chinese and instead of actually being 16 bit like it says right on here it's only 12 bits it's not actually an ads 1115 it's some kind of chinese imitation that is roughly compatible so for about 10 times the price i ordered a proper ads 1115 from adafruit and this is a genuine part and that one truly is a 16-bit part it's the real thing but i don't actually get any more sensitivity because by sampling a lot of times and correlating with a sine wave i can make up for the missing bits of resolution and the chinese part is actually a little bit faster which means i can get more samples out of it i also realized i could get more sensitivity by putting this metal sleeve around my coil like this adding this iron sleeve gives me more signal because it's about a thousand times more magnetically permeable than the air is and that means there's less reluctance to the magnetic flux which means there's more magnetic flux still has to go through the air in the middle here though but that means more magnetic flux goes through between the wires of my sense coil and that gives me more signal and that's why all power transformers use some kind of iron material for the core so that the magnetic flux can travel entirely within the core there's no air gap it has to go through so this is with the heater on medium calibration of course is wrong now and if i remove that sleeve the signal becomes much weaker but anytime you add some magnetically permeable material that material has hysteresis and a saturation point and that can affect the linearity so i'm not sure if it's a good idea and now with the sensitivity cranked up so more and averaging more samples i can actually detect when i'm plugging the ipad into a charger this uses more current and we can see there's more here than there was before now unplug it again and you can see it's gone down again a noisy signal but to be able to pick up inductively the current flowing through this wire from charging up an ipad i'm pretty pleased with that of course depending on how the coil is on here and whether i have the shield the gain changes and i keep calibrating that against my kilowatt meter by changing some constants in the code but unlike the kill a watt meter this can read much faster and it can also read much higher currents so i want to know how much current does this thickness planer use the instant that starts this thing is useless but this can capture it so this is now reading 30 times per second and look at this graph the current ends up settling at around 12 amperes but the instant that turn it on it's surged at over 70. that just seems unbelievable and it took me quite a while to trust those readings but if i plug in three space heaters into that same circuit it reads that correctly and of course that's also way overloading the circuit but the breaker is there to protect the wire so it won't actually pop until the current has been high enough for the wire to get warm but of course if you have a dead short on the line the breaker will pop right away but i don't know what current that is for the breaker to pop instantly now if i measure the dc resistance of this planer at the plug and turn it on that reads 1.0 or 1.1 ohms and the probes touched together is already 0.1 0.2 ohms which is to say this planer's dc resistance is less than 1 ohm that would work out to 120 amperes if it was dc but still it makes the 70 odd amperes of surge current more plausible that sauce surges at just 57 amperes now i'm curious if i plug both of those in at the same time while they're turned on will that pop the breaker both uh turn on and on this thing here if i plug those in together will that pop the breaker let's scroll back in here i peaked out at 108 amperes man those breakers are tough now the current was less than the sum of the individual tools because when i do that the voltage drops a lot and that cuts down on the current too next test three things circular saw planer plus a bandsaw [Music] that's surged at 118 amperes just what does it take to pop a breaker okay this band saw this bandsaw planar circular saw all right that's surged to 137 amperes but it wasn't the main breaker it was the breaker in this thing that popped all right without the power bar try again reading was 145 amperes this is getting ridiculous now i've got the jointer bandsaw bandsaw planar circular saw okay here goes finally pop that breaker and on here i'm only getting 47 amperes and that's because i averaged over two full cycles or about 30 milliseconds and the breaker popped before that and that was just a 15 amp breaker and having done all this experimentation with the coil around the wire i realized the current transformer can be used without splitting the wire if you just kind of clamp it so that the poles kind of between the wires and that is actually way more sensitive than my coil so i've got my ipad charger plugged in here and going continuous readings at a much smaller scale plugging in the ipad and you can see a very clean signal of the current going up this is about 120 milliamps of current so i didn't need to go through all that but the coil can still be useful for measuring very large currents or if you don't want to buy one of those clamp on current transformers you

Channel: Matthias random stuff

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Id: P47pjVyPP3w

Length: 12min 53sec (773 seconds)

Published: Thu Sep 09 2021