Hey @Robert93820,
Good catch, this looks like an error as ‘(RL)’ is probably referring to the load resistance that the sensor will place on the circuit it is measuring. We will get this changed to read as
“Load resistance (RL): 372Ω”.
The sampling resistance should be around 0-5Ω based on similar devices.
Hope this helps! 
I see 220 ohm across sleeve and tip of the SEN-1105.
Am now quite puzzled by what I see on the CRO. Placing a 12 ohm load across the coil, I now see a sinewave… but it doesn’t seem very stable. My DSO (Hantek DSO5102B with old firmware, but that’s another story !!) reports the frequency jumping all over the place (like ranging between maybe 20Hz up to 200Hz !!). P-P voltage of 72mV. I don’t really believe the reported changes in frequency…I have a stable sinewave showing…
My early attempts to get a handle on this included (perhaps foolishly) putting a diode in series with the load resistor. So… with that in place, my trace shows huge spikes, no longer a sinewave, with P-P of 33V !! Some traces attached.
Quite frankly, I don’t really know what to make of all this. I’ve only recently started messing with this DSO… and microcontrollers. Is my old Hantek a piece of er… rubbish? I’ve tried playing with trigger settings, but still get weird pics.
But, I digress… I’m happy to park this for now, pending arrival of the SCT-013-010 device. That said, any comments on my (dodgy?) Hantek would be welcome.
Pics…
12 ohm load across sleeve and tip…
This is trace with the IN4004 diode in series with 12 ohm R
Thanks to all who are replying…
Cheers,
T.
Hi Samuel
Any reflected resistance will be in series with the circuit being measured and although 372Ω would reflect a low series resistance if it is indeed the load it would be worth investigation. The different marking on the device raised my suspicion plus the close value to the 250Ω coil resistance quoted for a higher current device. I would have expected a burden resistor to be in the vicinity 35 to 40Ω. If you leave these coils unloaded there is a very good chance there will be close to an open circuit reflected into the effective single turn primary so nothing would get any further.
I had a practical experience with this sort of thing some years ago. Please refer to my reply above in this subject from Sept.2023.
I think this is worth confirming and checking.
Cheers Bob
Hi Trevor
I stated above DO NOT put the diode in series with the load resistor. I said it was in the wrong place and I meant it.
The top trace:
I don’t know where the 25Hz comes from. The time is set at 8mS per division so the period is clearly 20mS (50 Hz) which it should be. At 20mV per division the 72.8mV P-P looks right.
The bottom trace:
The period is still 20mS so I think the 2.5kHz could be the ringing frequency observed at the trailing edge of the spike.
Did you not read the links. There are warnings about operating these CTs open circuit. It looks as if the diode has allowed a termination one way (the negative part of the sine wave) and blocked it the other way so removing the terminating or as the text says the burden resistor so leaving the coil unloaded. That is why the high voltage. The CT has probably saturated or something just leaving the spikes. This result is completely wrong so get rid of the diode whatever type it is and re read the information in the links before you do any permanent damage. Your scope only shows about 34V but if you used a higher bandwidth these spikes could be many hundreds of V.
Be careful.
Cheers Bob
Thanks Bob… I have removed the diode, and yes I did read the links. I’ve made some progress with my project, but as said, I’m waiting on delivery of the other CT device(s) before I pursue this aspect further.
My worry re my DSO stems from acknowledging the firmware is way out of date… and when I see a stable, latched waveform displayed, yet the reported frequency jumps all over the place… literally changing every second or so by large amount, it makes you wonder. Yes, if I zoom in a see ringing following those spikes, but I don’t think that is what the DSO is reporting. Anyway, I know this is really kinda off-topic.
I have other issues to address that seem more fundamental. I am a newcomer to this whole environment, and although I have a lifetime of experience in IT at seniou levels, I am quite perplexed at some aspects of the Thonny IDE. That is my task for today… try to get a grasp on it. I have lost code updates, as Thonny does not seem to keep files on chip and files on disk insynch as it claims. I make some simple changes (eg to Liam’s Homestation code, altering the delay), and yet the delay I see occurring seems hit and miss which value it uses. It can’t be random… but even deleting the flash version, re-uploading from disk… I still get the old behaviour. Very frustrating/confusing what is going on! There seems to be little detailed documentation on some aspects of Thonny… I’ve found some on Wiki etc… but am seriously considering biting the bullet and moving to VS Code instead. I use VS Pro a lot, doing C# programming… so VS Code learning curve should be minimal.
Any comments?
Appreciate your responses
T.
Hi trevor
I think it is. As a matter of fact I am sure it is. This will change as the amplitude goes in and out of what amplitude values the DSO will report and I reckon this is the variation you are seeing. That is one of the things that have to be considered when using an instrument of this nature. Learning what is relevant and what to discard as irrelevant… In this case the 20mS period (50Hz) is important as is amplitude. All the other information is only there to worry you as you have let it. like what use is there in knowing the rise time of a sine wave, measured at 10% and 90% of the amplitude. Same with peak (or P-P) voltages on a signal that is a bit noisy. The little noise spikes are included in this value and in most cases mean nothing as you might be actually putting the irregularities there.
Some scopes have filters to remove some of this if it is unimportant.
I have said the before but I think it is important to be aware of any instruments limitations and quirks and get a feeling of what they are telling you. This will help discard mentally anything which has no real importance in the real world and save a lot of hair pulling in the process.
You won’t or should not be using a diode in this position so this won’t matter.
As far as your Thonny problems are concerned I am sorry I don’t know much about this. I know enough to try and stay away… All this RPi stuff does not seem to have a lot of backward compatibility and interchangeability and I don’t see myself chasing my tail too much at my stage in life. Just think what a mess it would be if your Windows or Mac PC was like that.
Cheers Bob
Cheers Bob
Hey @Robert93820,
Thanks for catching this, much appreciated!
This product data is obtained directly from our supplier’s website, and a quick check confirms that it is still listed at the same value on their end. I have attached a screenshot of this data below.
This specific image is obtained from the 20A variant, though the 5A variant is the same.
We have no physical stock of the 5A variant for testing purposes, however we do have 20A versions in supply. As seen in the image, the resistance is labelled at 93 Ohms. A multimeter test of the output terminals gives a reading of 46.3 Ohms. We will continue to investigate this to ensure it is not a mistake, or at the very least clarify the purpose of this value on our website.
Once again, thanks for bringing this up!
Hi Zach
The coil resistance will be in parallel with the 93Ω. The specs on Seed Studio web site are delightfully vague aren’t they. The info re Arduino is the same page I found previously from somewhere else. The 46.3Ω measured looks suspiciously like half the 93Ω quoted. I would have thought the coil resistance would have been more than that. One of these things (I forget which one now and I am not doing another search) was quoted at coil resistance 250Ω and a turns ratio of 2000:1 (which I had calculated earlier) so I would have expected this one at 1800:1 to be about 225Ω assuming the same wire gauge. That would mean your measurement should have been about 66Ω. But one never knows and I don’t think you are going to be able to find out. The only check would be to see if you do in fact get a result of 1V @ 20A. Then is this RMS, Peak or P-P. Not stated.
Cheers Bob
EDIT. I think this would be RMS, the same as the primary.
Hi Zach
This 93Ω terminating (some call it burden) resistor might be right.
This one. 20A is 1V output. 93Ω
10A one. 10A is 1V output. 186Ω (932)
5A one. 5A is 1V output. 372Ω (1862)
This looks like these units are all exactly the same with a different terminating resistor. Chosen to provide 1V output at the secondary current obtained with the primary current printed on the outside of the case. As the current goes down the resistor goes up in a linear fashion to always produce 1V at the secondary at the stated current. Cheeky eh but it must save heaps in tooling when manufactured. The same turns ratio with the same wire will produce this. Just change the printing on the outside to suit.
Cheers Bob
Finally got my 5A and 10A rated sensors, SCT-013-010, from YHDC (sourced elsewhere… but Core did not seem to have what I wanted).
I have attached a trace for this device, which is behaving just fine… with NOTHING across the terminals These are designed to put out voltage - not current. The 5A version is best for my needs… a 750 Watt pump motor.
So, my remaining challenge is how to massage this signal, or altenatively, sense the AC signal on a pin on my Pico. I remind you that all I need is an ON/OFF status…nothing facy.
The trace shown is actually from a 1KW heater I’ve been using as a test device. I did measure the actual pump, the trace is similar, only slightly lower P-P… and not such a clean sinewave, but I figure if I can get things working on this trace… the actual pump should be fine.
Are you gentlemen able to advise on how best to detect this on a Pico GPIO pin?
I thought I could use a smoothed half-wave rectifier… but so far am still unable to get that working… which does my ego no good at all! Seems it should be simple. Sigh…
Thanks in advance,
T.
In that case the important bit is what the trace looks like in the other state (I presume that trace is the “ON” state).
A rectifier, smoothing capacitor and resistor should work to provide a DC voltage the Pi can read. A Zener diode to ground would ensure clipping at a suitable voltage. With the setup you tried what was the trace for the signal at the Pi input, and how are you processing that input?
When the pump is OFF… the trace should be fairly close to zero… only noise. I have not yet writtten the code (or wired it up) to the Pico… doing that right now!
But really… if, as I expect, it shows maybe a few millivolts when off, and a decent number of millivolts when on… it should be trivial. And this is all really sloooow… timing is not at all critical.
I have an old diode (1N4004 ??), maybe even a zener, and a random assortment of capacitors, but initial attempts putting the diode and resistor across the leads ended in disappointment. I gave up at 2AM last night ;-( It’s been about 50 years since I played with stuff like this… I may have done something stupid… but seriously, this should NOT be hard… right?
BTW… what value capacitor would be about right? And R… if Indeed I need to make a little RC thing.
Happy to show you another trace, when I get it all hooked up again… maybe later today/tonight.
Thankyou for your response!
T.
It should, but considering that the last test didn’t work then that would be the first thing to confirm.
You haven’t indicated what you mean by “didn’t work”. Note that half-wave rectification won’t give you a useable analog voltage: you would need full-wave rectification and plenty of capacitor smoothing to get that. Half wave rectification can be used if you are reading the signal as digital and using triggering, or counting pulses over a period that would cover multiple cycles. That’s why the whole setup needs to be considered - the type of input to the Pi and the way you are reading it needs to match the output you are creating. The isolating capacitor can be small - tens of nF would work - while the smoothing capacitor would higher, in the uF range. None of the values is critical.
OK… some progress… some failure.
Got things looking good with my bench setup… using a blow heater to simulate pump motor. (I changed diode, amongst other issues)
Time for the real test… take it all outside and connect to my pressure pump. Dang… Seems to be on the edge of working. The simewave on the pump motor is less than perfect, after rectification, there just isn’t enough grunt to charge the cap, I’m seeing a trace on the CRO that sort of bounces between the OFF state (about 20mV mean) and ON… which SOMETIMES reads about 40mV.
I’m wondering if going full bridge might be the answer. I’ll get two diodes worth of voltage drop… but double the time charging the cap. Hoping that will bump up the mean measured voltage enough to be distinct from the OFF state.
PS My bench setup works great if I turn the heater on High… ie double the current. It works more or less on Low… but my pump draws even less current, hence my belief that I am on the edge of viability.
Tomorrow… need to buy more bits to test a full bridge rectifier.
Another PS… Counting the pulses interests me - as I have another pump to monitor also. Can you point me in the direction of how to code that on my Pico? I am not that experienced in microcontroller stuff… you may have noticed 
Hi Trevor,
Progress is still progress. It sounds like you’re nearly there and you have ideas on how to get yourself over the finish line.
As for the Pico, we have a wonderful free course put together that you may want to look at. It goes over the basics onwards to program a Pico. Chapters 2, 3, and 4 will show you how to read, interpret, and save data. Have a look and see what you think.
Looking forward to some more updates soon.
Thanks Jack. I am indeed getting closer to the finish line (it would help if I didn’t keep changing things… like adding in radio TX/RX… but that should be straightforward.
I just discovered I’ve made a basic mistake… somehow I moved the attenuation slider on my CRO probe to 1X not 10X… so all my measurements (and the trace above) are 10 times their actual value. I started out with it on 10X… so was OK. Now… not! So this means I really only have a 300 mV signal to work with… and that isn’t going to get rectified with a common diode, is it!
That means, I believe (tell me if I’m wrong) I really need to either:
Having only just started this journey, I’m not too sure if putting an AC voltage onto a GPIO pin is kosher… the basic guides about reading analogue inputs usually don’t talk about that… they assume a positive voltage.
I did read (and watch) a bit on using the PIO system, but that seems overkill. Something that counts zero crossings could work, but again, my naivity means slow progress! Can I set up an interupt handler that triggers on a pin going positive? If so, I reckon I could do it. Will I blow up my Pico if I put an AC signal on a pin? Dunno. Maybe I missed that bit in the documentation!!
An op amp could both shift and amplify my signal… so another option might to amplify, then rectify and smooth, get a simple positive DC voltage. I’ll try this too… when my next package of goodies from Core arrives
Anyway, that’s the direction I think I need to go… unless you advise me otherwise!
Thankyou for your patiebce with a new guy, who is no doubt making all the newbie mistakes possible ;-(
I’d do a mix of both. Use a summing op amp to bump the voltage out of the negatives and feed it into the analogue pins on a Pico. I’d also clamp the voltage with a Zener diode to be extra safe.
Hi Trevor
Firstly you mentioned earlier about some sort of residual signal present with the pump OFF.
Question: Are you switching the pump with a SSR (Solis State Relay)? If so this would be normal. A SSR has a “Snubber” circuit built in which is a capacitor and resistor connected in series ACROSS what would be called the relay “Contacts”. There will always be some leakage via this path and if you try to switch a very light load (in the mA range) with these they will appear to never switch off. This leakage is enough th keep a light load running. The best way to make sure everything is OFF is with unencumbered mechanical relay contacts.
I don’t quite know if this is even possible
I think this is what you will need to do. But to amplify AC you need a split supply. You can use a single supply if you level shift the AC zero up to half Vcc. The 3 links I suggested in an earlier reply on the Core web site will help here. The middle one on interfacing to Arduino goes into level shifting in detail. Please read.
You might have to consider rectifying to get a DC signal. If so you will have to amplify as a diode will need about 700mV just to turn on. If you use a bridge as you mentioned this means about 1.4V as you are aware. You can only use a bridge with isolated AC like a transformer secondary or similar. That is the voltage swings + and - WRT Ground. Once you level shift the AC voltage becomes a varying DC voltage WRT ground and a bridge will have ground conflicts. Not a good thing.
Rectification will require a capacitor being charged to peak value via a diode. Be it full or half wave this will involve charging current which has to be supplied by the source, in this case your sensor. As the capacitor is a short to ground at start up this inrush could be significant. A series resistor before the cap is used as a filter and will limit this current but I am not sure if the sensor is capable of supplying this. In this case an op amp will be needed with a high input Z and low output Z to isolate the sensor and supply this charging current.
Whatever you choose to do you will need significantly more that your measured 400mV peak (corrected to allow for probe setting mistake) which will mean amplification of some description.
Cheers Bob
Hey Bob… thanks for this.
I’ve made progress… but my kit is now someone disassembled…i am desperately trying to lock things down, but am spread a bit thin. Not enough hours in a day (and night)
The significant change since my last post: I have indeed massaged the signal from the 5A CT sensor using 1/4 of a quad op amp (HC17902… simply because thats all I had lying around). Put a trimpot in-circuit so I can tweak the gain… and after putting an electrolytic across the output, I indeed have a DC signal that clearly transitions between two different positive voltages, depending on pump on/off state. Woohoo. Should be trivial from here on. That said, I will go back and review the links you sent. I have so many tabs open its not funny…over 60! Sheesh…
In my “ïdle” time, I’ve been teaching myself how to use fritzing, and am well advanced in documenting the circuits, breadboard etc. Planning on replacing the breadboard with a Makers project board, as the breadboard connections are just too flakey. Who knows … might even go PCB once everything settles down!
Next challenge is to read input from my main pump… which I am switching on/off via an opto-isolated mains relay, driven from a GPIO pin. All works just fine, my remainng challenge is reading the opto-isolated relay-is-activated detect signal. I think I am almost there… probably just need to use a pull-up on the input pin. Am trying to master the technique of detecting zero crossings on a pin… and use interrupts, but I suspect I won’t need such fancy stuff when I hook it all up again… with a pull-up. Once again… all I want is pump ON/OFF status confirmation… nothing more. This relay unit is currently connected via wires - it’s several meters from my Pico… I have a couple RF boards on order to go wireless… maybe next week.
This project has been a real learning exercise, forcing me to dust off my 55+ years electronics skills (or… lack of!), learn about Pico, Python, microcontrollers generally, fritzing (I did wonder about using LTSpice… but fritzing seems OK for what I need).
Along the way, I made some huge improvements in the ability of the laser distance sensor (VL53L1X) to detect water levels… I plan to write it all up when I’m done, maybe it will help someone else.
Many thanks for your ongoing advice!
T.
Hi Trevor
No problem
Just interested. What is a “HC17902”. I have tries all sorts of combinations but can’t find anything. The 74HC series seem to be flip flops, Schmidt Trigger buffers and most other things digital, no mention of op amps. Tried juggling the rest around in case of a typo but no results.
Cheers Bob
And you can get our latest projects and tips straight away by following us on:
