How to read voltage from SCT-013-030

Thread Starter

beckmx

Joined Sep 18, 2021
8
I have a big doubt, I have tried to read voltage from a SCT-013-030 sensor with an esp32, ports 34 and 35, I created an offset voltage with a simple voltage divider and a capacitor (I have tried different values, 4.7uF, 10uF, 100uF), and I have noticed that at "low" current, around 9Amps, the readings look to be okay, however when the current increases, the readings seem to drop like if something related with the capacitor(just a guess) is cutting the frequency I am reading.
This is my circuit
sensor_current.png

and this is the behaviour, it starts reading the increasing Amps, but then it drops
IMG_0059.jpg
 

ericgibbs

Joined Jan 29, 2010
14,157
hi beck,
Welcome to AAC.

I see the sensor is rated for 30A = 1Vout.

Have you used an oscilloscope to check the actual voltage across the output terminals.?
I do not see a Vertical voltage scale on the image,?

E
ESP_ 811 Sep. 18 09.06.png
 

Thread Starter

beckmx

Joined Sep 18, 2021
8
@ericgibbs that graph is from my conversion to watts from the sensor output, in any case i attached an osciloscope directly to the sensor (i have 8) i tested all, and they all behave like this: the initial value is way too low, almost zero, then when i start to send power through the cable it is reading it increases correctly and then when it gets to .48v or .5v the direct output of the sensor drops to .3v and stays there.

so i disassambled the sensor and i see it has a 60ohm resistor and a capacitor, i suspect this might be the problem.

@ronsimpson if you see the graph there is a continuos growth in the beggining and then a drop, that drop shouldnt happen it should continue growing
 

ericgibbs

Joined Jan 29, 2010
14,157
hi beck.
Does the datasheet for the sensor specify a recommended minimum load impedance.?
It is possible your divider is too low an impedance.

E
 

Thread Starter

beckmx

Joined Sep 18, 2021
8
@ericgibbs well, the sensor by itself cuts off the voltage, in my test with the scope I didn’t connect my circuit and it drops its voltage, what I did notice was different is that when the sensor starts reading, the frequency is around 59.x hz and 60.x hz, when it starts reporting less voltage (by less voltage I mean not following the spec) the frequency gets a little crazy with values from 70 hz to 110hz when this happens the vmax stays around 0.3V. In the image I am attaching the sensor should report .6v the double, I have seen the same behavior measuring both a washing machine and a AC system, I am attaching an oscilloscope picture taken when the issue happens
 

Attachments

ronsimpson

Joined Oct 7, 2019
1,640
Many appliances do not have a sine wave of current. A heating element is good but a PC or a motor are not a clean loads.
1632007161689.png
Some things like a PC will draw power like the green trace.
 

Thread Starter

beckmx

Joined Sep 18, 2021
8
@ronsimpson i don’t know man, it is too much of a coincidence that it only happens (the voltage drop) when reaching 10Amps, also the Vpp at this current is 1v, which is what the manufacturer indicates but for 30A
 

Thread Starter

beckmx

Joined Sep 18, 2021
8
@ronsimpson well i read in another forum, the other element inside the sensor (it has a little pcb with a resistor and something that looks like a capacitor) it is a zener diode, my guess is that zener diode is being trigger at 1v and cuts off the voltage
 
I use some of those sensors to measure my house power consuption and solar power production. Designed a circuit and then with esphome and homeassistant magic happens. :)
You have to make sure that the current you are measuring stays below 30 A peak (21.2 A rms). Also, the 1V is peak, so 0.7 V peak.
Use a resistive heater. Then, use emonlib to calculate voltage, current, power, etc. There are plenty of examples in youtube. :)
 

Thread Starter

beckmx

Joined Sep 18, 2021
8
@fabiojdsoliveira very nice info, actually I already have my pcb and everything running online, what is strange is that I have another expensive device that measures the power consumption and I don’t exceed the amps not even in peak, so would you recommend to move to a higher sensor?
 
@fabiojdsoliveira very nice info, actually I already have my pcb and everything running online, what is strange is that I have another expensive device that measures the power consumption and I don’t exceed the amps not even in peak, so would you recommend to move to a higher sensor?
Without a circuit to analyse I can not help you anymore. Also, you should be testing with a resistive load. :)
I would not recommend moving to a higher sensor because if you are sure that you are not above your current sensor specification, the problem may not be the current sensor.
Another thing, I have never worked with ESP32's ADC because it seems to be pretty bad. Nonetheless, are you sure you are not putting an overvoltage in the ADC pin? Do not forget to add the 3.3V/2=1.65V + 1Vp = 2.65V. Does the ADC tolerates this voltage level?
 

Thread Starter

beckmx

Joined Sep 18, 2021
8
@fabiojdsoliveira from your comment I suspect you are using the sensor WITHOUT the burden resistor, are you?, because my sensor contains already a resistor inside 60ohms, so it is delivering voltage already, and the voltage drop is ocurring in the output of the sensor, without any circuit attached, I have seen the same behaviour on the rest of the sensors I bought
 
I use a sensot already with a resistor inside, 1 V output. Have you tried to desolder the TVS? THe other component besides the resistor?
I took another look at the image you shared.
It says 1800W. If you are in the US, for 120VAC -> 1800W/120VAC = 15 Arms. 15 Arms = 21 Apeak.
The problem is that the first peak is well above 1800W which means the peak current must be well above 30 A peak, which would explain the activation of TVS. You are using the sensor outside of its range. :)
 

Thread Starter

beckmx

Joined Sep 18, 2021
8
@fabiojdsoliveira actually is not 1800w, in the image, 1800w is the top of the image not the top of my graph, the most watts I can read are 1100 (where my wattage makes the first drop), however as you said, my device sometimes increases around 1600w, but I stop reading from 1100w. I guess I will get the 100A version, your responses have been great :)
 
Top