# A little help using this arduino AC current sensor / current transformer

#### zirconx

Joined Mar 10, 2010
130
I can easily measure the resistor on the board, now that I have the transformer removed. It's 1k.

I used my own 100R resistor. Then I get a sine wave. And about .8v RMS out for a 5A load.

If I switch to a 1k resistor, then I get the odd looking wave output again.

It acts like there is no resistor on the CT. (or a very high value)
I am using a different CT and getting a sign wave. 50 ohms and 100mV at 10A.
On a different type, 50 ohms, 1V @ 10A, seeing a good sign wave.
Yep, both the on-board resistor (1k) and having no resistor at all gives me the odd waveform. So does a 470R. 100R works. But only gives me .8v RMS output. Not much I can do with that except read it with an ADC. I'm going to see if I can rectify a higher voltage and then clamp it to 3v with a zener diode.

#### Attachments

• 48.7 KB Views: 2
Last edited:

#### MikeA

Joined Jan 20, 2013
260

#### zirconx

Joined Mar 10, 2010
130

#### MisterBill2

Joined Jan 23, 2018
9,308
A single diode half wave rectifier arrangement will charge up to the peak value and that may be adequate to work as a logic input. But I suggest adding a zener diode shunt any excess voltage as a protection for the input circuit. If the signal is spikey then the peak value is a lot more than the RMS value.

#### MikeA

Joined Jan 20, 2013
260
100 Ohms ÷ 500 turns × 5.3A = 1.06 AC * 0.707 = 0.75V AC RMS

0.8V AC RMS is pretty close to what math suggest it should be.

Now if you connect that to the ADS1115, you can actually monitor how many amps it is using since the ADS1115 has a resolution of 0.0078125mV.

You can actually tell if it's running or in a locked rotor situation.

#### MikeA

Joined Jan 20, 2013
260
But I suggest adding a zener diode shunt any excess voltage as a protection for the input circuit.
When I did some reasearch on using zeners for protection like that, from what I remember it "would not work" since currents are tiny. It takes some current to get the zener to properly clamp.

#### zirconx

Joined Mar 10, 2010
130
When I did some reasearch on using zeners for protection like that, from what I remember it "would not work" since currents are tiny. It takes some current to get the zener to properly clamp.
I came to this same conclusion also in my research.

So far in my experiments when I add the resistor & zener to the circuit, the voltage drops to almost nothing. From 4v or 5v DC (I'm using a 5k burden resistor to get the voltage this high) to less than 1v. I might be doing something wrong, or there may just not be enough current available to make it work.

Taking a step back, how might one design a simple circuit that could either ground a GPIO pin or supply 3.3v (from the 3.3v BUS most likely) to a GPIO pin, when current is flowing through the current transformer? Probably an op-amp to amplify the rectified voltage, then use the output to turn on a transistor? The transistor could ground a GPIO pin. Not that I'm planning on doing this, I have that other device ordered, but I'd like to learn. I might try turning on a transistor with the rectified output, but I don't think there will be enough power there to do it. A mosfet might work, since they are triggered by voltage rather than current? In a video I was watching on youtube, this guy's mosfet circuit was turning on just from touching the gate pin.

I have a couple transistors on hand (MPSA92, BC617, TIS-99), but no mosfets.

Last edited:

#### Sensacell

Joined Jun 19, 2012
2,893
You can always use a circuit like the one shown in this video- then it's easy to set the threshold.

Just substitute your CT for the washer thingie.

#### ronsimpson

Joined Oct 7, 2019
1,476
Different approach:
I have a well and my computer needs to know if there is pressure and volume.
>There is a float that signals if there is water in the well. If there is water then power can go to the pump and I use a simple cell phone charger to turn the 110V or 220Vac into 5Vdc for the computer.
>There is a pressure switch that turns the motor on/off at 30/50 pounds of pressure.
>Pressure switch This switch is different. It also does the on/off at pressure and it has a function where is the pressure drops below 10 pounds it opens up the switch. I do not power the pump with it. (it is good in that if it did power the motor it will kill the motor if no pressure because there is no water) My computer just monitors this switch. I have it set to open below 10 pounds and to open above 80 pounds. The computer can see if the water is in a good range of pressure.

I know this thread is about a sump pump and the pressure is likely very low and this 10 pound switch probably will not work.
The two points of interest 1) use a wall wort/cell charger to get 110/220 down to 5V for the computer. Put the charger across the motor to know when it is powered. 2) use a "low pressure" switch to see water.
Square D 9013FSG2J21M4 Well Tank Switch Low Pressure Cut-Off, 30-50 PSI

#### djsfantasi

Joined Apr 11, 2010
7,843
I can easily measure the resistor on the board, now that I have the transformer removed. It's 1k.

I used my own 100R resistor. Then I get a sine wave. And about .8v RMS out for a 5A load.

If I switch to a 1k resistor, then I get the odd looking wave output again.

Yep, both the on-board resistor (1k) and having no resistor at all gives me the odd waveform. So does a 470R. 100R works. But only gives me .8v RMS output.
But only gives me .8v RMS output. Not much I can do with that except read it with an ADC. I'm going to see if I can rectify a higher voltage and then clamp it to 3v with a zener diode.
You can use an op amp as a precision rectifier and set the gain to a larger range. This can then be read directly as a digital input.