Makes me wonder if I would be able to detect a power off condition.
Now making up a simple circuit hooking up the current module in series (ie I’ve got a short extension cord I’ve cut one line and hooked it up to the current module) using my cheap variable power soldering iron which I’ve had on my digital meter and at full power uses 0.5 Amps (the iron does come on and gets hot so my circuit is passing power). So at full power I see:
Socket 4: -0.422 (ie same as the zero case above)
Socket 2: -0.395 (ie same as the zero case above)
Socket 3: -0.411 (ie same as the zero case above)
Basically the readings just bounce around so I let them go for 39 readings and the average was -0.3898, min = -0.3985, max = -0.3814, this was for a zero state doing it again for a power situation avg = -0.3812, min = -4038, max = -0.3673
I tried a test where I read the module as DC ( current_ACS712.Read_DC_Current() ) in a zero power condition and the results were:
avg = 0.4007, min = 0.3871, max = 0.4192 (note these are positive values unlike above where they are all negative).
Other module tested with similar results.
Any suggestions as these aren’t the readings I was expecting.
I pretty much punted this module through the goal posts of life for 2 reasons, first it never did seem to work in the application I was trying to use it in and second when looking at the chip it is a:
ACS712T ELC-05B
and I believe the 05B means its designed for +/-5A range which is below what I was needing it for. A ACS712T ELC-20A would have likely worked better in my application.
As per the datasheet, the ACS712 “provides economical and precise solutions for AC or DC current sensing in industrial, commercial, and communications systems”. However, for AC current sensing, it will give you instant reading. I’m not 100% sure on that, as I’m waiting for my DSO to be shipped, and when I get it, I will be able to provide more information.
In the meantime, I have found several PDF files with information about current sensing. There are several methods, even micro-controllers that will do all the math for you and provide power measurements.
For my application, as I do not need extremely precise measurements, I am trying to go for a RMS-to-DC Converter, more precisely, an Analog Devices AD736. The chips are also being shipped, but they come from much closer and I hope to get them by the end of the week, and hopefully play a little during the weekend.
… can’t upload PDFs, so here’s the list of some useful parts/files/application notes that I found.
AD736 - True RMS-to-DC Converter
78M6610 - Energy measurement Processor
CRD5490-Z - Cirrus Logic Power monitor
CS5490 - Cirrus Logic Two-channel energy measurement
Current Sense Circuit Collection (by Linear Technology, application note 105) *** by far the most interesting read
Current Sensing Circuit Concepts and Fundamentals (by Microchip, application note AN1332)
It looks like the LTC1966 Precision Micropower ΔΣ RMS-to-DC Converter by Linear Technology is one of the mose precise chips available, but it’s not available for me to purchase easily, so I went for the AD736; I plan to test with the schematic if figure 26 on page 15 of the datasheet.
Ok, so here it goes; a few data based on my first tests (I was unable to save a hard copy of the wave from the DSO, but hopefully I can find a way to do that too).
Using a classic light bulb as load, I connected the ACS712 to 5V DC. With the load disconnected, the output is at 2.5V DC, as explained in the datasheet.
With the load connected, the output is a sine wave at 50Hz, in line with the mains.
So, the ACS712 will give instantaneous current reading; for AC, I will try to use an AD736 along with an AO to allow for low current measurements.