ADE7763 and ACS712(30A) energy monitoring current sensing error without opamp   - Page 1
Forum section: electronics / projects

author: getfast_kiran
author_rank: Regular Contributor
timestamp: July 21, 2017, 05:03:11 am
content: Hi,I have been struggling this for a month.I wanted to find current consumed through a Relay for a circuit that is supposed to accomodate load of 25A. Also here the dc offset removal has to be done with capacitor and not opamp as it requires a -ve rail for reference the current that goes to zero which our current smps is not build for to reduce the manufactuing complexity.The IC's used for this is an ADE7763 along with ACS712(30A version).ADE7763 ADC gain is used at 1.I made the PCB and tested the ACS712.Problem is : I tried three loads to start with a) Two resistive loads 100W & 40W bulbb) 1 Pedastal Fan with 3 speed control - Helps me with both harmonics and various power factor at different speeds(it even reaches to 0.47PF)max current consumption around 0.4A at 230V(rms)I first calibrated my circuit for Pedastal at 3 speed control for near to 0.03 error in PF and current error of 10mA and almost 0.5V error in Voltage measurement against a Tektronix Power Analyzer and then tested with an another product in the market.Then when I tried the resistive loads ie; 100watt and 40Watt incandescent bulb, the result gave was so out of range. Here if I somehow managed to calibrate it...then..I end up having non-consistent error. Either it started increasing in interval as time goes by or it will decrease with time...I had used this before directly connected to micro-controller to measure the current for feedback for random applications. But this specific application needed a specific IC for energy measurementlike ADE7763. I tried ADE 7763 along with Triac to measure upto 8A and had luck even at 17A reading by using an dynamic calibration using curve fitting method for resistive load.But due to problem with high current heating in Triac and my space constraint. I had to switch to Relay as switching element.That is when the problem started. I understand the magnetizing field of relay and switching noises from SMPS near might affect the the Hall Effect. So took relay out and shorted it and then replaced the SMPS with an external adapter to check it and still I had the same error. Can anyone point me to the issue that will cause the calibration problem in ADE7763 integrated with ACS712.Following are the schematic and PCB of the circuit that is related to the case. Please take a look and suggest the changes and correction that needs to be made to make it working again...

author: getfast_kiran
author_rank: Regular Contributor
timestamp: July 21, 2017, 05:09:39 am
content: I would also like to add that Hall effect current sensor was used because of space restraint. Also the ADE application note say we can use a hall effect sensor for the same byjust using a simple dc blocking capacitor to remove the offset.

author: capt bullshot
author_rank: Super Contributor
timestamp: July 21, 2017, 06:12:15 am
content: The problem is: any kind of open loop hall sensor isn't suitable for this purpose. They cannot be used to reliably measure light loads (your 40W lamp is about 1% of the full scale 25A), you need a better sensor for this.Open loop sensors are highly sensitive to all kind of environment changes (temperature, magnetic fields, EMC) influencing offset and gain. You rule the offset out by using the coupling capacitor, but you cannot compensate for the gain changes and nonlinearity at small currents.Consider using a proper shunt, a 3 mOhm shunt gives about 40A peak capability (to fulfil 25Arms) with the ADE 0.125V range, having a 1:1000 dynamic range with good accuracy, which is way much better than anything an open loop hall sensor can deliver, at 1.8W disspation at 25A, if this is too large, you can still lower the shunts value without sacrifying too much accuracy.

author: getfast_kiran
author_rank: Regular Contributor
timestamp: July 21, 2017, 06:42:59 am
content: Thank so much for the reply and information about the sensitivity issues.Thank for verifying the coupling capacitor ability to rule offset off as a cause. I was quite confused about it. Since sometimes while removing offset, Voltage at the output of capacitor would sometime go below zero.I am assuming it might not be a problem as the ADE has differential input to read current. Do you think this might be a problem. If so tell me if I could correct it.I wanted to use the shunt method, But I am worried about the isolation problem cause I am also going to remove the isolation problem in voltage sensing using another method so if I use current sensing using shunt method again I will have to find a way to isolate it. Otherwise I would have liked to go ahead as you have told. Please do confirm this observation too if possible.Do you think will any closed loop current sensor help me with it. Could you suggest me any closed loop hall effect sensor that could do the same with isolation...at lower footprint and height clearance. Only one I found was ACS 724KMA with reinforced isolation of around 565Vrms..(I am thinking its a closed loop current sensor...I do not know if its closed loop or not by definition of it...All I know its differential and is suppose to eject all environment noises upon it)..Did you find any other problem or mistakes I made with my current sensing and any better method to do it given my constraints....of isolation, low pcb footprint, low height clerance...

author: linkinchester
author_rank: Newbie
timestamp: July 21, 2017, 06:53:15 am
content: Kiran are you doing undergrad course in elec enggSent from my SM-J700F using Tapatalk

author: capt bullshot
author_rank: Super Contributor
timestamp: July 21, 2017, 07:14:09 am
content: Regarding the input: these pins are allowed to go below ground (see Analog inputs section of datasheet), so your circuit is OK here.Yes, using a shunt places an isolation problem onto you. Depends on the application if this is an issue, if the whole thing is placed inside an isolated enclosure etc., you'll be fine with circuit GND at line voltage. Otherwise, the most common way to achieve isolation would be to isolate the digital port of the ADE to the MCU, having the ADE related circuit on line voltage, and the MCU isolated.The ACS724 is an open loop sensor. A closed loop sensor has a magnetic core and a secondary winding plus a hall sensor for DC feedback. You don't need DC here, so a passive CT would do the job.Using a closed loop sensor wouldn't be a good recommendation for your project, since these are rather bulky and may have considerable power consumption.You may be able to find a good passive CT, specially made for this purpose, but these also tend to be rather large. Smallest size I can imagine would be 1.5 to double the size of a sugar cube. The rogowski coil type sensors mentioned in the ADE datasheet may be an alternative, since they do not need a magnetic core, but I don't know any source for them. These may have problems with waveforms of a typical TRIAC driven light dimmer circuit.

author: dmills
author_rank: Super Contributor
timestamp: July 21, 2017, 11:32:54 am
content: Where does the V1P input bias current go?I cannot see it specified anywhere but the diagram shows an opamp so I would expect there to be some having to go somewhere.Agreed that open loop hall does not have great dynamic range, either use a shunt (and isolate on the digital side) or use a current transformer and a suitable burden.I would probably do the separate isolator thing in the digital domain, something from SI Labs or TI, maybe LT, and float the measurement chip one one or the other of the mains conductors.Something like a 10m Ohm shunt in the neutral with AGnd tied to neutral (To meet common mode limit of +- 0.5V), use a 4 terminal resistor (Bourns make some suitable looking things). You will need an isolated power supply, just use one of those little encapsulated switchers.Here be dragons, you will have circuitry floating at live potential, so be careful!

author: getfast_kiran
author_rank: Regular Contributor
timestamp: July 21, 2017, 12:39:47 pm
content: Quote from: linkinchester on July 21, 2017, 06:53:15 amHi...I am not..I have messaged you the details. Why asked ?

author: getfast_kiran
author_rank: Regular Contributor
timestamp: July 21, 2017, 01:13:16 pm
content: Quote from: capt bullshot on July 21, 2017, 07:14:09 amOkay thanks for the input especially confirming my doubt regarding the voltage going below ground. And as you told, yes its planned to be in a enclosure connected to earth. Here I am trying to isolate the ADE and MCU completely from the AC side. Tested it for Voltage reading already..I have to test it just for the current side..but I have not still got a good current sensing method with stability and accuracy under the constraints I told you...along the range upto 25A.Quote from: capt bullshot on July 21, 2017, 07:14:09 amThanks so much for that explanation of closed loop sensor. I will try to find the passive CT as you have told and check if the size could fit the constraint.Quote from: capt bullshot on July 21, 2017, 07:14:09 amHave not used rogowski coil type sensor before...I will try the same..and see if it work for my purpose...And as you told if it have trouble with triac base light dimmers then it will come as a problemsince I have one of those in queue for testing.Will try to find the passive CT as you have told and get back to you...Thanks...again..

author: getfast_kiran
author_rank: Regular Contributor
timestamp: July 24, 2017, 10:44:10 am
content: Quote from: dmills on July 21, 2017, 11:32:54 amSorry I do not know why the schematic caused confusion. In this particular schematic I have not added opamp. I actually wanted to avoid opamp since it needsdual rail and already SMPS which has been included only has single rail supply. In this particular case the current date from Vout of ACS712 reaches to V1P of ADE directly.Quote from: dmills on July 21, 2017, 11:32:54 amSince both of you guys suggested shunt. If ACS724KMA arrives within a week and that experiment too fails. I will definitely start working with the shunt as you guys have told and work out anisolation technique for both the current and voltage sensing.Quote from: dmills on July 21, 2017, 11:32:54 amYes will look into this configuration as the testing with ACS724KMA finishes.Thanks so much for your help.