Understanding ACS7xx Current Sensor

Using Arduino General Electronics
#1

Hey guys! So I have this ACS714 board made by Pololu: http://www.pololu.com/catalog/product/1187 After spending some time with it, there are a couple of things I don't quite understand....

First of all, the output fluctuates. For example, when I apply a constant 2A of current across the terminals, here is what happen: Then I hook up an oscilloscope to the output, and here is what happen: Note that the Delta_V is 248mV. I figured this might be the problem as the ADC on the Arduino has a sensitivity of 5V/1024=.0048V=4.8mV... But this doesn't add up either because the Delta_V on the oscilloscope is 248mV, which represents 248mV/4.8mV=52 digits of fluctuation. I am only getting 4 digits of fluctuation at most (from 175 to 179). So what exactly cause the fluctuation?

I tried a software approach by using a for-loop and take 100 samples and then the average. The final value still fluctuates by around 2-3 digits. So I decided to look for a way to reduce the noise from the output pin. According to the website, there is already a RC low pass filter implemented on the board, but as you can see, I still get a lot of noise to a point that it affect the ACD reading. I am wondering if it is possible to build another filtering circuit at the output pin to further reduce the noise? If it is possible, what kind of circuit can be use to filter the noise?

Last, I am intended to use this sensor for sensing a 100V DC source. If I am understanding it correctly, this hall effect IC can achieve isolation for up to 2.1kV at its current path (pin1+2 to pin3+4). But, basically the Pololu website states that if the voltage is higher than 30V I will get killed: "Warning: This product is intended for use below 30 V. Working with higher voltages can be extremely dangerous and should only be attempted by qualified individuals with appropriate equipment and protective gear." Now I am confuse. What exactly is restricting the voltage from go over 30V? Is that an issue with the board or the IC? Cause I can't find this limitation on the data-sheet of the ACS714 IC. http://www.pololu.com/file/0J196/ACS714-Datasheet.pdf If it is an issue with the Pololu board, what area should I pay more attention if I am to make on own PCB using the same IC that can enable me to sense a 100V DC source safely?

Any input would be appreciated!

#2

Those sensors are noisy. I have even more noise than you. Those sensors pick up any magnetism from transformers nearby. Both normal 50Hz/60Hz transformers and also high frequency switching power supply transformers.

I live in Europe, with 50Hz mains. So I use the average during 20ms, to measure DC current. But it still is not very stable.

If you look at the Pololu breakout board, the current side copper on the pcb is near the sensor output side. For high voltages it should be as far as possible away from the sensor output side.

#3

Krodal: Those sensors are noisy. I have even more noise than you. Those sensors pick up any magnetism from transformers nearby. Both normal 50Hz/60Hz transformers and also high frequency switching power supply transformers.

I live in Europe, with 50Hz mains. So I use the average during 20ms, to measure DC current. But it still is not very stable.

If you look at the Pololu breakout board, the current side copper on the pcb is near the sensor output side. For high voltages it should be as far as possible away from the sensor output side.

please can you explain by what you mean noisy? do you mean emit noise / electrical noise?

#4

They don’t emit, but the values read with those current sensors go up and down a lot.

If the average of a number of samples is used, and the current sensor is not near other devices, it should be possible to get a nice stable reading of the measured current. But that is not always possible.

#5

Thanks for the responses. Does anyone else have experience using the ACS7xx series sensor? I would love to hear how you deal with the fluctuation. Making another RC filter at the output maybe?

#6

Or place the sensor board inside a steel enclosure (old style tobacco tin) to minimise external magnetic influence If you do remember to fully insulate the inside to prevent connection with the live circuits

#7

what does the output look like with zero load? should be 2.5v

#8

"Warning: This product is intended for use below 30 V. Working with higher voltages can be extremely dangerous and should only be attempted by qualified individuals with appropriate equipment and protective gear." Now I am confuse. What exactly is restricting the voltage from go over 30V? Is that an issue with the board or the IC? Cause I can't find this limitation on the data-sheet of the ACS714 IC.

I think it's just a general caution warning meant to protect inexperienced people from using it with voltages that could cause them shock hazard and injury because of their inexperience. The chip is certainly rated at being able to measure the AC current of household AC power, so again in my opinion it's not a technical requirement but more a personal warning and a way for Pololu to try and lower their exposure to liability claims if someone kills themselves trying to wire this up to measure the current draw on their refrigerator. ;)

Lefty

#9

I briefly considered using these chips, as others have. In the end, I dumped them on account of the inability on my part to see how these chips would not turn to crispy fritters during sustained high loads. The heat sinking requirements are 'interesting'.

I much prefer the LTSR series from LEM - excellent stable signal output, and a reference output for those of us who use differential ADCs. If you don't need the reference output, use the LTS series instead. Tamura also makes some nice ones.

Hall effect-compensated current sensors of this sort are more expensive, for sure but offer almost no insertion resistance, virtually no current limit, etc. The only 'downside' is the working voltage - i.e. they operate on 5V whereas I am making the transition to 3.3V systems. So I use a voltage divider to bring things down to a level that makes my Teensy 3.0 ADC happy.

#10

retrolefty, your answer scares me. Look at the pcb, how short the distance is between the current side and the signal side of the sensor. There is even a signal line under the chip.

#11

Krodal: retrolefty, your answer scares me. Look at the pcb, how short the distance is between the current side and the signal side of the sensor. There is even a signal line under the chip.

Yes the PCB trace spacing could be a factor for Pololu warning also, but the trace spacing is the same as the pin spacing of the device and the datasheet certainly doesn't include any voltage limit caution between pins 2 and 3.

SparkFun sells a 5 amp version of the this chip in a module/breakout form: https://www.sparkfun.com/products/8882

A couple of users asked if it was suitable for AC power current measurements and other users answered:

Anyone know if this is suitable for the mains? say 3A max @ 240v AC (UK). I had a quick skim through the datasheet, and didn’t seem to find anything :/

Chiel | about 3 years ago 1 the ACS712 is rated to 5A and after searching around on the web for projects using this ic at 220v i would say its usable in europe. the datasheet mentions a Peak Basic isolation voltage of ~380 Volt which could be the maximum voltage the ic can handle. i hope this answers your question. mattkenny | about 3 years ago 1

I’m using the 20A version of this chip in a device to sense mains current @240V/50Hz (Australia). It handles it just fine.

Of course these are just Internet 'answers' and don't carry any authority.

Lefty

#12

retrolefty, a resistor of 1.2mOhm is between pin 2 and 3. So at 30A there is 36 mV between 2 and 3.

#13

Krodal: retrolefty, a resistor of 1.2mOhm is between pin 2 and 3. So at 30A there is 36 mV between 2 and 3.

So what scares you about the module or my thoughts stated so far? The voltage isolation rating from the datasheet from pins 1-4 and 5-8 pins is stated in the datasheet and would seem to support household AC power usage?

I own a similar +/- 5 amp product from an Asian e-bay seller that I've played around with a little, but only on DC voltages so far.

http://www.ebay.com/itm/5A-ACS712-Module-range-Current-Sensor-Module-/370669739382?pt=LH_DefaultDomain_0&hash=item564da35d76

Lefty

#14

retrolefty, look at the pcb, both sides. The copper of the high current (high voltage) side is very close to the sensor signal lines to the Arduino. That’s scary. I will work, but not within the safety regulations.

#15

Krodal: retrolefty, a resistor of 1.2mOhm is between pin 2 and 3. So at 30A there is 36 mV between 2 and 3.

Well I would reword that as "the maximum resistance between current terminals is specified as 1.2mOhm" - the resistance is incidental to the operation of the device since it's a hall-effect sensor - some of that resistance may be the pins, the solder on the pins, there will be some from the PCB traces too (for 1oz copper board the ohms-per-square is about 0.5mOhm).

Or put another way there will be at least tens of millivolts everywhere at those current levels, and 30A continuous wouldn't be a sensible application for these sensors.

#16

Hmm… well since this is a +/-30A sensor I do assume it is safe to run 30A through the chip… But for my application, the current actually won’t go over 5A, so it is not really a concern to me. The thing I am worry about is the voltage cause it is going to be constantly around 100V.

To be more specific, I want to connect 3 solar panels in series and measure the current of the string using this sensor. Each panel has a Vmp of 35.2V and Imp of 4.95A. Connecting them in series yields 105.6V and 4.95A at max.

So you guys think it is okay to use this IC to take current measurement as far as safety goes? My plan is to make my own PCB with a couple of this IC on board (I have a couple of strings connected in parallel) as well as a voltage divider for voltage measurement across the two nodes. The trace is probably going to be 2mm wide at least, but I have no idea how the spacing between each trace should be for 100V.

#17

There are better breakout boards: https://www.sparkfun.com/products/8882 http://www.ebay.com/itm/ACS712-current-sensor-module-5A-range-/160730888420

Your board will work with 100V, and the risk is very low with 100V. But if it is about safety, I would say: Do it well, or don't do it at all.

Sparkfun parts are available from Sparkfun but also from other sellers in many countries.

#18

ive been using that 30a chip up to 9 amps and not noticed any heat issues as discussed earlier in the thred, i suspect my heat is sunk through the connector i am using as its so close to the ic, i am measuring a single ac 230 phase

they are very sensitive to how you lay them out on the pcb, if you dont get it right they dont work, took me a few attempts

in the data sheet there is an explanation on the best method of board design

note, obviously make sure the connector you use is capable of what current you are measuring, not all the small breakout boards that are sold everywhere are capable of handling their intended currents (the connectors). on my board i fill the track with extra solder from the ic pin to the pin of the connector just to be safe.

if you want i can send you the pcb design file, i use Design Spark free its a really great package cant recommend it enough.

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#19

MarkT:
30A continuous wouldn’t be a sensible application for these sensors.

My interpretation of the datasheet is even more conservative than that. I am happy to be wrong, but my recollection of the datasheet and the engineering examples was that the sensor would require a substantial heat sink not to turn into blue smoke at sustained amperages well below the maximum. The breakout boards do a good job minimizing the PCB around the chip (good from a $$$ perspective) but at the very real risk of frying the chip if it encounters sustained loads.

Since my application has the possibility of such loads (think dryers, water heaters, and like products that can pull 5kW), I simply gave up on these chips. They may work well for some applications (like detecting abnormal current draws on motors necessitating the shutdown of an IGBT before it blows up) but for power measurement, they offer neither the accuracy nor the power capacity of other solutions.

#20

ok had another look at the data sheet

i cant find any mention of heatsinking,

i really dont understand where this heat would be coming from in the first place, the current passes between IP+ (pins 1 and 2) and IP- (pins 3 and 4) the resistance is 0.3 ohms how can this generate any heat?