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### Topic: Understanding ACS7xx Current Sensor (Read 24233 times)previous topic - next topic

#### dominicfhk

##### Feb 07, 2013, 09:29 am
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!

#### Krodal

#1
##### Feb 07, 2013, 11:59 am
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.

#### jonisonvespaa

#2
##### Feb 07, 2013, 01:11 pm

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?

#### Krodal

#3
##### Feb 07, 2013, 03:57 pm
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.

#### dominicfhk

#4
##### Feb 08, 2013, 08:14 pm
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?

#### jackrae

#5
##### Feb 08, 2013, 08:39 pm
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

#### jonisonvespaa

#6
##### Feb 08, 2013, 10:04 pm
what does the output look like with zero load? should be 2.5v

#### retrolefty

#7
##### Feb 09, 2013, 12:17 am
Quote
"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

#### Constantin

#8
##### Feb 09, 2013, 12:29 am
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.

#### Krodal

#9
##### Feb 09, 2013, 12:33 am
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.

#### retrolefty

#10
##### Feb 09, 2013, 01:08 am

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:

Quote

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

#### Krodal

#11
##### Feb 09, 2013, 01:32 am
retrolefty, a resistor of 1.2mOhm is between pin 2 and 3. So at 30A there is 36 mV between 2 and 3.

#### retrolefty

#12
##### Feb 09, 2013, 01:35 amLast Edit: Feb 09, 2013, 01:43 am by retrolefty Reason: 1

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

#### Krodal

#13
##### Feb 09, 2013, 02:03 am
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.

#### MarkT

#14
##### Feb 09, 2013, 03:24 am

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.
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

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