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Using Arduino => General Electronics => Topic started by: dominicfhk on Feb 07, 2013, 09:29 am

Title: Understanding ACS7xx Current Sensor
Post by: dominicfhk on 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:
(http://s3.postimage.org/yk7iwof5b/image.jpg) (http://postimage.org/image/yk7iwof5b/)
Then I hook up an oscilloscope to the output, and here is what happen:
(http://s11.postimage.org/6r5c5cvnz/image_1.jpg) (http://postimage.org/image/6r5c5cvnz/)
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!
Title: Re: Understanding ACS7xx Current Sensor
Post by: Krodal on 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.

Title: Re: Understanding ACS7xx Current Sensor
Post by: jonisonvespaa on 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?
Title: Re: Understanding ACS7xx Current Sensor
Post by: Krodal on 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.
Title: Re: Understanding ACS7xx Current Sensor
Post by: dominicfhk on 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?
Title: Re: Understanding ACS7xx Current Sensor
Post by: jackrae on 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
Title: Re: Understanding ACS7xx Current Sensor
Post by: jonisonvespaa on Feb 08, 2013, 10:04 pm
what does the output look like with zero load? should be 2.5v
Title: Re: Understanding ACS7xx Current Sensor
Post by: retrolefty on 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
Title: Re: Understanding ACS7xx Current Sensor
Post by: Constantin on 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.
Title: Re: Understanding ACS7xx Current Sensor
Post by: Krodal on Feb 09, 2013, 12:33 am
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.
Title: Re: Understanding ACS7xx Current Sensor
Post by: retrolefty on Feb 09, 2013, 01:08 am

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:

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
Title: Re: Understanding ACS7xx Current Sensor
Post by: Krodal on 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.
Title: Re: Understanding ACS7xx Current Sensor
Post by: retrolefty on Feb 09, 2013, 01:35 am

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
Title: Re: Understanding ACS7xx Current Sensor
Post by: Krodal on 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.
Title: Re: Understanding ACS7xx Current Sensor
Post by: MarkT on 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.
Title: Re: Understanding ACS7xx Current Sensor
Post by: dominicfhk on Feb 09, 2013, 04:50 am
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.
Title: Re: Understanding ACS7xx Current Sensor
Post by: Krodal on Feb 09, 2013, 09:13 am
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.
Title: Re: Understanding ACS7xx Current Sensor
Post by: jonisonvespaa on Feb 09, 2013, 10:13 am
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.
Title: Re: Understanding ACS7xx Current Sensor
Post by: Constantin on Feb 10, 2013, 12:25 am

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.
Title: Re: Understanding ACS7xx Current Sensor
Post by: jonisonvespaa on Feb 10, 2013, 12:42 pm
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?  
Title: Re: Understanding ACS7xx Current Sensor
Post by: retrolefty on Feb 10, 2013, 05:44 pm

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?  


Can't be .3 ohms and still rated for 30amps. Amps squared X R loss would blow the whole module up.

Lefty
Title: Re: Understanding ACS7xx Current Sensor
Post by: dc42 on Feb 10, 2013, 06:14 pm

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.


If the current won't go above 5A then you should use a more sensitive sensor to reduce the noise. As others have said, you can get boards that provide better isolation than the one you have. I suggest you use this http://www.ebay.co.uk/itm/1x-ACS712-Module-Current-Sensor-5-Amp-Range-Halleffect-Current-Arduino-PIC-ATMEL-/221087104462?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item3379d17dce&_uhb=1 (http://www.ebay.co.uk/itm/1x-ACS712-Module-Current-Sensor-5-Amp-Range-Halleffect-Current-Arduino-PIC-ATMEL-/221087104462?pt=UK_BOI_Electrical_Components_Supplies_ET&hash=item3379d17dce&_uhb=1) or something similar.

EDIT: changed URL to refer to one with a better PCB layout.
Title: Re: Understanding ACS7xx Current Sensor
Post by: jonisonvespaa on Feb 10, 2013, 07:21 pm


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?  


Can't be .3 ohms and still rated for 30amps. Amps squared X R loss would blow the whole module up.

Lefty


here is a pic of the internals of the ic, of the ic pins 1,2 connected to 3,4 i measured it this morning 0.3 ohms
Title: Re: Understanding ACS7xx Current Sensor
Post by: dc42 on Feb 10, 2013, 07:33 pm
What resistance does your meter read:

(a) when you press the probes directly on to the leads of the chip;

(b) when you press the probes against each other?
Title: Re: Understanding ACS7xx Current Sensor
Post by: jonisonvespaa on Feb 10, 2013, 08:00 pm

What resistance does your meter read:

(a) when you press the probes directly on to the leads of the chip;

(b) when you press the probes against each other?


ok, slight change probably due to temp.

leads shorted 0.2 ohms,

across ic pins and connector 0.2 ohms, on my fairly new fluke meter

this is a pic of my application of the ic
Title: Re: Understanding ACS7xx Current Sensor
Post by: dominicfhk on Feb 10, 2013, 09:18 pm
Oh the reason I use the +/-30A IC is that I got them for free :D

@jonisonvespaa Thanks for posting the layout. I assume you try to minimize the traces surrounding the IC to reduce magnetic field. What capacitors do you use for the RC low pass filters for pin 6 and 7? Are you able to get a nice stable reading? According to page 14 on the data-sheet, it appears that we either put a capacitor on pin 6, or a capacitor on pin 7 and leave pin 6 alone. But I guess having 2 wont hurt.
Title: Re: Understanding ACS7xx Current Sensor
Post by: jonisonvespaa on Feb 10, 2013, 09:37 pm
yes the output is very stable im measuring ac current, pvm out of a frequency inverter, my output is really stable nice sign wave no noise.

im using the suggested circuit in the data sheet, 1nf across pin 5 and 6
Title: Re: Understanding ACS7xx Current Sensor
Post by: Constantin on Feb 18, 2013, 11:32 pm
All I will point out is the size of the eval pcb heat sink vs what the commercial resellers typically design and sell. The datasheet mentions a typical internal resistance of 1.2 mOhm for the acs712 and a 5x over current survival.

The datasheet also mentions a atypical 2oz copper thickness for the pcb used on the eval board. The FAQ (http://www.allegromicro.com/en/Products/Current-Sensor-ICs/Zero-To-Fifty-Amp-Integrated-Conductor-Sensor-ICs/ACS712/ACS712-ACS713-Frequently-Asked-Questions.aspx#Q7) also shows die temperatures assuming optimal conditions (ie eval board) reaching 165 C at 20amps in hot ambient conditions. I expected my device to potentially get hot inside as it did not feature active convection and the enclosure is small.

So, by all means go and use this sensor but between the relatively high error and the heat issues, if was not suitable for me. For those measuring short impulse loads in particular, this chip family features very attractive attributes like small size and relatively low cost. I went for the LTSR series from LEM instead. It's leads are significantly beefier, you get a reference voltage for differential measurements, etc.
Title: Re: Understanding ACS7xx Current Sensor
Post by: dominicfhk on Feb 19, 2013, 03:22 am
Thanks for all the responses. I just begin to make a shield for the Mega with a couple of these ACS714 sensors on board. Hopefully it won't blow up :D
Title: Re: Understanding ACS7xx Current Sensor
Post by: jonisonvespaa on Feb 19, 2013, 10:13 am

All I will point out is the size of the eval pcb heat sink vs what the commercial resellers typically design and sell. The datasheet mentions a typical internal resistance of 1.2 mOhm for the acs712 and a 5x over current survival.

The datasheet also mentions a atypical 2oz copper thickness for the pcb used on the eval board. The FAQ (http://www.allegromicro.com/en/Products/Current-Sensor-ICs/Zero-To-Fifty-Amp-Integrated-Conductor-Sensor-ICs/ACS712/ACS712-ACS713-Frequently-Asked-Questions.aspx#Q7) also shows die temperatures assuming optimal conditions (ie eval board) reaching 165 C at 20amps in hot ambient conditions. I expected my device to potentially get hot inside as it did not feature active convection and the enclosure is small.

So, by all means go and use this sensor but between the relatively high error and the heat issues, if was not suitable for me. For those measuring short impulse loads in particular, this chip family features very attractive attributes like small size and relatively low cost. I went for the LTSR series from LEM instead. It's leads are significantly beefier, you get a reference voltage for differential measurements, etc.


hi, was a little worried when you quoted the internal resistance ive looked at the data sheet many times and never came across this figure, ive been using the acs 712 30amp chip for some time and never noticed a heat issue, im using their suggested circuit.

those internal resistance in the chart marked  "Typical Leadframe Resistance at Various Ambient Temperatures"

Are defiantly wrong ive just gone through all my 20amp and 30amp chips and measured this resistance
results as follows, these were measured on the pins nothing attached to them on ip+ (pins 1 and 2) and ip- (pins 3 and 4).

acs 712 20a  total 6 ics,  all 0.3 ohms
acs 712 30a  total 13 ics  all 0.3 ohms

maby the figure they quote in their chart has a typo (m?) sould be just (?)

they do mention that the data was made using theAllegro ASEK712 demo board maby this resistance comes from extra circuitry there maby? but looking at it cant see any components that would suggest this

be really great if someone else could also confirm this resistance as well

Title: Re: Understanding ACS7xx Current Sensor
Post by: retrolefty on Feb 19, 2013, 01:21 pm
Quote
Are defiantly wrong ive just gone through all my 20amp and 30amp chips and measured this resistance
results as follows, these were measured on the pins nothing attached to them on ip+ (pins 1 and 2) and ip- (pins 3 and 4).

acs 712 20a  total 6 ics,  all 0.3 ohms
acs 712 30a  total 13 ics  all 0.3 ohms

maby the figure they quote in their chart has a typo (m?) sould be just (?)

they do mention that the data was made using theAllegro ASEK712 demo board maby this resistance comes from extra circuitry there maby? but looking at it cant see any components that would suggest this

be really great if someone else could also confirm this resistance as well


Are you aware of how difficult it is to accurately measure resistance values down to the milliohm value? It usually takes using a 4 wire Kelvin set-up with a constant current sent through the unknown resistance and then measuring the resulting voltage drop across the unknown resistance. So describe your measurement method.

Lefty
Title: Re: Understanding ACS7xx Current Sensor
Post by: jonisonvespaa on Feb 19, 2013, 02:31 pm
ok im stumped there, will look up 4 wire Kelvin set-up, thanks lefty.

ok i understand now thanks for pointing that out lefty

on a more practacle note ran my acs 712 30amp version, loaded it up to 10amps  ran constant for 5 mins, then checked it for its temp,  there was absolutely no heat in the ic casing what so ever.

according to the chart id be getting 80 ish dec C, in the chart it mentions Die temperature whatever this is? maby it the core of the ic itself maby? really not sure, but i didnt notice any heat generated at all @ 10 Amps, be nice to know what this die temp is though, i often hear case temp or package temp, but never die temp.
Title: Re: Understanding ACS7xx Current Sensor
Post by: Constantin on Feb 19, 2013, 05:54 pm
For all I know, this chip puts up a lot less resistance than the manufacturer warrants in their documentation. And 10A through the 30A version may be perfectly safe, generating no heat in your application, etc. I am not criticizing your use of this chip. I was merely constrained by a design that requires high accuracy, low insertion losses, and the ability to withstand long duration exposure to high currents.

This sensor design with its SO8 frame and a mere two leads each carrying that current gave me the willies. That does not mean it doesn't work. I simply wanted something with more meat and some other attributes.

Please also note how the Allegro eval board offers a wide land for the leads going to the sensor and very heavy-duty connecting posts. It also features double-sided high-voltage PCB traces and 'stitched' construction. In other words, Allegro is using the copper on both sides of the board for the power signal and the conductors are thoroughly bonded to each other using lots and lots of vias to reduce insertion losses and improve thermal heat dissipation from the chip.

I am not convinced that many of the commercially-available boards out there go to the same lengths to ensure a wide, thick path or use the same extra-heavy copper trace thickness (2oz copper is uncommonly used, costs extra) as the Allegro board. Just something to keep in mind as you source your gear. Relying on conductors attached to the terminals to dissipate heat (as some design apparently expect to) seems like bad engineering practice.

On the other hand, if the chip stays super cool no matter what currents are being passed, then so much the better for you.
Title: Re: Understanding ACS7xx Current Sensor
Post by: Zapro on Feb 19, 2013, 06:15 pm
I wouldn't recommend using Polou's board for anything above 50V - the missing isolation on the board is not too clever.

Running 230V on those pads would be very dangerous

// Per.
Title: Re: Understanding ACS7xx Current Sensor
Post by: Constantin on Feb 20, 2013, 02:50 am
There are a couple of designs out there that I find odd. This one (http://www.hobbytronics.co.uk/image/cache/data/sparkfun/acs712_breakout-500x500.jpg), features no on-board caps and filters, a tiny land, etc. The adjustable Sparkfun board (http://www.hobbytronics.co.uk/image/cache/data/sparkfun/acs712_lc_breakout-500x500.jpg) features a tiny load path and a giant nearby GND path nearby. I don't get it for safety reasons. The pololu board (http://www.hobbytronics.co.uk/image/cache/data/pololu/acs714_breakout_2-500x500.jpg) at least features some serious high-voltage lands though the distance to the low-voltage components seems questionable.
Title: Re: Understanding ACS7xx Current Sensor
Post by: jonisonvespaa on Feb 20, 2013, 01:26 pm
yes i agree those designs are poor, not sure if those tracks are man enough for their intended currents tough, it took me about 6 attempts to get to this this design, i did a few versions where i had ground fill didnt work well, either. not a good idea to place tracks under the ic either, you have to isolate the pins either side.

the current path pins are as close as possible to the connector as possible and i then fill that track with solder from the connector to the 2 pins both sides to be sure  
Title: Re: Understanding ACS7xx Current Sensor
Post by: dominicfhk on Mar 01, 2013, 06:28 pm
Hey guys, sorry for digging up this old post. I am making the PCB right now and I am wondering what is the purpose of the 0.1uF cap.

Let say I am going to include 10 ACS714 on a single board. According to the data sheet, a 0.1uF cap is suppose to be connected from VCC to GND. Can I just use one cap for all the ACS714? Should it be 0.1uF or 1uF? Thank you so much!
Title: Re: Understanding ACS7xx Current Sensor
Post by: retrolefty on Mar 01, 2013, 06:42 pm

Hey guys, sorry for digging up this old post. I am making the PCB right now and I am wondering what is the purpose of the 0.1uF cap.

Let say I am going to include 10 ACS714 on a single board. According to the data sheet, a 0.1uF cap is suppose to be connected from VCC to GND. Can I just use one cap for all the ACS714? Should it be 0.1uF or 1uF? Thank you so much!


It's the typical .1ufd bypass chip that all ICs should have mounted as close to each IC as possible. Don't try to skimp or combine into one or few caps.

Lefty
Title: Re: Understanding ACS7xx Current Sensor
Post by: dominicfhk on Mar 01, 2013, 06:45 pm
Got it, thanks!  :)
Title: Re: Understanding ACS7xx Current Sensor
Post by: pavelp on Oct 26, 2013, 11:24 am
Hello guys,
@jonisonvespaa: it is absolutely not possible to measure resistances around 0.001 Ohm with any standard multimeter. Even the resistance of the multimeter wires is much higher (~200  times!) than the measured value.

In this case

a) just believe the datasheet, the internal resistance should be around  0.0012 ohm

b) make a simple test - solder one precise value 0.001 ohm resistor as close as possible in parrallel to the ACS7xx current sensor and check the reported current value. You can then calculate the internal chip resistance precisely. For example with 10A current the reported current should be 4.55 A after the modification, if the internal chip resistance is really 0.0012 ohm. (for ACS715 0-30A sensor)

@to this topic: I was experimenting with many current sensors lately, and no one met my expectations:

a) ACS7xx based hall sensors : highly sensitive to magnetic fields, very high noise. Not usable for measuring precise values under 1 Amp.

b) AMPLOC hall current sensors (amploc.com) : not suitable for DC currents - the core gets magnetized and zero value shifts by +-0.5 Amps after each day of measurement. Zero internal resistance. Not usable for measuring precise values under 2 Amps. So the error of this sensor may be over 0.5 Amp in any moment, that is not good at all. To be used in cars or such equipment, where it's enough to know the result with +- 1 Amp error.

c) ATTOPILOT current sensor (45A) : shunt resistor type sensor. Not isolated from the measured circuit! Stable zero value. Does not measure currents under 0.22 A (reports zero). All reported values are lower by this 0.22A offset (you have to add 0.21-0.22 Amps to all results). Internal resistance 0.001 Ohm.

d) LTC6102 based shunt current sensor (I haven't seen any sensor using it, I had to make my own). Same as ATTOPILOT, but the offset error is lower - around 0.1 A. Internal resistance 0.0005 Ohm.

So if you do not care about currents under 0.1 resp. 0.2 Amps and you do not care about the electrical isolation, use the ATTOPILOT 45 or LTC6102 based sensor. The LTC6102HV is able to measure on voltages up to 105 Volts or something like that (see the datasheet). Main advantage is, that the results are very precise (above the offset mentioned), low noise and not influenced by any magnetic fields.
Title: Re: Understanding ACS7xx Current Sensor
Post by: MarkT on Oct 27, 2013, 02:14 am
If you have a bench power supply with a current control its easy to measure small
resistances in combination with a multimeter:

Set the PSU to 1A limit, connect to the resistor under test (it is a small resistor so I-squared-R
will be small and thus won't fry).

Measure the voltage directly across the resistor right on the terminals (not along the wires),
this forms a 4-wire Kelvin measuring setup.

Since the current is known to be 1A, the resistance in milliohms = the voltage in millivolts,
easy!

You can separately calibrate the current setting on your PSU using the amps range on
the multimeter if you want to check its accuracy.

Remember just ordinary bits of wire lying around will have resistances of from a
few milliohms (thick cables) to 100's of milliohms (small signal wires).  Oxidation on
plug and sockets can add tens to hundreds of milliohms too...
Title: Re: Understanding ACS7xx Current Sensor
Post by: michinyon on Oct 28, 2013, 01:07 pm
How are you sure that the 2 amp current you  are measuring,  is not noisy ?
Title: Re: Understanding ACS7xx Current Sensor
Post by: thedoubless on Oct 23, 2014, 11:12 am
hi

how can i use ACS712 current sensor with 220 volt
i tested it with dc and it works well, nut with ac it didn't give any response :/
regards ty
Title: Re: Understanding ACS7xx Current Sensor
Post by: thedoubless on Oct 23, 2014, 11:28 am
hi
can some one helps me to measure AC current with ACS712 sensor
it just worked fine with dc but with ac i cant find a solution
ty