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Topic: Understanding ACS7xx Current Sensor (Read 5561 times) previous topic - next topic

dominicfhk

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.

Krodal

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.

jonisonvespaa

#17
Feb 09, 2013, 10:13 am Last Edit: Feb 09, 2013, 11:14 am by jonisonvespa Reason: 1
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.

Constantin


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.

jonisonvespaa

#19
Feb 10, 2013, 12:42 pm Last Edit: Feb 10, 2013, 01:48 pm by jonisonvespa Reason: 1
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?  

retrolefty


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

dc42

#21
Feb 10, 2013, 06:14 pm Last Edit: Feb 10, 2013, 06:17 pm by dc42 Reason: 1

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 or something similar.

EDIT: changed URL to refer to one with a better PCB layout.
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

jonisonvespaa



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

dc42

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?
Formal verification of safety-critical software, software development, and electronic design and prototyping. See http://www.eschertech.com. Please do not ask for unpaid help via PM, use the forum.

jonisonvespaa


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

dominicfhk

#25
Feb 10, 2013, 09:18 pm Last Edit: Feb 10, 2013, 09:22 pm by dominicfhk Reason: 1
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.

jonisonvespaa

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

Constantin

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 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.

dominicfhk

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

jonisonvespaa

#29
Feb 19, 2013, 10:13 am Last Edit: Feb 19, 2013, 10:47 am by jonisonvespaa Reason: 1

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 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


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