Understanding ACS7xx Current Sensor

There are better breakout boards:

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.

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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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.

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?

jonisonvespa:
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

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.

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.

retrolefty:

jonisonvespa:
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

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?

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?

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

Oh the reason I use the +/-30A IC is that I got them for free :smiley:

@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.

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

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.

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 :smiley:

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.

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

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

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.

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.

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.

There are a couple of designs out there that I find odd. This one, features no on-board caps and filters, a tiny land, etc. The adjustable Sparkfun board features a tiny load path and a giant nearby GND path nearby. I don't get it for safety reasons. The pololu board at least features some serious high-voltage lands though the distance to the low-voltage components seems questionable.

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