Pages: 1 2 [3]   Go Down
Author Topic: Understanding ACS7xx Current Sensor  (Read 3990 times)
0 Members and 1 Guest are viewing this topic.
Left Coast, CA (USA)
Offline Offline
Brattain Member
*****
Karma: 361
Posts: 17262
Measurement changes behavior
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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 (mΩ) sould be just (Ω)

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
Logged

uk
Offline Offline
Sr. Member
****
Karma: 0
Posts: 314
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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.


* die.png (45.92 KB, 699x476 - viewed 35 times.)
« Last Edit: February 19, 2013, 08:49:53 am by jonisonvespaa » Logged

Peoples Republic of Cantabrigia
Offline Offline
God Member
*****
Karma: 6
Posts: 719
Arduino happiness
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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.
Logged

Denmark
Offline Offline
God Member
*****
Karma: 15
Posts: 654
Please don't send questions as PM!
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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.
Logged

Peoples Republic of Cantabrigia
Offline Offline
God Member
*****
Karma: 6
Posts: 719
Arduino happiness
View Profile
WWW
 Bigger Bigger  Smaller Smaller  Reset Reset

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.
Logged

uk
Offline Offline
Sr. Member
****
Karma: 0
Posts: 314
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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  


* acs.png (16.84 KB, 649x541 - viewed 31 times.)
« Last Edit: February 20, 2013, 07:31:05 am by jonisonvespaa » Logged

Offline Offline
Jr. Member
**
Karma: 0
Posts: 61
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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!
Logged

Left Coast, CA (USA)
Offline Offline
Brattain Member
*****
Karma: 361
Posts: 17262
Measurement changes behavior
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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
Logged

Offline Offline
Jr. Member
**
Karma: 0
Posts: 61
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

Got it, thanks!  smiley
Logged

Czech Republic, Europe
Offline Offline
Newbie
*
Karma: 0
Posts: 20
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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.
« Last Edit: October 26, 2013, 07:43:36 am by pavelp » Logged

0
Offline Offline
Shannon Member
****
Karma: 200
Posts: 11690
Arduino rocks
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

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...
Logged

[ I won't respond to messages, use the forum please ]

Offline Offline
Faraday Member
**
Karma: 60
Posts: 2814
View Profile
 Bigger Bigger  Smaller Smaller  Reset Reset

How are you sure that the 2 amp current you  are measuring,  is not noisy ?
Logged

Pages: 1 2 [3]   Go Up
Jump to: