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I purchased 3 ACS758 Hall Effect Current Sensors datasheet on ebay. A 50,100 and 200A, all three are bi-directional. They look like this.

With no current the 50A outputs 477 on the analogue input (2.33v), the 100A outputs 509 (2.48v) and the 200A outputs 512 (2.50v). At no current they should all be at exactly mid range i.e. 512 (2.50v). The datasheet says they're factory calibrated as such. I've tested them one after the other, both with 5v from the Audrino and with a separate 5v supply. As I've tested one after the other it's not likely to be stray magnetism or VCC.

I've then put them in a circuit drawing about 9amps. Accounting for the offset of zero amps the 50A and 100A measured the increased voltage correctly. The 200A doesn't change.

Is there any other tests I can do to see if I'm doing something dumb or are they all damaged?

  • Did you measure VIOUT with anything else than an Arduino? A precision digital voltmeter, for example? Are you aware that precision measurements require precautions such as insulation against noise? Also make sure that your reference voltage is extra stable. All AVR micro-controllers have a noise reduction mode that increases accuracy. The ADC accuracy of the ATmega328P is also highly dependent on the ADC clock: the slower the more accurate. Refer to the ATmega328P datasheet for more information. – user16306 Jun 27 '17 at 14:10
  • Thanks for the reply Nasha. I will try measure VIOUT with my cheap multimeter, I don't have a precision one. I (think) I understand your point about noise. With zero load I'm getting quite a consistent reading, just a different one from each device. It should be 2.50v from each device and instead it's 2.33v and 2.48v (according to the arudino). Even if they're not the correct voltages they're different to each other. The chip is supposed to come factory calibrated to 2.50v for zero load. Do you think noise or ADC accuracy could be the cause of the calibration problem? – Squats Jun 27 '17 at 23:00
  • If you have an oscilloscope, you will see how noise does affect any measurement. And since calibration is one... Repetitive measurements, averaged will reduce the effect of noise, as if you filtered the signal through a RC low-pass filter, more or less. Anyway if you read different values (with your multimeter as the only load) then the accuracy of the current sense devices may be the reason. Doesn't mean they're faulty (check the datasheet). Means you may have to account for a certain offset in your software, which you measure through calibration; that's what it's for. Also check the 5V... – user16306 Jun 28 '17 at 7:37
  • Also the 0.1µF bypass capacitor is key in stability. I don't know if the PCB you've shown includes one. Bypass capacitors shall be as close to the chip as possible. Certainly plays a role, too. Be sure to check that. – user16306 Jun 28 '17 at 7:41
  • Thanks again! For the calibration measurement I did take hundreds of readings and they were very consistent (within 1/1024). I checked VIOUT with my cheap multimeter and I got the same 2.33v and 2.48v. VCC was 4.98v. The board has a 0.1µF capacitor right next to the chip, I bought the whole board but just looked and they are. – Squats Jun 28 '17 at 10:30
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I contacted Allegro they were really helpful which was pleasantly surprising.

The 200A has almost certainly failed. The 2.48v is within tolerance and the 2.33v is probably out.

I've been able to better understand the datasheet sections on 0A calibration. "VIOUT(Q) may drift over the lifetime of the device by as much as ±25 mV." and there is an electrical offset voltage noted. @25C it's +/-5mv. It's +/-20mv between 0c and 50c. So still only 45mv. Even allowing for noise it's only 55mv.

About 2.445V at zero amps would be the limit.

  • Don't you mean the 50A did fail? Glad you sorted out this issue that said :-) . – user16306 Jun 28 '17 at 11:57
  • Annoyingly the 200A was the one with the perfect calibration but ......"I've then put them in a circuit drawing about 9amps. Accounting for the offset of zero amps the 50A and 100A measured the increased voltage correctly. The 200A doesn't change." .... The 200A was perfect at zero amps and stayed at 2.50v regardless of current.... completely broken. – Squats Jun 28 '17 at 12:06
  • Ouch! Rest In Pieces... – user16306 Jun 28 '17 at 12:07

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