need help with ACS754

TC

Hello, I hope someone can help me. I am somewhat a newbie to programming. But that is another story. Right now I need a little help. I am working on a little project for my car I am tying to monitor my battery and how much power is being used. I am thinking of using Allegro’s “ACS754xCA-050” http://www.allegromicro.com/datafile/0754-050.pdf . I am having a little problem understanding what the out put would be. From what I understand,

VOUT = 2.5V when VCC = 5V at 0 current.

Is this true? Or do I understand it wrong? Also could someone please help me understand “Amp hours” and how I could possibility use it to estimate how much time I have left on my battery?
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Thanks in advance TC

Nate

TC,

The datasheet is confusing: From the graph in the datasheet (pg. 5), it looks like the output voltage would be 0 when the current through IP+/- is 0, the output would be +V when there is forward current through IP+/-, and output would be -V when the current is reverse biased through IP+/-.· However, maximum reverse voltage output is listed as -.1v -·To me this would indicate that the maximum negitive output voltage would be -.1v.· Not sure why the datasheet doesn't label the V axis relative to Vcc (or whatever Vout is relative to).

If you have one on hand, the easiest way to tell is bench test it.

Amp hours is a measure of energy stored - a batterry that is rated at 100 amp hours means a fully charged battery·could theoretically run a 1 amp load for 100 hour, or a 2 amp load for 50 hours, or a 4 amp load for 25 hours, etc (with no charging).

If you can track current in and out of a battery, you can·get a rough idea of·how much·energy is stored in the battery.· Be aware that using current, time, and the rated capacity of the battery to predict how much energy is left in the battery is gestimation at best - though it can work ok if you have a zeroing of the charge amount (ie the battery gets fully charged fairly often).· Of course, a battery's storage capacity also changes with age and use.· I belive most battery monitoring systems use the voltage of the battery to predict remaining charge.

Nate

Post Edited (Nate) : 7/10/2005 2:50:52 PM GMT

TC

Thank you Nate, you would not believe how confusing the datasheet is to someone with basic electronic knowledge. I have one on order, just wanted to see if anyone else used it before. But I thank you for you input. Also thank you for helping me understand amp hour. Now since I know it I could get a ruff idea of how much time I have left on my battery. I am also going to monitor battery voltage, to get a good idea about how good the battery is. Thanks for you input Nate

Dave Paton

I used the older ACS750 series for a few things last year (the output is the same as the 754 series), and the current is ratiometric about VCC/2, so 0A is 2.5V in your case. Imax+ will be 5V, Imax- will be 0V. Pay careful attention to layout, with an emphasis on guard rings and low impedence traces for these. At high currents, the induced noise can be incredible.

-dave

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TC

Thanks Dave. I was going to use the Evaluation Kit. I thought it would be the best idea. All I would have to do is add a filter cap and shilded 3 conductor wire to connect to the ADC. I thought I was reading the data sheet right. Do you think a 8 bit ADC would work to get a 1 amp resulation?
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Thanks TC