Re: mA-hour meter

>I'm working on an amp-hour meter (actually milliamp-hour) and am
>planning on using an Allegro current sensor to measure current from a
>discharging battery under load. I plan on displaying the accumulated
>amp-hours on an LED display, and a PIC will be running the show.
>
>Is the Allegro the best method for doing this, or is there another way?
>I haven't gotten into it yet, but it looks like the Allegro takes some
>calibration to make it accurate.
>
>Any suggestions? Thanks.
>
>

Since you are measuring mA (you don't mention how many) you should check
out the FAQ on the allegro part
http://www.allegromicro.com/faq/acs704faq/index.htm#Q6

Given the low resolution and accuracy issues, and assuming that you are
sub 1Amp, I would reccomend that you measure the voltage across a current
shunt. Maxim has a part that will allow an external current shunt,
National has one with an internal one. I think TI (Burr Brown) & Linear
Tech also have products. Here's a start at Maxim.
http://para.maxim-ic.com/compare.asp?Fam=CS_Amp&Tree=PowerSupplies&HP=PowerSupplies.cfm&ln=


But you don't need a dedicated part- you simply need a difference
amplifier that can handle input voltages beyond the supply rail. You can
make your own with resistive dividers at the input, or use a product like
the AD626, an Instrumentation amplifier from Analog Devices. Of course
this approach is not isolated.

I am currently using the Allegro ACS704xx-005 current sensor and I have
developed I problem where the device "locks up". I am awaiting tech
support to find out where the problem lies.

-Aubrey

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