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Forum Index : Electronics : opamp?

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windman1000
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Joined: 21/06/2009
Location: United States
Posts: 91
Posted: 01:17pm 03 May 2020
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hi i built me a power monitor from a arduino mega so i have 8 voltage dividers to 8 pins , right before i went to hook it up i relies that all ground on arduino mega is 1 ground and my battery bank is in series so my grounds have voltage between them so i need to isolate each battery.
looking threw the internet i think i found away by using opamp , im not a electronic person so my question is, is there a opamp thing premade into a module by china on ebay that i can just buy?

my battery bank is 48v with 8 6v batteries
Edited 2020-05-03 23:21 by windman1000
 
Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 3254
Posted: 10:07pm 03 May 2020
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Quite a few different ways to do that, but none are going to be cheap or simple if you require high resolution and high accuracy.
https://en.wikipedia.org/wiki/Isolation_amplifier

The simplest and crudest way would be to configure each of your eight inputs to measure 0v to +60v with a very carefully matched voltage divider on each input with respect to ground.
Measure between the common ground and the +ve terminal of each battery.
First input might measure 6v
Second input 12v
Third input 18v, and so on up to 48v (which might actually be much closer to 60v with a fully charged battery).

Then do the subtraction in software to calculate the voltage across each battery.
The problem with that, is you only have ten bits of resolution to work with, or one part in 1,024.
So steps of roughly 60mV resolution will be about the best you can do for the voltage of each battery.

You can do much better than that, by averaging many readings in software, and achieve a usefully higher resolution that way.  It trades off speed of response for increased resolution.

Initially connect all eight voltage dividers to the full battery voltage, and tweak your voltage divider adjustment potetiometers to read the exact same identical voltage on every input, so you know they all read the same.  The rest is just number crunching in software.
Cheers, Tony.
 
windman1000
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Joined: 21/06/2009
Location: United States
Posts: 91
Posted: 11:57pm 03 May 2020
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thanks tony i did think about the 6v 12v 18v 24v to 48v and calculating threw the program but thought it wouldn't be very accurate but since i do have the voltage dividers at 70vdc it sure wouldn't hurt to try it that way.
Edited 2020-05-04 09:57 by windman1000
 
Warpspeed
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Joined: 09/08/2007
Location: Australia
Posts: 3254
Posted: 12:49am 04 May 2020
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You should be able to get some pretty good results if a very large number of readings are taken and averaged. Noise and randomness will produce some very nice scattering of the individual readings, and if those variations are averaged out, its possible to greatly increase the resolution by a lot more than you might reasonably expect.

Nicest way to average is with a revolving stack.
Replace the oldest reading with the newest, and then average all of the values stored in the stack. The length of the stack can be a very few or thousands !

That provides a very smooth constantly updating average, and the longer the stack, the slower and smoother and higher resolution the result will be.

Another hint, you can purchase 0.1% resistors. These sometimes come up on e-bay in funny odd values.  These can be quite expensive if bought new, but its sometimes possible to get lucky on e-bay. Try not to overheat them when soldering, and it should be possible to make eight very closely matched voltage dividers without having any potentiometers to adjust.

The ratio of the voltage dividers is not critical, final voltage calibration will be adjusted in software anyway.

*edit*
Just spotted these on flea bay, this is the type of thing to look out for, 0.1% and a temperature coefficient of 25ppm rather than the more usual 40ppm per degree Celsius.
With a bit more looking, might be possible to find something else to produce a usable step down voltage ratio at a sane cost.
https://www.ebay.com.au/itm/10x-1-69k-Ohm-1k69-0-1-25ppm-0207-Metal-Film-Resistor-Metal-Layer-Resistor/281959564902?hash=item41a6198666:g:hGUAAOSwdzhcMWT~

If you still wish to try eight isolation amplifiers, these are available at around ten dollars each.
https://docs.broadcom.com/doc/AV02-3563EN
Edited 2020-05-04 12:02 by Warpspeed
Cheers, Tony.
 
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