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Gizmo

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Joined: 05/06/2004
Location: Australia
Posts: 5015

Posted: 02:16pm 15 Jul 2015

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Hiya

rebuilding my Maximite based solar charge controller, adding an extra 8 solar panels so need to beef it up a bit.

Currently using a 50 amp Alegro current sensor chip to measure charging current into my battery bank. With the new panels and wind generator, my charging current could go over 60 amps. There are a couple of (free) options for me.

I could solder a shunt wire across the Alegro to reduce the current through the sensor. This means the Alegro is no longer calibrated, but I do the calibration in the Maximite software anyway. This is a simple fix.

Or I could use a old 100amp 100mV shunt resistor I've had lying around for years just waiting for a chance to get used. If I fit this in the -ve line, it would directly read mV - Amps, so 1mV = 1 amps, up 100mV = 100 amps.

Question is, just how sensitive/reliable is the Maximite analogue input at these low readings. Would it measure down to less than 1mV, or would the values be too erratic? I dont want to add a op-amp to voltage multiply, thats getting fiddly compared to the Alegro option.

Glenn

The best time to plant a tree was twenty years ago, the second best time is right now.
JAQ

TassyJim

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Joined: 07/08/2011
Location: Australia
Posts: 5905

Posted: 02:38pm 15 Jul 2015

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The best that the maximite can resolve is 3.2mV

3.3V / 1024 = 0.00322265

and that's assuming everything can stay calibrated.

If you do use the 100A shunt, you would need to use an instrument amplifier to increase the output and let the maximite control the gain. That will give you the best of both worlds.

The shunt across the Alegro or a higher rating unit would be the simplest.

Jim

VK7JH
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Gizmo

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Posted: 02:52pm 15 Jul 2015

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Thanks Jim.

I suspected as much. A google couldn't find the minimum 3.2mV reading, but have to admit I didn't look that hard.

I'll add a shunt to the Alegro, found some documentation about it here.

Glenn

The best time to plant a tree was twenty years ago, the second best time is right now.
JAQ

panky

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Joined: 02/10/2012
Location: Australia
Posts: 1094

Posted: 01:46pm 16 Jul 2015

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Hi Gizmo,

Have you looked at just upgrading the Allegro to a 100 amp version? Straight drop in replacement and although a few extra bucks, would certainly be simplest solution.

By the way, my system uses the 50amp allegros but I would love to have a 12 to 16 bit a2d in the mm to get resolution down to sub millivolt readings. I am looking around for external 16bit a2d with it's own reference for calibration using i2c or spi - any thoughts anyone?

Also, has anyone done anything with a plain d2a with accururatacy better than 1mV resolution? Eg. I just want a 2.125v output or a 1v output etc but adjustable under program control

Doug.

... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it!

twofingers
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Joined: 02/06/2014
Location: Germany
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Posted: 01:51pm 16 Jul 2015

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@Panky

how about the MCP3424?

Regards
Michael

panky

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Posted: 12:22pm 17 Jul 2015

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@twofingers,

Thanks Michael, exactly what I am looking for - must have missed that thread somehow,

Regards, Doug.

... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it!

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