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Forum Index : Microcontroller and PC projects : Maximite voltage sensitivity below 100mV
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Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5015 |
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 |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 5905 |
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 MMedit MMBasic Help |
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Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5015 |
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 |
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panky Guru Joined: 02/10/2012 Location: AustraliaPosts: 1094 |
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! |
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twofingers Guru Joined: 02/06/2014 Location: GermanyPosts: 1133 |
@Panky how about the MCP3424? Regards Michael |
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panky Guru Joined: 02/10/2012 Location: AustraliaPosts: 1094 |
@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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