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Forum Index : Microcontroller and PC projects : Current Shunt Measurements
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| nasi Newbie  Joined: 08/07/2015 Location: AustraliaPosts: 16 |
G'Day to all. Thank you in advance for any help on short notice. I have a 24 Volts Solar Off Grid System in operation. 2 kilo watt capacity. My MPPT controller can push up to 80 amps into my battery bank. I have a manganin current shunt FL-2/0.5 300 Amp 75 mV. It came with a small analogue gauge that works well. I have installed an 18m2 picaxe near the bank which is currently taking voltage measurements through a resistor divider network and sending that data back to my maximites via rs485. I realise there are some other current sensing components out there I can order, however being as impatient as I am and wanting to test something out, I was wondering if I can use some components I have here to amplify that teensy voltage drop across that shunt to make it useful for the Picaxe ADC. I have LM358 opamps and I have an IN126P TI Instrumentation amp, Maybe some other opamps. Can I use these with relative effectiveness to get a reasonable gain and accuracy from this shunt that will make the effort worth while? I did try a hall sensor but found it to be quite fiddly and ineffective. I realise the shunt maybe a bit too big for my current setup but its all I have right now. Thank you all again in advance. Regards Richard |
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panky Guru  Joined: 02/10/2012 Location: AustraliaPosts: 1098 |
Richard, Any decent open amp would be OK but because of the low voltages (near zero for lower current), you would need to have positive and negative rails for the op amp and to also fine tune the offset. Even the newer rail-to-rail op amps loose linearity close to either rail. I find the hall effect sensors like the Allegro range by far and away the best way to go as they are reasonably cheap, at least as accurate as the 10 bit ADC's in micros and as they are isolated, can be used in either high side or low side measurement. I have used them extensively in a caravan monitoring system and recommend them as the best way to go. Doug. ... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it! |
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| nasi Newbie Joined: 08/07/2015 Location: AustraliaPosts: 16 |
Hey Doug Thanks heaps for the reply. In my case the face that I probably will not exceed 80 amps in my system for now. Can you recommend which Allegro part I should use in this scenario? Thanks Again Richard |
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redrok Senior Member  Joined: 15/09/2014 Location: United StatesPosts: 209 |
Hi Nasi; I like the venerable single supply LM324 op-amps. However, today there are a number single supply of op-amps that have outputs that can go from rail to rail. And the inputs can go from slightly below the negative rail to nearly the positive rail. I like the MICROCHIP MCP609-I/P and MCP619-I/P in the DIP package. Better yet these run nicely at 3.3V. While not a cheap as the LM324 they should be in everyone's junk box. They cost about $1.55us. I just experimented yesterday with a thermocouple amplifier running at a gain of 1000 and read by the the AtoD. You only need a gain of 44 or so to go full range from 75mV. redrok |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 5914 |
I use the ACS712 for low currents 5, 20 and 30 A bidirectional and the ACS758 for 50A and above They have a +/-100A and 100A unidirectional, one of which would be ideal for your 80A The advantage over op-amps is they are isolated and can be put in either the positive or negative rail. They require a 5V supply. Jim VK7JH MMedit MMBasic Help |
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| panky Guru Joined: 02/10/2012 Location: AustraliaPosts: 1098 |
Nasi, I agree with Jim, the ACS758 would be ideal for you. You can buy them as individual chips or as a module - the latter available on ebay for around $15. I have used a number of these with a Maximite and they work a treat. Just a resistor divider to bring the 0 to 5V output of the ACS758 down to a voltage that the Maximite can handle and possibly a capacitor to stabilise any noise. Doug. PS. I have code to read and graph data from ACS modules and graph output. I have both colour Maximite driving a normal VGA display and Micrmite plus driving a 7" display versions which I am happy to provide to you. Just PM me if you want a copy of either. Doug. ... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it! |
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| nasi Newbie Joined: 08/07/2015 Location: AustraliaPosts: 16 |
Gentlemen. Thank you very much this is great. Ordering underway for these parts. And a very generous offer from Doug. Redrok, Jim and Doug its great to involve myself with people of common interests. Speak more soon Richard |
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| Phil23 Guru Joined: 27/03/2016 Location: AustraliaPosts: 1664 |
I got a couple back last year to add to my Heat Pump conversion.  It normally uses 750 to 820 Watts; Maximum rating is 1300W. But when things go wrong, like lost water flow it quickly climbs to the 100W mark. Just not sure what range the resistors should be in for the Voltage divider to take it's 5V output down to 3.3V. I know the calculation & 5.1k & 10k would work, as would 50k & 100k etc. Just not clear on what's the ideal overall resistance of the divider. What's the recommendation? Thanks Phil. |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 5914 |
The ASC712 units output 2.5V with zero amps flowing. If you install them the right way around, the output will go lower with increased current. I am using 30A ones on my solar monitor. With 11 amps flowing the 'wrong' way, the output reaches 3.3V One sensor I have can go that way when an AC charger is running but it's output is less than 11A so I have no problems. Under normal current flow 30A would have the output at about 0.5V so there is no need for any voltage conversion. I had 'fun' getting reliable zero current readings. I will post a description of my efforts 'soon' Jim VK7JH MMedit MMBasic Help |
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| panky Guru Joined: 02/10/2012 Location: AustraliaPosts: 1098 |
Like Jim, I had trouble getting the zero current reading correct and reliable. My approach ended up using ACS758 modules with the bi-directional version. As I only needed to monitor current to 50A, I used the +/- amp version. Each module has an LM317 variable voltage regulator on it and I modified the output adjust resistor divider chain by adding a 250 ohm 25 turn trimpot into the chain such that I could adjust the supply to the ACS758 from 4.9volts to 5.1 volts. This in turn allowed me to "tune"the zero current point to exactly 2.5 volts. I have 5 ACS modules set up this way to read a range of charging and load currents in the van. In the umite, I read the output of the ACS758 directly with 2.5 volts equal to 0 amps and 0 volts equal to 50 amps. So, input pin minus 2.5 times 20 gives a direct current reading. There is a bit of loss of linearity as the ACS758 output gets close to 0 volts (50 amps) but as I only need to work from around 0 amps to a max of 30 or so, it is not a problem for me. The supply to the mite needs to be exactly 3.3 volts with as little system noise as possible (as you would expect). For voltage measurement, I use the standard resistor divide chain but add in the 25 turn trimpot to "tune" the values as needed. Checking with a good quality meter, both voltage and current readings are good to about 1%, more than sufficient for my requirements. cheers, Doug. ... almost all of the Maximites, the MicromMites, the MM Extremes, the ArmMites, the PicoMite and loving it! |
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