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Forum Index : Microcontroller and PC projects : can the maxi read current
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sparkey Senior Member  Joined: 15/06/2011 Location: AustraliaPosts: 819 |
would it be possible for the maxi to read a potential difference across two pins and givea calculated report accross the two pins allowing for a millivolt of difference so that a reported difference would be re calculated in "amp/hours"be across a shunt....////.../// technicians do it with least resistance |
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thetinkerer Regular Member  Joined: 16/06/2011 Location: AustraliaPosts: 68 |
Perhaps you could use a differential amp across the shunt and feed it into the Maximite. There is an Example of using an OP Amp in one of the Silicon chip articles. regards |
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| sparkey Senior Member Joined: 15/06/2011 Location: AustraliaPosts: 819 |
thank you i will have to dig up the mags thanks..regards sparkey...but the problem is with my floor less software i cant code to save my self from drowning...lol...regards sparkey... technicians do it with least resistance |
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| Ray B Senior Member Joined: 16/02/2007 Location: AustraliaPosts: 219 |
Why measure across 2 pins? Just measure the volt drop agross a known R. If it is DC then use a resistor in series with the load and measure the volt drop across it. Bit of ohms law will tell you the R you need. If it is AC use a transformer maybe even with the primary & secondary reversed depending of the voltage / current your dealing with. You may even find an old CT (current transformer) transducer floating around in that shed of yours. Watch out for open circuit voltages from transformer they may bite - once again calculate expected monitored voltage and consider amplification through turns ratio. This will give an instantaneous voltage proportional to current - from then just factor the time period by sampling over a period of interest and calculate an average, minimum, maximum over the time period. Look at the Data logger example re saving readings that you can then periodically read back and average etc. don't blow yourself up with this one.... RayB from Perth WA |
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| thetinkerer Regular Member Joined: 16/06/2011 Location: AustraliaPosts: 68 |
Try page 69 in the May issue 2011. Here an example : Current measuring with a shunt You can put a 10k resistor between the "5v=5A" and the Maximite input. |
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jman Guru  Joined: 12/06/2011 Location: New ZealandPosts: 711 |
Hi This one works quite well http://www.sparkfun.com/products/8883 John |
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| Ray B Senior Member Joined: 16/02/2007 Location: AustraliaPosts: 219 |
follow this link to info on the same subject started by me on Picaxe forum with reference to op amps for low current reading & read between the lines... http://www.picaxeforum.co.uk/showthread.php?17961-Measuring- Low-AC-Voltages&highlight=tong RayB from Perth WA |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
In the past I've tried a few different ways to measure current with a micro chip. I'll link to a few projects here. First up was a picaxe based windmill/solar controller with LCD display and menu setup. http://www.thebackshed.com/Windmill/articles/GizmoPicAxeChar ger.asp I used a hall sensor and op-amp to measure current, it worked ok, but wasn't very linear. Later I wanted a more accurate sensor for logging purposes, so built a picaxe based logger http://www.thebackshed.com/Windmill/PicLog.asp. It used a low side shunt resistor, good accurate results but not practical, as the circuit is slightly above ground, and it only measures current in one direction. Later I started using current transducers and the Allegro current sensor chips. Both of these options provide isolation and will measure current in both directions. Some Allegro current sensor info here. These will typically provide a 0 to 5 volt output, with 2.5 volts equal to 0 amps. To feed into a Maximite you would need a couple of resistors to get into the Maximites 3.3 volt input range. The Allegro chips have a little drift around 0 amps, and this can do weird things to a amp hour reading. At 0 amps, the output may drift from 2.49 to 2.51 volts. Two tricks to fix it, electrical filtering and software filtering. Fit a 22uF or 47uF cap across the microchip input, and a 47k resistor in series with the input and the sensor. This will filter out any little fluctuation in the sensors output, you may need to experiment with the component values for best results. For software filtering, if the 0 amp reading equals a figure of 1.65 from the Maximites AD converter, then you would ignore any readings between 1.64 and 1.66, treat them as 1.65. It just filters out any slight changes around the 0 amp value. Once it gets past that dead zone, it will treat the values are real data. I do want to try a high side difference amp like thetinkerer suggested. Yes you can use the Maximite directly to do this with two inputs, but you will loose a lot of resolution. The op amp increases the resolution lots. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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Keith @ Senior Member Joined: 19/06/2011 Location: AustraliaPosts: 167 |
Sparkey As per usual you're rather lacking in descriptive detail. What sort of amps? DC or AC ... low amps <5 or high amps >100? What resolution and accuracy? For shunts Jaycar have QP-5410,12,14,16 For AC contactless use a Jaycar clampmeter QM-1565 These may or may not meet your requirements or budget but for an indication of AC amps and portability without breaking into a circuit I don't think you can beat the clampmeter ... 0.1A to 300A AC RMS output as 1mV/0.1A Keith The more we know, the more we know we don't know ! |
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