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Forum Index : Microcontroller and PC projects : Measurements using Micromite?
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lew247 Guru  Joined: 23/12/2015 Location: United KingdomPosts: 1676 |
Is it possible to measure mA (milliAmps) using MMBasic and the Micromite? I have a NovaLynx 200-WS-23 Current Loop Wind Sensor which measures wind speed and direction and outputs both as 4 - 20mA 2 wire measurement loop. I know I could probably dispose of the electronics box that came with the anemometer and the wind speed unit and just use the pulses generated by the reed switch in the anemometer and count them and use the 20K pot on the direction sensor but I was thinking as the unit already outputs 4-20mA directly proportional to speed or direction was it possible to use it somehow Anyone got any ideas/thoughts? |
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| viscomjim Guru Joined: 08/01/2014 Location: United StatesPosts: 925 |
You can get a module like THIS and use the uMites a to d input. I'm sure there are other ways to go. Maybe there is a 4-20ma receiver that can do I2C. |
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| Chris Roper Senior Member  Joined: 19/05/2015 Location: South AfricaPosts: 280 |
Using Ohm's Law if you place a 160 Ohm resistor in series with the current loop and measure the Voltage across it with the MicroMite A2D function then 640 mV = 4 mA and 3.2 V = 20 mA. I have not physically done that but the theory should form a basis for experiment if you need a simple solution. Cheers Chris http://caroper.blogspot.com/ |
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MicroBlocks Guru Joined: 12/05/2012 Location: ThailandPosts: 2209 |
The 'normal' resistor used in PLA's is 250 ohm, but the closer you can get to zero and still have a good enough range to measure, it will be better. All is dependent on the precision you need. You can go low as 0.1 ohm. Adding a resistor with a tolerance will vary the calculated values. It is only to get a ballpark value to start with. You would need to calibrate (scale) the voltages measured. Another gotcha is the ground, make sure your ground and the instruments ground is at the same potential. Low side is safer then high side. Have a look here: http://www.dataq.com/data-acquisition/general-education-tutorials/4-20-ma-current-loop-data-acquisition.html Microblocks. Build with logic. |
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| Chris Roper Senior Member Joined: 19/05/2015 Location: South AfricaPosts: 280 |
250 ohm is the standard with 5V but the PIC32 being a 3V3 part needs a smaller value. 160 Ohm being the best fit from the standard E24 Series. If the device properly implements the current loop it should be a constant current source and not change with a 160 Ohm Load. That said you will get better accuracy if you use an OpAmp to take the reading. Cheers Chris http://caroper.blogspot.com/ |
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