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17

Using Arduino / Programming Questions / MQ135 Value Conversion

on: December 27, 2012, 01:55:59 pm

i've a mq135  http://www.futurlec.com/Datasheet/Sensor/MQ135.pdfplease anyone correct me if i'm wrong: what i've undestand from datasheet is that ppm = Rs/Ro Rs = mq135 readed resistence Ro = sensor resistance at 100ppm of NH3 in the clean air (according to datasheet 30Kto200K) i wire the sensor according to datasheet (see attached image, downloaded is from http://www.staceyk.org) set RL = pulldown resistor (in the example 39K, according to datasheet any between 10KΩ to 47 KΩ) so Rs = 1024*RL/adc  RL the point here is to setup Ro, which means calibrate the sensor. any help? thank you, davide



19

Using Arduino / General Electronics / Re: ac current meter using acs758 hall sensor

on: November 22, 2012, 09:30:15 am

i've made further test it seems that with a sensitivity of 0.034/2 it works from 4A to 12A quite well. 0.034 seems ok, because my supply is 4.7V, sensitivity should be 0.03824 + 5mV (Electrical offset voltage from datasheet) but i've to use half of it.
p.s checked also my multimeter and it has no problem.
have you any suggestion, or anyone here that can test my code on other chip?
tks!



25

Using Arduino / General Electronics / Re: ac current meter using acs758 hall sensor

on: November 20, 2012, 05:20:48 am

damned paranthesis! later i will use #define or const for my code, code here is only for test purpose my new code has a simple filter for adc to remove offset uint16_t i = 0; int16_t adc_raw = 0; int16_t adc_rawlast = 0; int16_t adc_filtered = 0; int16_t adc_lastfiltered = 0; for(i=0; i<100; i++) { adc_rawlast = adc_raw; adc_raw = (512  adc_read(2)); adc_lastfiltered = adc_filtered; //digital high pass filters to remove offset. adc_filtered = 0.996*(adc_lastfiltered+adc_rawadc_rawlast); s = adc_filtered * adc_filtered; ssum += s; _delay_us(100); } d = sqrt(ssum/100) * 5 / (0.018 * 1024.0); ssum = 0; now with a sensitivity of 0.018 (0.036 (close to 0.04) /2) things seems right from 0.5A to 12A my chip is ACS758 LCB050B because d = sqrt(ssum/100) * 5 / (0.018 * 1024.0); is like d = 2 * sqrt(ssum/100) * 5 / (0.036 * 1024.0); isn't that there is a 2 multiply left?



27

Using Arduino / General Electronics / Re: ac current meter using acs758 hall sensor

on: November 20, 2012, 03:48:12 am

it's a 1500w hair dryer with three position (stop, 3.4A, 6.4A, as read by my multimeter, which is not true rms). i've also try attaching another heater, from 6.4A the current sucked it sine wave form BUT, and the extimated current measure is right IF i change the sensitivity A LOT. uint16_t i = 0; double sensitivity = 0.0016; int16_t adc_raw = 0; int16_t adc_zero = 512; //as read when there is no current on the load for(i=0; i<100; i++) { adc_raw = adc_read(2); l += (adc_raw  adc_zero)*(adc_raw  adc_zero); _delay_us(100); } d = (sqrt(l)/100) * 5 / (0.0016 * 1024.0)  0.5; l = 0; d = (sqrt(l)/100) * 5 / (0.0016 * 1024.0)  0.5; with sensitivity of 0.0016 and an offset of 0.5 i've almost right result on 0A and above 3.4A (tried only 6.4A / 8A / 12A) but from datasheet sensitivity is 0.04 ( = 25 * 0.0016)



28

Using Arduino / General Electronics / Re: ac current meter using acs758 hall sensor

on: November 19, 2012, 02:08:55 pm

l += pow(adc_raw  adc_zero, 2);
You don't want to use pow() here, it uses float, double and is far too slow (it does logarithms, antilogarithms). long val = adc_raw  adc_zero ; l += val * val ;
You need to be sampling the waveform at 1000Hz or more for good accuracy so its important the maths isn't too slow. Lose the delays  if you want to control the timing you should use millis() or micros() so you can synchronize to a multiple of 50 or 60Hz. First look at a set of raw values from the sensor to check they are what you'd expect (sample a bunch into an array, then print out the array, printing out is too slow also). You should see a clear sinusoidal pattern with the correct amplitude. Then test your calculation code with known correct raw values to see if the arithmetic is correct. attached a plot of raw values, just analogRead of the pin. it has a strange behavior+ the first part (y=512) is a line, device is power off. + the second part, some sort of 1/2 sine wave, only + part is present, device @ 3.4Amp, [strange here, why??] + the third part is a sine wave, device @ 6.4Amp sensor has the same circuit as the datasheet propose, it does not change if Cf and / or Rf are present or not.



29

Using Arduino / General Electronics / Re: ac current meter using acs758 hall sensor

on: November 19, 2012, 11:11:57 am

Almost. You need to convert (adc_raw  adc_zero) to type 'long' before you square it, because the square may exceed the range of an int. You will need to declare currentac with type 'long' too. After taking the sqrt, you need to adjust it for the sensitivity etc. just as you do for DC.
i've left all untyped i know typed code is below. i've tried two ways, but it doesn't seams to works. test 1 subtract the zerocurrent value (512 or thereabouts) from the raw readings, square that, and add it to an accumulator variable. When you have accumulated enough samples, take the square root if the accumulator and divide by the number of readings.
double d = 0; long l = 0; for(;;) { uint16_t i = 0; int16_t adc_raw = 0; int16_t adc_zero = 512; for(i=0; i<100; i++) { adc_raw = adc_read(2); l += pow(adc_raw  adc_zero, 2); _delay_us(100); } d = (sqrt(l)/100) * 5 / (0.04 * 1024.0); l = 0;
test 2 average the square of the current, and take the square root of the average
double c = 0, d = 0; for(;;) { uint16_t i = 0; int16_t adc_raw = 0; int16_t adc_zero = 512; for(i=0; i<100; i++) { adc_raw = adc_read(2); adc_raw = adc_raw  adc_zero; c += pow(adc_raw * 5 / (0.04 * 1024.0), 2); _delay_us(100); } d = sqrt(c)/100; c = 0; ... print c... } i'm testing a 6.4 Amp 220V 50hz device both code i obtain almost the same value, which is 0.27Amp, but is not correct



30

Using Arduino / General Electronics / Re: ac current meter using acs758 hall sensor

on: November 19, 2012, 09:49:03 am

The max value you can read is indeed 1023, but the correct value to use in the calulation is 1024. See section 24.7 of the atmega328p datasheet.
Thank you for this info! Are you trying to measure alternating current? If so, you need to average the square of the current, and take the square root of the average (that's what RMS means). Easiest way is to subtract the zerocurrent value (512 or thereabouts) from the raw readings, square that, and add it to an accumulator variable. When you have accumulated enough samples, take the square root if the accumulator and divide by the number of readings.
Yes ac current. so, for AC current something like this samplesnum = 100; adc_zero = 512; //as read when there is no current on the load sensitivity = 0.04; //as read from datasheet for(i=0; i<samplesnum ; i++) adc_raw = analogRead(1); currentacc += (adc_raw  adc_zero) * (adc_raw  adc_zero); //subtract the zerocurrent value (512 or thereabouts) from the raw readings, square that, and add it to an accumulator variable } currentac = sqrt(currentacc)/samplesnum; //take the square root if the accumulator and divide by the number of readings. for DC current something like this adc_zero = 512; //as read when there is no current on the load sensitivity = 0.04; //as read from datasheet currentdc = (adc_raw  adc_zero) * 5/(sensitivity * 1024) right?



