ostaquet
commented
4 years ago See below some answers.
1° I changed the original schema with a MOSFET to have a higher voltage on the heater. Using a BC337 is fine but, even saturated, you don't have 5V on the heater. There is always a drop of voltage in the transistor (Vbe). For the BC337, you end with 4.3V for the heater in ideal conditions. With the MOSFET, you have 5V for the heater, it is better the reach the temperature.
3° The voltage on the analog input is calculated by using the analog to digital from the Arduino. The analog input returns a value between 0 and 1023 which is corresponding the 0V to 5V. So, voltage (VRL) = analog value / 1024 x 5V.
The second part needs a bit more calculation...
The Rs and the RL in the circuit are a voltage divider. The concept of a voltage divider is by putting two resistors is series. The VRL measured on the Arduino analog pin is the final voltage coming from the voltage divider (usually called Vout). We also know the Vin (5V).
Vout = Vin * (R2 / (R1 + R2))
Where:
- Vout = VRL
- Vin = 5V
- R2 = RL
- R1 = Rs
We try to find Rs (so, R1)... The first line is the formula of a voltage divider to know the Vout (not easy reading in GitHub...)
Vout = Vin (R2 / (R1 + R2)) Vout / Vin = R2 / (R1 + R2) Vin / Vout = (R1 + R2) / R2 (Vin R2) / Vout = (R1 + R2) ((Vin R2) - R2) / Vout = R1 R1 = ((Vin R2) - R2) / Vout R1 = ((Vin / Vout) - 1) * R2
So... Rs = ((5V / VRL) - 1) * RL
4° R0 can vary, of course. According to some readings on the internet, the calibration process should be done every 3 months but I don't have specific experience on this. Calibration should be in clean fresh air, you should not do it at every sample.
5° No articles published (it was already a long time ago and not about this discipline ;-) ).
marinajg
Hi Oliver!!! Thank you so much for this library, it's helping me a lot!
1) I noticed that you've changed the sketch and insted of BC337 it's now MOSFET IFR840. But I have the first one, so I want to use it! But in this old scheme, you use a 5,1kOhm resistor on the base to ensure saturation. But, by my calculations, we have to use 2,5kOhm or less, since Ic is 150 mA, than Rb is Hfe(Vbb-Vbe)/Ic = 100(5-1.2)/0.15 = 2500 Ohm. Right? Than I would use a 2,2kOhm resistor to guarantee a higher Ib.
3) The rS was calculeted at readRs function. About this one, could you explain to me the math? I just undestand that the analogue pin gives a number between 0 to 1023 and thats why you divide by 1024. But the rest? // Compute the voltage on load resistance (for 5V Arduino) float vRL = ((float)valueSensor) / 1024.0 5.0; // Compute the resistance of the sensor (for 5V Arduino) float rS = (5.0 / vRL - 1.0) valueRL;
5) I saw that R0 can change a bit with use, so is good to keep thack on it. You recomend to calibrate every time before sampling?
6) Do you have any article published?
I'm doing this as my final work to graduate in environmental engineering, so I don't understand much of eletronics/programing. Sorry for the english too, I'm brasilian.
Thanks a lot already!
XOXO, Marina.